CA1062824A - Two-way telephone transmission system utilizing opto-couplers - Google Patents
Two-way telephone transmission system utilizing opto-couplersInfo
- Publication number
- CA1062824A CA1062824A CA244,517A CA244517A CA1062824A CA 1062824 A CA1062824 A CA 1062824A CA 244517 A CA244517 A CA 244517A CA 1062824 A CA1062824 A CA 1062824A
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- input
- coupled
- amplifier
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- output
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/44—Arrangements for feeding power to a repeater along the transmission line
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/36—Repeater circuits
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optical Communication System (AREA)
- Amplifiers (AREA)
- Dc Digital Transmission (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
Abstract
Abstract of the Disclosure A two-way transmission system, which utilizes solid state circuitry and a pair of opto-couplers, achieves high linearity and provides high electrical isolation between input and output signals without the use of the traditional hybrid transformer. The opto-coupler consists essentially of a single light-emitting diode (LED), two light detector diodes (LDDs), and an operational amplifier. The LED is connected to the output of the amplifier and is physically located with respect to the two LCDs such that light emitted by the LED impinges on the photosensitive areas of the LDDs.
The first LDD is coupled to the input of the amplifier, and the second LDD is coupled to output circuitry. This opto-coupler is relatively temperature insensitive and compensates for the inherent nonlinearity of an LED. These characteristics facilitate the use of the above-described opto-coupler in systems which require relatively high linearity and temperature stability.
The first LDD is coupled to the input of the amplifier, and the second LDD is coupled to output circuitry. This opto-coupler is relatively temperature insensitive and compensates for the inherent nonlinearity of an LED. These characteristics facilitate the use of the above-described opto-coupler in systems which require relatively high linearity and temperature stability.
Description
Backqround of the Invention This invention relates to linear communications systems which use optical coupler circuitry and, in particular, to optically coupled circuitry which has sufficient linearity and temperature insensitivity to be used in a telephone system.
Various attempts have been made to eliminate transformers with the use of solid-state electronics. One solid-state combination suggested is the light-emitting diode (LED) - light-detecting diode (LDD) pair. One serious disadvantage of an LED-LDD optical coupler is the inherent non-linear transfer characteristics and the poor temperature sensitivity. Various attempts have been made to compensate for these undesirable characteristics of the LED-LDD pair to allow the use thereof in anàlogue sys-tems which require a relatively high degree of linearity (low distortion).
One such attempted solution is the use of two substantially identical serially-connected LEDS connected to the output of an operational amplifier. Two LDDs are physically located with respect to the LEDS such that light from the first LED impinges on the first LDD, and light from the second LED impinges on the second LDD, The first LDD is coupled to the input of the operational amplifier so as to create a negative feedback path. The second LDD iS coupled to output circuitry. The negative feedback path serves to help linearize the output signal with respect to an input signal. One serious problem associated with this configuration is that the two serially-connected LEDs must have essentially identical electro-optical characteristics in order to obtain reasonably good linearity. The requirement of matching the characteristics of two LEDs closely make such an optical coupler economically unattractive.
One solution to the above problem of matching diodes is the use of two parallel LEDS with separate series resistors coupled to each. The LEDs are matched electrically and optically as closely as possible. Differences in the electrical and optical characteristics can be somewhat attenuated at any one operating point by varying one or the other of the resistors to vary the current through either LED. A major disadvantage of th~~s configuration is that while one setting of resistor values may produce a high-quality linear response at one operating point, the same resistor values will result in unsatisfactory response at other operating pGints.
It would be desirable to have a solid state optical coupler which doesn't require very close matching of the electro-optical characteristics of components but does provide relatively high linearity and is relatively insensitive to temperature variations.
Summary of the Invention One preferred embodiment of the invention is an optically coupled two-wire to two-wire bilateral communication system which comprises essentially two amplifiers (1 and 2j, two line termination impedances, two cancellation (hybrid subtraction) impedances, and two opto-coupler combinations which each comprise a dual input amplifier, a light-emitting diode (LED) and two light-detecting diodes (phQto-diodes) ( LDDs ) .
In each opto-coupler the LED and one LDD are coupled to the output and first input, respectively, of the amplifier associated with that opto-coupler. The second LDD
of each opto-coupler is coupled to an input of one of amplifiers 1 or 2. The first LED of each opto-coupler is positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the photosensitive areas of the first and second LDDs and gives rise to a feedback current signal which is coupled to the input of the amplifier of the opto-coupler.
A first input/output terminal is coupled through a first summing impedance to the first input terminal of the amplifier of the first opto-coupler. The output of amplifier 1 is coupled through the first line termination impedance to the first input/output terminal and to the second input of the amplifier associated with the first opto-coupler through a first cancellation thybrid-subtraction) impedance.
A second input/output terminal is coupled through a se`cond summing impedance to the first input of the amplifier of the second opto-coupler. The output of amplifier 2 is coupled to the second input/output terminal through a second line termination impedance and to the second input of the amplifier of the second opto-coupler through the second cancellation (hybrid subtraction) impedance.
Each opto-coupler employs negative feedback which modulates the output signal of the amplifier in such a manner that the output signal current induced in the second LDD is essentially linearized with respect to an input signal applied to the first input of the amplifier associated with the opto-coupler. The opto-couplers, in -~` 106Z8Z4 addition to providing linear output signals, provide relatively high electrical isolation between the two input/output terminals.
The amplifiers (1 and 2) provide gain such that a signal appearing at either input/output terminal is amplified at the other input/output terminal. The above-described communication system provides gain, and relatively high electrical isolation and linearity. This is all achieved without the use of the traditional hybrid transformer and without the necessity of closely matching diode characteristics.
In accordance with one aspect of the present invention there is provided a bidirectional communication system comprising:
first and second amplifiers, each having two input terminals and an output terminal;
third and fourth amplifiers, each having at least one input terminal and an output terminal;
first and second light-emitting diodes (LEDs), the first and second LEDs being coupled to the output terminals of the first and second amplifiers, respectively;
first, second, third and fourth light-detecting diodes (LDDs), the first LDD being coupled to a first input terminal of the first amplifier and the third LDD being coupled to a first input terminal of the second amplifier;
the first LED being positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the first and second LDDs;
the second LED being positioned with respect to the third and fourth LDDs such that light emitted by the second LED impinges on the third and fourth LDDs;
~ _ 4 _ ~- 106Z8Z4 the second and fourth LDDS being coupled to the input terminals of the third and fourth amplifiers, respectively;
a first input/output terminal being coupled to the first input terminal of the first amplifier through a first summing impedance and to the output of the fourth amplifier through a first line matching impedance;
a second input/output terminal being coupled to the .
first input of the second amplifier through a second summing impedance means and to the output of the third 0 amplifier through a second line matching impedance means;
the output terminal of the fourth amplifier being coupled to the second input terminal of the first amplifier through a first cancelling impedance means; and the output terminal of the third amplifier being coupled to the second input terminal of the second amplifier through a second cancelling impedance means.
In accordance with another aspect of the present invention there is provided an optically coupled bidirectional communication system comprisi.ng:
a first amplifier circuit means having an output and an input;
a first light-emitting means coupled to the output of the first amplifier circuit means;
first and second light detector means, the first and second light detector means being positioned so as to receive light from the first light-emitting means;
the first and second light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
- 4a -,~ ~
- ` 106Z824 the first light detector means being coupled to the input of the first amplifier;
second amplifier circuit means having an output and an input;
the second light detector means being coupled to the input of the second amplifier circuit means;
third amplifier circuit means having an input and an output;
a second light-emitting means coupled to the output of the third amplifier circuit means;
third and fourth light detector means, the third and fourth light detector means being positioned so as to reeeive light from the second light-emitting means;
the third and fourth light detector means being charaeterized in that incident light thereon is proportional to the eurrent indueed therethrough and the ratio of the indueed eurrents is essentially constant;
the third light detector means being coupled to the input of the third amplifier circuit means;
fourth amplifier cireuit means having an input and an output;
the fourth light deteetor means being eoupled to the input of the fourth amplifier eircuit means;
a first summing impedance and a first eancellation impedance;
a first input/output terminal being eoupled to the input of the third amplifier eireuit means through the summing impedance and being coupled to the output of the second amplifier eircuit means; and the first cancellation impedance being coupled to the output of the second amplifier circuit means and to the input of the third amplifier circuit means;
- 4b -a second summing impedance and a second cancell.ation impedance;
a second input/output terminal being coupled to the input of the first amplifier circuit means through the second summing impedance and being coupled to the output of the fourth amplifier circuit means;
the second cancellation impedance being coupled to the output of the fourth amplifier circuit means and to the input of the first amplifier circu;t means.
These and other features of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the following drawings: -Brief Description of the Drawings FIG. 1 illustrates an embodiment of an opto-coupler circuit;
FIG. 2 illustrates an embodiment of another opto-coupler circuit;
FIG. 3 illustrates an optically coupled bilateral communication system;
FIG. 4 illustrates another optically-coupled bilateral communication system;
FIG. 5 illustrates a circuit embodiment of an input and power circuit of FIG. 4; and FIG. 6 illustrates a circuit embodiment of another input and power circuit of FIG. 4.
- 4c - t Detailed Description Now referring to FIG. 1, there is illustrated an optical coupler circuit 10 which comprises a Darlington pair of transistors Ql and Q2, a light-emitting diode LEDl, and light detector diodes LDD1 and LDD2. An input terminal 12 is connected to one terminal of a resistor Rl. The other terminal of Rl (14) is connected to the base of Ql and the cathode of LDDl. The anode of LDD1 is coupled to a potential Va. The emitter of Q2 is connected to the anode of LEDl.
The cathode of LEDl is connected through a resistor R2 to a potential Vb. The collectors of Q1 and Q2 are connected together to a potential Vc. The value of potential Va is selected such that LDDl is reverse-biased during the entire operation of circuit 10. The potentials of Vc and Vb are selected such that LEDl is forward-biased and current flow can occur through the series combination of LEDl and R2. The anode of LDD2 is coupled to a terminal 16 (the output terminal) and through a resistor R3 to potential Vd. The cathode of LDD2 is coupled to potential Ve. Potentials Vd and Ve are selected such that LDD2 is always operating in reverse bias.
LEDl is positioned with respect to LDDl and LDD2 such that light emitted by LEDl impinges on the photo-sensitive areas of LDDl and LDD2.
An input signal applied to terminal 12 is amplified by Ql and Q2 and gives rise to a current through Q2, LED~
and R2. The current flow through LEDl causes LEDl to emit light that impinges on the light-sensitive areas of LDDl and LDD2. The current flow through LDDl acts as negative feedback to the input of the Darlington pair Ql and Q2.
This feedback modifies the output signal at the emitter of Q2 and causes the current level through Q2 and LED 1 to vary in response thereto. Accordingly, the light output of LEDl is varied such that the photo-induced current flow through LDD2 is linearized with respect to the input signal applied to terminal 12.
Photo-induced current in LDD2 flows through R3 and thus gives rise to an output voltage signal at terminal 16 which in linearized with respect to the input signal voltage applied to terminal 12.
The relationship between incident light impinging on a light-detecting diode (photo-diode) and the resulting conduction therethrough is known to be a very linear one.
This characteristic of photo-diodes makes it unnecessary that LDDl and LDD2 be identical. This feature allows circuit 10 to be manufactured with reasonable economy.
In one operating condition LDDl and LDD2 receive substantially equal amounts of light-signal energy from LEDl. The feedback to the input of the Darlington pair is a function of the light received by LDDl. As the amount of light received by LDDl decreases, the feedback decreases.
Correspondingly, distortion caused by the nonlinearity of LEDl is cancelled to a lesser extent. This results in somewhat less linearization (more distortion) between output and input signals. If the amount of light incident of LDDl increases, the negative feedback signal amplitude increases and the linearization improves (distortion decreases).
Considerable design flexibility is available such that the amount of feedback can be varied over wide limits.
Now referring to FIG. 2 there is illustrated an optical coupler circuit 18 which comprises an operational . -` 1062824 amplifier 24, a light-emitting diode LED2, and light detector diodes LDD3 and LDD4. An input terminal 20 is connected through a resistor R4 to the anode of LDD3 and inverting input terminal 22 of operational amplifier 24.
The output of amplifier 24 iS connected to the anode of LED2. The cathode of LED2 is coupled through a resistor R5 to potential Vg. The cathodes of LDD3 and LDD4 are coupled to potentials Vf and Vh, respectively. The anode of LDD4 iS
connected to output terminal 26 and to potential Vi through a resistor R6.
LED2 iS positioned with respect to LDD3 and LDD4 such that light emitted by LED2 impinges on the photosensitive areas of LDD3 and LDD4. Potentials Vf, Vh and Vi are selected such that LDD3 and LDD4 always operate in a reverse-bias condition. The potential of Vg is selected such that LED2 is able to operate in a forward-bias condition.
An input signal applied to terminal 20 is amplified by 24 and gives rise to a current through LED2. The current flow through LED2 causes LED2 to emit light that impinges on the light-sensitive areas of LDD3 and LDD4. This causes photo-induced current to flow in LDD3 and LDD4. The current flow through LDD3 acts as negative feedback to input terminal 22 of 24. This feedback modifies the output signal of 24 and causes the current level through LED2 to vary in response thereto. Accordingly, the light output of LED2 is varied such that the photo-induced current which flows through LDD4 is linearized with respect to the input signal applied to input terminal 20.
Photo-induced current in LDD4 flows through R6 and thus gives rise to an output voltage signal at terminal 26 which is linearized with respect to -the input signal voltage - applied to terminal 12.
The relationship between incident light impinging on a light-detecting diode (photo-diode) and the resulting conduction therethrough is known to be a very linear one.
This characteristic of photo-diodes makes it unnecessary that LDD3 and LDD4 be identical. This feature allows circuit 10 to be manufactured with reasonable economy.
In one operating condition, LDD3 and LDD4 receive substantially equal amounts of light signal energy from LED2. The feedback to the input of 24 is a function of the light received by LDD3. As the amount of light received by LDD3 décreases, the feedback decreases. Correspondingly, distortion caused by the nonlinearity of LED2 iS cancelled to a lesser extent. This results in somewhat less lineariza-tion (more distortion) between output and input signals.
If the amount of light incident on LDD3 increases, the negative feedback signal amplitude increases and the system linearization improves (distortion decreases). Considerable design flexibility is available such that the amount of feedback can be varied over wide limits.
The input signal applied to terminal 20 can be dc or ac. The corresponding output signal at terminal 26 iS
corresponding dc or ac.
Now referring to FIG. 3 there is illustrated an optically coupled bidirectional communica~ion system 28 which comprises two light-emitting diodes LED3 and LED4, four`light-detecting diodes (photo-diodes) LDD5, LDD6, LDD7 and LDD8, two dual input differential operational amplifiers 34 and 52, and two single input operational amplifiers 38 and 44. A first input/output terminal 30 is connected through a line-terminating impedance Zl to terminal 40 (the output of amplifier 38) and to input ~erminal 36 of amplifier 34 through a summing resistor R7. A cancellation (hybrid subtraction) impedance Z2 is connected between terminal 40 and input terminal 32 of amplifier 34. LDD5 is coupled by the anode to terminal 36 and by the cathode to potential Vi.
The output of amplifier 34 is coupled to the anode LED3.
The cathode of LED3 is coupled through a resistor R8 to potential Vj.
lQ LDD6 is couplea by the anode to input terminal 42 of amplifier 44 and to a resistor R9. R9 is coupled to potential Vl. The cathode of LDD6 iS coupled to potential Vk. Output terminal 46 of amplifier 44 is connected to input/output terminal 54 through line termination impedance Z4 and to input terminal 50 of amplifier 52 through cancellation (hybrid aubtraction) impedance Z3. Terminal 54 is coupled to input terminal 48 of amplifier 52 and the anode of LDD8 through summing resistor R10. The cathode of LDD8 iS coupled to potential Vm. The output of amplifier 52 iS connected to the anode of LED4. The cathode of LED4 is coupled to potential Vn through resistor Rll.
Again control feedback impedance Zol is connected across terminals 42 and 46 of amplifiers 44 and a gain control feedback impedance Zo2 iS connected across terminals 51 and 40 of amplifier 38. The anode of LDD7 is connected to input terminal 51 of amplifier 38 and a resistor R12.
The cathode of LDD7 is coupled to a potential Vo and R12 is coupled to a potential Vp.
LED3 is positioned with respect to LDD5 a~d LDD6 such that light emitted by LED3 impinges on the photosensitive areas of LDD5 and LDD6. LED4 is positioned with respect to LDD7 and LDD8 such that light emitted by LED4 impinges on the photosensitive areas of LDD7 and LDD8.
An input signal applied to terminal 30 can propagate through two input paths. The first path is through Zl to terminal 40 (the output terminal of amplifier 38). The input signal cannot propagate past terminal 46 since the output of operational amplifier 38 acts as a virtual ground. The second path is through R7 to terminal 36. The combination of R7, amplifier 34, LED3, LDDS and LDD6 acts in substantially the same manner as the combination of R4, amplifier 24, LED2, LDD3 and LDD4 of FIG. 2. Thus an input signal applied to the input terminal 30 is optically coupled to terminal 42 and the inherent nonlinearity of LED3 is effectively cancelled because of the optical feedback to LDD5.
The signal at terminal 42 is amplified by amplifier 44 and is coupled from terminal 46 to the input/output terminal 54 through impedance Z4 and then to input terminal 48 through R10. In addition, the signal at terminal 46 is coupled to the input terminal 50 through Z3.
Input/output terminal 54 has coupled thereto sender/receiver circuitry (not illustrated) which has an inherent impedance associated therewith. Z3 and Z4 are designed, taking into account the impedance characteristics of the sender/receiver circuitry (not illustrated) coupled to terminal 54, such that substantially equal signals are coupled to input terminals 48 and 50 of amplifier 52 from output terminal 46 of amplifier 44.
Equal signals appearing at the two input terminals (48 and S0) of differential amplifier 52 results in no output signal. ~onsequently the current level in LED4 does ~ 106Z824 not change. Therefore, the input signal from terminal 30 is not coupled back through LDD7, amplifier 38 and Zl to input/output terminal 30.
Thus, an input signal applied to terminal 30 is transmitted to terminal 54. It is effectively cancelled before the signal can propagate back to terminal 30 from where it originated.
An input signal applied to input/output terminal 54 propagates through two input paths. The first path is through Z4 to terminal 46. The second path is through R10 to an input terminal 48 of amplifier 52. The output terminal 46 of operational amplifier 44 acts as a virtual ground and thus prevents the signal from propagating further.
The input signal at 4 8 causes an output signal to appear at the output of 52 which causes LED4 to emit light in response thereto. This results in a signal being induced in LDD7 and LDD8. The light signal to LDD8 is a feedback signal and the light signal to LDD7 iS the output signal.
The output of amplifier 38 is coupled through Zl to input/output terminal 30 which is coupled through R7 to the input terminal 36 of amplifier 34. The output signal at terminal 40 is also coupled through Z2 to input terminal 32 of amplifier 34. The sender/receiver circuitry (not illustrated) coupled to terminal 30 has an inherent impedance characteristic associated therewith. Zl and Z2 are designed, taking into account the impedance characteristics of the sender/receiver connected to terminal 30, to insure that substantially equal signals are applied to input terminals 32 and 36 of amplifier 34. This insures that there is no output signal from 34 which could be coupled back 106Z8;~L
to input/output 54 where it originated.
Thus an input signal applied to terminal 54 is transmitted to terminal 30. It is effectively cancelled before the signal at 30 can propagate back to terminal 54 from where it originated.
The potentials Vi, Vk, Ve, Vm, Vo and Vp are selected to be of sufficient amplitude and polarity to cause LDDs 5, 6, 7 and 8 to operate in a reverse bias condition.
Potentials Vj and Vn are selected to be of sufficient polarity and magnitude to permit forward conduction through LED3 and LED4. The power supplies which provide potentials Vi, Vj, Vo and Vp are electrically separate from the power supplles that supply potentials Vk, Vl, Vm and Vn.
It is thus apparent that an input signal applied to terminal 30 or 54 will be transmitted from one terminal to the other but will not return to the terminal at which the signal originates. Circuit 28 provides both physical and electrical isolation between terminals 30 and 54 and provides for the linear transfer of information between the respective terminals. The gain of amplifiers 38 and 44 can be varied by adjusting Zol and Zo2. Thus signals can be linearly transmitted between terminals 30 and 54 with gain introduced and relatively high electrical isolation. The output light signals of LED3 and LED4 can be coupled to LDD6 and LDD7, respectively, through optical fibers. The use of low-loss high-linearity optical fibers permits system 28 to be used as an optical transmission system.
Now referring to FIG. 4, there is illustrated an optically coupled two-wire to two-wire bilateral communication system 55. System 55 has been designed as a replacement for the transmission path with voice frequency gain that 106282~
is illustrated by reference No. 115 in FIG. 4 of U.S. Patent No. 3,671,676 in which the assignee is the same as in this present application. The input terminals denoted as T and R of input and power circuitry 57 are connected thrbugh a central telephone office (not illustrated) to a first telephone set (not illustrated). The input terminals denoted as Tl and Rl are connected to a second telephone set (not illustrated) through the same central telephone office (not illustrated). These two telephone sets are thus coupled together through system 55. The input and power supply circuitry 57 and 59, which are illustrated in block form, will be described later in detail.
The output of 57 is intermediate input/output terminal 56 and the output of 59 is intermediate input/
output terminal 61. Terminal 56 is coupled through the series combination of a capacitor C12, a resistor R50, and a resistor R57 to an input terminal 60 of a differential two-input operational amplifier 62. Resistors R58 and R59 are connected to the common connection of R50 and RS7. A
resistor R60 is connected between terminal 60 and the anode of a light-detecting diode 64. The cathode of 64 is coupled to a potential +Vx. R59 is connected to a potential -Vx.
Output terminal 76 of amplifier 62 is connected to the anode of light-emitting diode 78. The cathode of 78 is coupled to a potential -Vx through the parallel combination of a resistor R64 and a capacitor C16. Terminal 56 is further connected through the series combination of a capacitor Cll and a resistor R49 to output terminal 66 of a two-input differential operational amplifier 68.
Terminal 66 is coupled to the series combination of a capacitor C13 and a resistor R54 to resistor R55 and a -"` 1062824 capacitor C15. C15 and a resistor R56 are both coupled to input terminal 70 of operational amplifier 62. R55 and R56 are both coupled to a potential Vx ref. Terminal 66 is also connected to the series combination of resistors R51, R53 and R98. R98 is coupled to Vx ref and to a switch Sl. Sl is a two-position switch which shorts R53 to Vx ref or connects R53 to Vx ref through R98. The common node of R53 and R51 is coupled through resistor R52 to capacitor C14 and to an input ~erminal 72 of operational amplifier 68.
Terminal 72 is coupled through a resistor R63 to the anode of light-detecting diode 74. C14 is also coupled to terminal 66. Resistor R61 is coupled to a resistor R62, and a capacitor C28 to input terminal 74 of operational amplifier 68. C28 is coupled to a potential Vy ref. R62 is coupled to Vx ref, and R61 is coupled to +Vx.
` A light-detecting diode 80 is coupled by the anode through a resistor R67 to an input terminal 82 of an operational amplifier 84. The cathode of 80 is coupled to a potential +Vy. A resistor R66 is coupled between +Vy and an input terminal 88 of amplifier 84. A capacitor C29 is coupled between Vx ref and terminal 88. A resistor R75 is coupled between Vy ref and terminal- 88. Terminal 82 is connected through the series combination of resistors R78 and R80 to the output terminal 86 of operational amplifier 84. A capacitor C23 is connected between terminals 82 and 86. R78 and R80 are connected to the series combination of a resistor R79 and a resistor R99. R99 is coupled to a potential Vy ref and a two-position switch S2. In one position S2 shorts R79 directly to Vy ref. In the other position R79 is coupled to Vy ref through R99.
Terminal 86 is connected through the series combination of resistor R77 and capacitor C22 to intermediate input/output terminal 61 and to an input terminal 90 of an operational amplifier 92 through the series combination of a capacitor C21 and a resistor R69.
Terminal 86 is also connected to input terminal 94 of 92 through the series combination of a capacitor C20, a resistor R76, and a capacitor C18. A capacitor Cl9 and a resistor R74 are both connected between C18 and Vy ref. A
resistor R73 is connected between terminal 94 and Vy ref.
Resistors R70 and R71 are both connected to R69. R70 and R71 are connected to -Vy and Vy ref, respectively.
Terminal 90 is connected through resistor R68 to the anode of a light-detecting diode 96. The cathode of 96 is coupled to +Vy. Output terminal 98 of amplifier 92 is coupled to the anode of a light-emitting diode 100. The cathode of 100 is connected to a potential-Vy through the parallel combination of a resistor R65 and a capacitor C17.
The combination of Cll and R49 and C22 and R77 (contained within dashed-line rectangles Zl' and Z4', respectively) act as line terminations. The combination of C13, C15, R54 and R55 (contained within dashed-line rectangle Z2') acts as a first hybrid substraction or cancellation impedance. The combination of C18, Cl9, C20, R74 and R76 (contained within dashed-line rectangle Z3') acts as a second hybrid subtraction or cancellation impedance.
The combination of C14, R51, R52, R53, R98 and Sl (contained within dashed-line rectangle Zol') serves as a gain control feedback impedance across input terminal 72 and output terminal 66 of operational amplifier 68. The combination of C23, R78, R79, R80, R99 and Sl (contained within dashed-line rectangle Zo2') serves as a gain control feedback impedance across input terminal 82 and output terminal 86 of operational amplifier 84.
C12 and C21 serve to block DC input signals from reaching input terminals 60 and 90 of operational amplifiers 62 and 92, respectively.
R50 serves to attenuate input signals coupled to terminal S6. R58 and R59 serve as a voltage divider network. R70 and R71 likewise serve as a voltage divider network. R57 and R69 serve as signal summing resistors.
Csl represents the inherent parasitic capacitance between light-emitting diode 78 and light detector diode 80.
Cs2 represents the inherent parasitic capacitance between light-emitting diode 100 and light-detecting diode 74. As has already been described, C28 is coupled between input terminal 74 of operational amplifier 68 and potential Vy ref. In addition, C29 is coupled between input terminal 88 of operational amplifier 84 and potential Vx ref. C28 and C29 serve to inject common mode signals into the respective first input terminals of the operational amplifiers 68 and 84 so as to cancel common mode signals injected into the second input terminals by the two parasitic capacitances Csl and Cs2. This effectively prevents common mode signals from propagating through the system 55.
LED78 is positioned with respect to ~DD64 and LDD80 such that light emitted by LED78 impinges on the photo-sensitive areas of LDD64 and LDD80. This incident light causes a photo-induced feedback current through LDD64 and an output signal current through LDD80.
` 10628Z4 LED100 is positioned with respect to LDD96 and LDD74 such that light emitted by LED100 impinges on the photosensitive areas of LDD96 and LDD74. This incident light causes a photo-induced feedback current through LDD96 and an output signal current through LDD74.
Potentials +Vx and +Vy are of such amplitude and polarity that LDD64, LDD80, LDD96 and LDD74 are operated in reverse bias. R60, R63, R67 and R68 are included to insure that if by mistake LDD64, LDD80, LDD96 or LDD74 are forward-biased, that the current flow is limited.
Potentials -Vx and -Vy are of such amplitude and polarity that LED78 and LED100 can be forward-biased and thus emit light. R56 and R73 serve to insure that input terminals 70 and 94 do not float in potential to limit the pickup of noise signals.
+Vx and -Vx are supplied to amplifiers 62 and 68.
+Vy and -Vy are supplied to amplifiers 84 and 92.
The series combination of R66 and R75 set up a dc votage level at terminal 88. The series combination of R61 and R62 set up a dc voltage level at terminal 74.
Undesirable common mode signals in the power supply which generates potential Vx ref are coupled to 88 through C29. Csl couples these same common mode signals to terminal 82 through R67. The value of C29 is selected to equalize the undesirable common mode signals which reach 88 and 82. These equal input signals effectively cancel each other and the output signal of operational amplifier 84 is essentially free of these undesirable signals.
Undesirable common mode signals in the power supply which generates Vy ref are coupled to 74 through C28. Cs2 couples these same common-mode signals to 72 through R63.
The value of C28 is selected to equalize the undesirable common mode signals which reach 72 and 74. These equalized signals effectively cancel each other and the output signal of operational amplifier 64 is essentially free of these undesirable signals.
An input voice signal applied to the T and R input terminals of 57 propagates to terminal 56 and then to terminal 60 through C12, R50 and R57. The applied signal also propagates through the series combination of Cll and R49 to termlnal 66 which acts as a virtual ground because it i9 the output terminal of operational amplifier 68. The impedance within Zl' is that impedance which a central telephone office line and telephone set is nominally terminated in. The input signal applied to T and R does not reach input terminal 70 because terminal 66 is a virtual ground.
The combination of amplifier 62, LED 78, LDD64 and LDD80 functions in essentially the same manner as amplifier 24, LED2, LDD3 and LDD4 of FIG. 2. Thus an ac input signal applied to input terminal 60 is optically coupled to terminal 82 and the inherent nonlineari~y of LED78 is effectively cancelled due to the optical feedback to LDD64.
The signal appearing at terminal 82 is amplified by amplifier 84 and is coupled from output terminal 46 to intermediate input/output terminal 61 through R77 and C22, and then is coupled through C21 and R69 to input terminal 90 of amplifier 92. In addition, the signal appearing at terminal 86 iS coupled through the combination of C18, Cl9, C20 and R74 and R76 to input terminal 94 of amplifier 92.
Z3' and Z4' are designed, taking into account the impedance characteristics of a transmission line and telephone set coupled to Tl and Rl such that substantially equal signals from terminal 86 are coupled to 90 and 94. secause equal input signals appear at the two inputs of 92, there is no output signal and the current level in LEDlO0 does not change. Consequently, the input signal applied to 56 cannot propagate through LED100 to LDD74 and then through 68 back to 56 where it originated.
An input voice signal applied to the Tl and Rl terminals of 59 propagates to terminal 61 and then to terminal 90 through C21 and R69. The signal also propagates through the series combination of C22 and R77 to terminal 86 which acts as a virtual ground since it is the output terminal of operational amplifier 84. Z4' is that impedance which a transmission line (subscriber loop) and telephone set is nominally terminated in. The input signal at 61 does not reach input terminal 94 since terminal 86 is a virtual ground.
The combination of amplifier 92, LED100, LDD96 and LDD74 ,functions in essentially the same manner as amplifier 62, LED78, LDD64 and LD~80. Thus an input signal applied to terminal 90 is optically coupled to terminal 72 and the inherent nonlinearity of LEDlO0 is effectively cancelled due to the optical feedback through LDD96.
The signal appearing at terminal 72 iS amplified by amplifier 68 and is coupled from output terminal 66 to terminal 56 through Zl' and then through C12 and R50 to input terminal 60 of amplifier 62. In addition, the signal at 66 iS coupled through Z2' to input terminal 70 of amplifier 62. Zl' and Z2'` are designed, taking into account -lg--~062824 the impedance characteristics of the transmission line (subs¢riber loop) and telephone set coupled to T and R, such that substantially equal signals from terminal 66 are coupled to 90 and 94. Because equal input signals appear at the two inputs of 62, there is no output signal change and the current level in LED78 does not change. Consequently, the input signal applied to 61 cannot propagate through LED78 to LDD80 and then through 84 back to 61 where it originated.
An LED can be coupled to an LDD through an optical fiber. This allows the LED to be separated from the LDD by substantial distances which reduces the parasitic capacitance betweén the LED and LDD. The use of low loss high linearity optical fibers permits system 55 to be used as an optical transmission system.
R64 and C16 serve as a gain and frequency shaping feedback impedance with respect to amplifier 62. R65 and C17 serve the same basic function with respect to amplifier 92.
With Sl positioned to short out R98 the gain of signals being amplified by 68 is higher than when R78 is n~t shorted. With S2 positioned to short out R99 the gain of signals being amplified by 84 is higher than when R99 is not shorted. The length of a subscriber telephone loop determines which of the two positions Sl and S2 are set to.
Mispositioning of LEDs 78 and 100 with respect to - LDD64 and 80 and LDD74 and 96 respectively, causes the amplitude of signals which reach 72 and 82, respectively, to vary. This in turn leads to variations in gain between terminals 56 and 61. Resistors R53 and R79 can be varied in value to compensate for gain variations due to any mispositioning.
It is thus apparent that system 55 is a bilateral communication system in that an input signal applied to T
and R or Tl and Rl is transmitted with the desired gain from one input/output to the other, but does not return from where it originated. System 55 allows for the linear transfer of voice signal information with gain and provides relatively high electrical isolation between two sending/
receiving units.
Referring now to FIG. 5 there is illustrated a circuit embodiment of the input and power circuitry 57 of FIG. 4. The T and R input terminals are coupled through battery feed inductor coils (not illustrated) to a potential Vz and a reference potential, which are both typically available in a central telephone office. Vz is typically +
or - 48 volts and the reference potential is typically ground potential. Input terminals T and R are connected through resistors R43 and R44, respectively, to a polarity guard which is shown within dashed-line rectangle 102. The polarity guard consists essentially of diodes Dl, D2, D3 and D4, which are interconnected as illustrated. The cathode of Dl is coupled to intermediate input/output terminal 56, one terminal of a resistor R45, the collector of a transistor Q3, the collector of a transistor Q4, and the cathode of a zener diode Dzl. The second terminal of R45 is coupled to the base of Q3 through a resistor R46 and is coupled to one terminal of a capacitor C8 and a resistor R93. The second terminals of C8 and R93 are connected to the first terminals of a resistor R92 and a capacitor C9. This common terminal is connected to the second terminals of resistors R47 and R48 and the anode of Dzl. This terminal supplies the 1062~324 potential +Vx.
The first terminal of R47 iS connected to the emitter of Q3 and the base of Q4. The second terminals of R92 and C9 are connected together to the first terminal of a resistor R94 and a capacitor C10. This terminal supplies the Vx ref potential. The second terminals of R94 and C10 are coupled together to the anode of D4. This terminal supplies the potential -Vx.
One of the requirements of a central telephone office is that a specified essentially constant current flow between the T and R terminals when a telephone set coupled to the T and R terminals is off-hook. The dc current serves as the carrier for voice signals. Because the battery feed inductors have a certain finite dc resistance, it is necessary that the effective resistive loading between the cathode of Dl and the anode of D4 be equivalent to a value -which results in the desired current flow through T and R.
A simple resistor across these terminals would meet this particular electrical requirement.
The impedance of the telephone set and transmission line coupling it to the T and R input terminal is typically 900 ohms in series with 2~F. A relatively low resistance coupled between terminal 56 and the anode of D4 would greatly attenuate any voice input or output signals.
The circuitry which comprises R45, R46, R47, R48, R92, R94, C8, C9, C10, DZ1 and Q3 and Q4 acts as an essentially high impedance constant current sink and ac filter combination. R43 and R44 are utilized in order to limit excessive power dissipation. Dzl normally operates in reverse bias. Avalanche breakdown operation occurs only if the potential of 56 becomes excessive. The three potentials -1~)6Z8Z4 +Vx, +Vx ref, and -Vx are all DC potentials which are utilized by the bidirectional communication system 55 of FIG. 4.
Now referring to FIG. 6 there is illustrated a circuit schematic embodiment of the input and power circuitry circuit 59 of FIG. 4. Input terminal Tl is connected to intermediate input/output terminal 61 and the first terminal of a resistor R84. The second terminal of R84 is coupled to the first terminal of a thermistor Thl and the first terminal of a capacitor C24. The second terminal of Thl is coupled to a reference potential which exists in the central telephone office. The second terminal of C24 is coupled to the first terminal of a second thermistor Th2 and -to the first terminal of a resistor R85. The second terminal of Th2 is coupled to a potential Vv which exists in the central office. Vv is typically - 78 volts and the reference potential is typically the ground potential of a central telephone office.
The second terminal of R85 is connected to input terminal Rl and to the second terminals of a capacitor C26 and a resistor R85, and to the first terminals of a resistor R97~and a capacitor C27. This terminal supplies the Vy ref potential which is utilized by system 55 of FIG. 4.
The second terminals of R97 and C27 are connected to the second terminal of a capacitor C6 and an output terminal of a full-wave rectifier A. This terminal suppli~es the -Vy potential which is utilized by the system 55 of FIG. 4. The first terminals of R83 and C26 are connected together to the second terminal of a resistor R82. This terminal supplies the +Vy potential utilized by the system 55 of FIG. 4. The first terminal of R82 is connected to an output of full-wave rectifier A.
The output terminals of a transformer Tl are coupled to the inputs of full-wave rectifier A. A first input terminal of Tl is connected through a thermistor Th3 to a potential Vw which is available in a central telephone office. The second input terminal of Tl is connected to a switch S3. An oscillator circuit Osc is coupled to and controls switch S3.
The oscillator circuit causes S3 to open and close such that an alternating current is established through the primary of Tl. This gives rise to an alternating current in the secondary of Tl. This alternating current is rectified by full-wave rectifier A and then filtered by C6 so as to create a DC voltage at the first terminal of R82, R83 and R97 are selected to be of substantially equal values such that the potential drop at their common junction is one-half the potential value applied across both resistors. C26 and C27 serve to filter the DC voltage appearing across R83 and R97.
An input voice signal from a telephone-set coupled between Tl and Rl causes a voice signal to be transmitted to intermediate input/output terminal 61. Power supply Vv supplies DC bias current for the telephone set connected to input terminals Tl and Rl. Capacitor C24 serves as an effective AC short. Voice input signals coupled to intermediate input/output terminal 61 effectively bypass the potential Vv.
The embodiment of the invention illustrated in FIGS. 4, 5 and 6 has been constructed and tested in a working telephone system. Western Electric 502AR dual input differential operational amplifiers were used for amplifiers 62, 68, 84 and 92. Below are listed values of resistors and capacitors utilized. All resistor values are in ohms and all capacitive values are in microfarads except as indicated:
R43 - 10 R52 - 147k R44 - 10 R53 - 7.5k R45 - 22k R54 - 18k R46 - 1.78k R55 - 3.9k R47 - 21.5k R56 - lOOk R48 - 147 R57 - lOOk R49 - 909 R58 - 17.8k R50 - 27k R59 - 147k R51 - 56.2k R60 - lOk R61 - 21.5k R76 - lOk R62 - 348k R77 - 464 R63 - lOk R78 - 147k R64 - 147 R79 - 5.62k R65 - 147 R80 - 56.2k R66 - 38.3k R82 - 287 R67 - lOk R83 - 133k R68 - lOk R84 - 215 R69 - lOOk R85 - 215 R70 - 237k R92 - 511 R71 - 17.8k R93 - 17.8k R73 - lOOk R94 - 511 R74 - 8.2k R97 - 133k R75 - 4.64k R99 - 1.62k C6 - .1 C18 - .047 C8 - .22 - Cl9 - .001 C9 - 40 C20 - .047 10628~4 C10 - 40 C21 - .05 Cll - 2.0 C22 - 2.0 C12 - .011 C23 - 18pF
C13 - .022 C24 - 4 C14 - 18pF C26 - 40 C15 - .022 C27 - 40 C16 - .015 C2~ - 4.75pF
C17 - .015 C29 - 51pF
The LEDs used in system 55 were GaP diodes and the LDDs were silicon diodes.
With a standard -10dsm telephone voice signal applied to the T & R inputs, the measured full harmonic distortion level at the Tl and Rl outputs was 60 to 65 dB
down from the input signal level. The third harmonic distortion level was significantly smaller than the second.
Undesirable common mode signals, for example the "60-cycle hum" of AC power lines, are coupled through stray capacitances or inductances into the T & R or the Tl or Rl terminals. It is desirable that these stray signals not be coupled between T & R and Tl and Rl. One test measure of a communication system is the rejection or attenuation of such undesirabie common mode signals. With a 20dBm-3000Hz common mode input signal applied to input terminals T & R, the measured output signal appearing at Tl and Rl is 77dB down from the level of the input signal. When the input signal frequency is 100Hz, the corresponding output signal is 95dB
down.
While the system of FIG. 4 has been specifically designed for use within the telephone voice frequency range, it is relatively easy to redesign component values for other frequency ranges including dc operation signal.
` 106Z~Z4 The embodiments described herein are intended to be illustrative of the general embodiments of the invention.
Various modifications are possible consistent with the spirit of the invention. For example, a three-transistor-type Darlington amplifier can be substituted for the Darlington pair amplifier or the operational amplifier. In a system which uses the triple Darlington pair amplifier, the cancellation impedances are coupled between the outputs of the emitters of the tripl~e Darlington which replace amplifiers 68 and 84 and the input base terminals of the triple Darlingtons which replace amplifiers 62 and 92, respectively. A resistor is coupled between a potential and the collectors of the three transistors of the triple Darlington amplifiers which replace amplifiers 68 and 84.
The collectors of the triple Darlingtons which replace 68 and 84 are coupled to terminals 56 and 61, respectively, through appropriate line termination impedances. Still further, LEDl of FIG. 1 could be coupled in series with the collector of Q2 instead of the emitter.
Various attempts have been made to eliminate transformers with the use of solid-state electronics. One solid-state combination suggested is the light-emitting diode (LED) - light-detecting diode (LDD) pair. One serious disadvantage of an LED-LDD optical coupler is the inherent non-linear transfer characteristics and the poor temperature sensitivity. Various attempts have been made to compensate for these undesirable characteristics of the LED-LDD pair to allow the use thereof in anàlogue sys-tems which require a relatively high degree of linearity (low distortion).
One such attempted solution is the use of two substantially identical serially-connected LEDS connected to the output of an operational amplifier. Two LDDs are physically located with respect to the LEDS such that light from the first LED impinges on the first LDD, and light from the second LED impinges on the second LDD, The first LDD is coupled to the input of the operational amplifier so as to create a negative feedback path. The second LDD iS coupled to output circuitry. The negative feedback path serves to help linearize the output signal with respect to an input signal. One serious problem associated with this configuration is that the two serially-connected LEDs must have essentially identical electro-optical characteristics in order to obtain reasonably good linearity. The requirement of matching the characteristics of two LEDs closely make such an optical coupler economically unattractive.
One solution to the above problem of matching diodes is the use of two parallel LEDS with separate series resistors coupled to each. The LEDs are matched electrically and optically as closely as possible. Differences in the electrical and optical characteristics can be somewhat attenuated at any one operating point by varying one or the other of the resistors to vary the current through either LED. A major disadvantage of th~~s configuration is that while one setting of resistor values may produce a high-quality linear response at one operating point, the same resistor values will result in unsatisfactory response at other operating pGints.
It would be desirable to have a solid state optical coupler which doesn't require very close matching of the electro-optical characteristics of components but does provide relatively high linearity and is relatively insensitive to temperature variations.
Summary of the Invention One preferred embodiment of the invention is an optically coupled two-wire to two-wire bilateral communication system which comprises essentially two amplifiers (1 and 2j, two line termination impedances, two cancellation (hybrid subtraction) impedances, and two opto-coupler combinations which each comprise a dual input amplifier, a light-emitting diode (LED) and two light-detecting diodes (phQto-diodes) ( LDDs ) .
In each opto-coupler the LED and one LDD are coupled to the output and first input, respectively, of the amplifier associated with that opto-coupler. The second LDD
of each opto-coupler is coupled to an input of one of amplifiers 1 or 2. The first LED of each opto-coupler is positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the photosensitive areas of the first and second LDDs and gives rise to a feedback current signal which is coupled to the input of the amplifier of the opto-coupler.
A first input/output terminal is coupled through a first summing impedance to the first input terminal of the amplifier of the first opto-coupler. The output of amplifier 1 is coupled through the first line termination impedance to the first input/output terminal and to the second input of the amplifier associated with the first opto-coupler through a first cancellation thybrid-subtraction) impedance.
A second input/output terminal is coupled through a se`cond summing impedance to the first input of the amplifier of the second opto-coupler. The output of amplifier 2 is coupled to the second input/output terminal through a second line termination impedance and to the second input of the amplifier of the second opto-coupler through the second cancellation (hybrid subtraction) impedance.
Each opto-coupler employs negative feedback which modulates the output signal of the amplifier in such a manner that the output signal current induced in the second LDD is essentially linearized with respect to an input signal applied to the first input of the amplifier associated with the opto-coupler. The opto-couplers, in -~` 106Z8Z4 addition to providing linear output signals, provide relatively high electrical isolation between the two input/output terminals.
The amplifiers (1 and 2) provide gain such that a signal appearing at either input/output terminal is amplified at the other input/output terminal. The above-described communication system provides gain, and relatively high electrical isolation and linearity. This is all achieved without the use of the traditional hybrid transformer and without the necessity of closely matching diode characteristics.
In accordance with one aspect of the present invention there is provided a bidirectional communication system comprising:
first and second amplifiers, each having two input terminals and an output terminal;
third and fourth amplifiers, each having at least one input terminal and an output terminal;
first and second light-emitting diodes (LEDs), the first and second LEDs being coupled to the output terminals of the first and second amplifiers, respectively;
first, second, third and fourth light-detecting diodes (LDDs), the first LDD being coupled to a first input terminal of the first amplifier and the third LDD being coupled to a first input terminal of the second amplifier;
the first LED being positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the first and second LDDs;
the second LED being positioned with respect to the third and fourth LDDs such that light emitted by the second LED impinges on the third and fourth LDDs;
~ _ 4 _ ~- 106Z8Z4 the second and fourth LDDS being coupled to the input terminals of the third and fourth amplifiers, respectively;
a first input/output terminal being coupled to the first input terminal of the first amplifier through a first summing impedance and to the output of the fourth amplifier through a first line matching impedance;
a second input/output terminal being coupled to the .
first input of the second amplifier through a second summing impedance means and to the output of the third 0 amplifier through a second line matching impedance means;
the output terminal of the fourth amplifier being coupled to the second input terminal of the first amplifier through a first cancelling impedance means; and the output terminal of the third amplifier being coupled to the second input terminal of the second amplifier through a second cancelling impedance means.
In accordance with another aspect of the present invention there is provided an optically coupled bidirectional communication system comprisi.ng:
a first amplifier circuit means having an output and an input;
a first light-emitting means coupled to the output of the first amplifier circuit means;
first and second light detector means, the first and second light detector means being positioned so as to receive light from the first light-emitting means;
the first and second light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
- 4a -,~ ~
- ` 106Z824 the first light detector means being coupled to the input of the first amplifier;
second amplifier circuit means having an output and an input;
the second light detector means being coupled to the input of the second amplifier circuit means;
third amplifier circuit means having an input and an output;
a second light-emitting means coupled to the output of the third amplifier circuit means;
third and fourth light detector means, the third and fourth light detector means being positioned so as to reeeive light from the second light-emitting means;
the third and fourth light detector means being charaeterized in that incident light thereon is proportional to the eurrent indueed therethrough and the ratio of the indueed eurrents is essentially constant;
the third light detector means being coupled to the input of the third amplifier circuit means;
fourth amplifier cireuit means having an input and an output;
the fourth light deteetor means being eoupled to the input of the fourth amplifier eircuit means;
a first summing impedance and a first eancellation impedance;
a first input/output terminal being eoupled to the input of the third amplifier eireuit means through the summing impedance and being coupled to the output of the second amplifier eircuit means; and the first cancellation impedance being coupled to the output of the second amplifier circuit means and to the input of the third amplifier circuit means;
- 4b -a second summing impedance and a second cancell.ation impedance;
a second input/output terminal being coupled to the input of the first amplifier circuit means through the second summing impedance and being coupled to the output of the fourth amplifier circuit means;
the second cancellation impedance being coupled to the output of the fourth amplifier circuit means and to the input of the first amplifier circu;t means.
These and other features of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the following drawings: -Brief Description of the Drawings FIG. 1 illustrates an embodiment of an opto-coupler circuit;
FIG. 2 illustrates an embodiment of another opto-coupler circuit;
FIG. 3 illustrates an optically coupled bilateral communication system;
FIG. 4 illustrates another optically-coupled bilateral communication system;
FIG. 5 illustrates a circuit embodiment of an input and power circuit of FIG. 4; and FIG. 6 illustrates a circuit embodiment of another input and power circuit of FIG. 4.
- 4c - t Detailed Description Now referring to FIG. 1, there is illustrated an optical coupler circuit 10 which comprises a Darlington pair of transistors Ql and Q2, a light-emitting diode LEDl, and light detector diodes LDD1 and LDD2. An input terminal 12 is connected to one terminal of a resistor Rl. The other terminal of Rl (14) is connected to the base of Ql and the cathode of LDDl. The anode of LDD1 is coupled to a potential Va. The emitter of Q2 is connected to the anode of LEDl.
The cathode of LEDl is connected through a resistor R2 to a potential Vb. The collectors of Q1 and Q2 are connected together to a potential Vc. The value of potential Va is selected such that LDDl is reverse-biased during the entire operation of circuit 10. The potentials of Vc and Vb are selected such that LEDl is forward-biased and current flow can occur through the series combination of LEDl and R2. The anode of LDD2 is coupled to a terminal 16 (the output terminal) and through a resistor R3 to potential Vd. The cathode of LDD2 is coupled to potential Ve. Potentials Vd and Ve are selected such that LDD2 is always operating in reverse bias.
LEDl is positioned with respect to LDDl and LDD2 such that light emitted by LEDl impinges on the photo-sensitive areas of LDDl and LDD2.
An input signal applied to terminal 12 is amplified by Ql and Q2 and gives rise to a current through Q2, LED~
and R2. The current flow through LEDl causes LEDl to emit light that impinges on the light-sensitive areas of LDDl and LDD2. The current flow through LDDl acts as negative feedback to the input of the Darlington pair Ql and Q2.
This feedback modifies the output signal at the emitter of Q2 and causes the current level through Q2 and LED 1 to vary in response thereto. Accordingly, the light output of LEDl is varied such that the photo-induced current flow through LDD2 is linearized with respect to the input signal applied to terminal 12.
Photo-induced current in LDD2 flows through R3 and thus gives rise to an output voltage signal at terminal 16 which in linearized with respect to the input signal voltage applied to terminal 12.
The relationship between incident light impinging on a light-detecting diode (photo-diode) and the resulting conduction therethrough is known to be a very linear one.
This characteristic of photo-diodes makes it unnecessary that LDDl and LDD2 be identical. This feature allows circuit 10 to be manufactured with reasonable economy.
In one operating condition LDDl and LDD2 receive substantially equal amounts of light-signal energy from LEDl. The feedback to the input of the Darlington pair is a function of the light received by LDDl. As the amount of light received by LDDl decreases, the feedback decreases.
Correspondingly, distortion caused by the nonlinearity of LEDl is cancelled to a lesser extent. This results in somewhat less linearization (more distortion) between output and input signals. If the amount of light incident of LDDl increases, the negative feedback signal amplitude increases and the linearization improves (distortion decreases).
Considerable design flexibility is available such that the amount of feedback can be varied over wide limits.
Now referring to FIG. 2 there is illustrated an optical coupler circuit 18 which comprises an operational . -` 1062824 amplifier 24, a light-emitting diode LED2, and light detector diodes LDD3 and LDD4. An input terminal 20 is connected through a resistor R4 to the anode of LDD3 and inverting input terminal 22 of operational amplifier 24.
The output of amplifier 24 iS connected to the anode of LED2. The cathode of LED2 is coupled through a resistor R5 to potential Vg. The cathodes of LDD3 and LDD4 are coupled to potentials Vf and Vh, respectively. The anode of LDD4 iS
connected to output terminal 26 and to potential Vi through a resistor R6.
LED2 iS positioned with respect to LDD3 and LDD4 such that light emitted by LED2 impinges on the photosensitive areas of LDD3 and LDD4. Potentials Vf, Vh and Vi are selected such that LDD3 and LDD4 always operate in a reverse-bias condition. The potential of Vg is selected such that LED2 is able to operate in a forward-bias condition.
An input signal applied to terminal 20 is amplified by 24 and gives rise to a current through LED2. The current flow through LED2 causes LED2 to emit light that impinges on the light-sensitive areas of LDD3 and LDD4. This causes photo-induced current to flow in LDD3 and LDD4. The current flow through LDD3 acts as negative feedback to input terminal 22 of 24. This feedback modifies the output signal of 24 and causes the current level through LED2 to vary in response thereto. Accordingly, the light output of LED2 is varied such that the photo-induced current which flows through LDD4 is linearized with respect to the input signal applied to input terminal 20.
Photo-induced current in LDD4 flows through R6 and thus gives rise to an output voltage signal at terminal 26 which is linearized with respect to -the input signal voltage - applied to terminal 12.
The relationship between incident light impinging on a light-detecting diode (photo-diode) and the resulting conduction therethrough is known to be a very linear one.
This characteristic of photo-diodes makes it unnecessary that LDD3 and LDD4 be identical. This feature allows circuit 10 to be manufactured with reasonable economy.
In one operating condition, LDD3 and LDD4 receive substantially equal amounts of light signal energy from LED2. The feedback to the input of 24 is a function of the light received by LDD3. As the amount of light received by LDD3 décreases, the feedback decreases. Correspondingly, distortion caused by the nonlinearity of LED2 iS cancelled to a lesser extent. This results in somewhat less lineariza-tion (more distortion) between output and input signals.
If the amount of light incident on LDD3 increases, the negative feedback signal amplitude increases and the system linearization improves (distortion decreases). Considerable design flexibility is available such that the amount of feedback can be varied over wide limits.
The input signal applied to terminal 20 can be dc or ac. The corresponding output signal at terminal 26 iS
corresponding dc or ac.
Now referring to FIG. 3 there is illustrated an optically coupled bidirectional communica~ion system 28 which comprises two light-emitting diodes LED3 and LED4, four`light-detecting diodes (photo-diodes) LDD5, LDD6, LDD7 and LDD8, two dual input differential operational amplifiers 34 and 52, and two single input operational amplifiers 38 and 44. A first input/output terminal 30 is connected through a line-terminating impedance Zl to terminal 40 (the output of amplifier 38) and to input ~erminal 36 of amplifier 34 through a summing resistor R7. A cancellation (hybrid subtraction) impedance Z2 is connected between terminal 40 and input terminal 32 of amplifier 34. LDD5 is coupled by the anode to terminal 36 and by the cathode to potential Vi.
The output of amplifier 34 is coupled to the anode LED3.
The cathode of LED3 is coupled through a resistor R8 to potential Vj.
lQ LDD6 is couplea by the anode to input terminal 42 of amplifier 44 and to a resistor R9. R9 is coupled to potential Vl. The cathode of LDD6 iS coupled to potential Vk. Output terminal 46 of amplifier 44 is connected to input/output terminal 54 through line termination impedance Z4 and to input terminal 50 of amplifier 52 through cancellation (hybrid aubtraction) impedance Z3. Terminal 54 is coupled to input terminal 48 of amplifier 52 and the anode of LDD8 through summing resistor R10. The cathode of LDD8 iS coupled to potential Vm. The output of amplifier 52 iS connected to the anode of LED4. The cathode of LED4 is coupled to potential Vn through resistor Rll.
Again control feedback impedance Zol is connected across terminals 42 and 46 of amplifiers 44 and a gain control feedback impedance Zo2 iS connected across terminals 51 and 40 of amplifier 38. The anode of LDD7 is connected to input terminal 51 of amplifier 38 and a resistor R12.
The cathode of LDD7 is coupled to a potential Vo and R12 is coupled to a potential Vp.
LED3 is positioned with respect to LDD5 a~d LDD6 such that light emitted by LED3 impinges on the photosensitive areas of LDD5 and LDD6. LED4 is positioned with respect to LDD7 and LDD8 such that light emitted by LED4 impinges on the photosensitive areas of LDD7 and LDD8.
An input signal applied to terminal 30 can propagate through two input paths. The first path is through Zl to terminal 40 (the output terminal of amplifier 38). The input signal cannot propagate past terminal 46 since the output of operational amplifier 38 acts as a virtual ground. The second path is through R7 to terminal 36. The combination of R7, amplifier 34, LED3, LDDS and LDD6 acts in substantially the same manner as the combination of R4, amplifier 24, LED2, LDD3 and LDD4 of FIG. 2. Thus an input signal applied to the input terminal 30 is optically coupled to terminal 42 and the inherent nonlinearity of LED3 is effectively cancelled because of the optical feedback to LDD5.
The signal at terminal 42 is amplified by amplifier 44 and is coupled from terminal 46 to the input/output terminal 54 through impedance Z4 and then to input terminal 48 through R10. In addition, the signal at terminal 46 is coupled to the input terminal 50 through Z3.
Input/output terminal 54 has coupled thereto sender/receiver circuitry (not illustrated) which has an inherent impedance associated therewith. Z3 and Z4 are designed, taking into account the impedance characteristics of the sender/receiver circuitry (not illustrated) coupled to terminal 54, such that substantially equal signals are coupled to input terminals 48 and 50 of amplifier 52 from output terminal 46 of amplifier 44.
Equal signals appearing at the two input terminals (48 and S0) of differential amplifier 52 results in no output signal. ~onsequently the current level in LED4 does ~ 106Z824 not change. Therefore, the input signal from terminal 30 is not coupled back through LDD7, amplifier 38 and Zl to input/output terminal 30.
Thus, an input signal applied to terminal 30 is transmitted to terminal 54. It is effectively cancelled before the signal can propagate back to terminal 30 from where it originated.
An input signal applied to input/output terminal 54 propagates through two input paths. The first path is through Z4 to terminal 46. The second path is through R10 to an input terminal 48 of amplifier 52. The output terminal 46 of operational amplifier 44 acts as a virtual ground and thus prevents the signal from propagating further.
The input signal at 4 8 causes an output signal to appear at the output of 52 which causes LED4 to emit light in response thereto. This results in a signal being induced in LDD7 and LDD8. The light signal to LDD8 is a feedback signal and the light signal to LDD7 iS the output signal.
The output of amplifier 38 is coupled through Zl to input/output terminal 30 which is coupled through R7 to the input terminal 36 of amplifier 34. The output signal at terminal 40 is also coupled through Z2 to input terminal 32 of amplifier 34. The sender/receiver circuitry (not illustrated) coupled to terminal 30 has an inherent impedance characteristic associated therewith. Zl and Z2 are designed, taking into account the impedance characteristics of the sender/receiver connected to terminal 30, to insure that substantially equal signals are applied to input terminals 32 and 36 of amplifier 34. This insures that there is no output signal from 34 which could be coupled back 106Z8;~L
to input/output 54 where it originated.
Thus an input signal applied to terminal 54 is transmitted to terminal 30. It is effectively cancelled before the signal at 30 can propagate back to terminal 54 from where it originated.
The potentials Vi, Vk, Ve, Vm, Vo and Vp are selected to be of sufficient amplitude and polarity to cause LDDs 5, 6, 7 and 8 to operate in a reverse bias condition.
Potentials Vj and Vn are selected to be of sufficient polarity and magnitude to permit forward conduction through LED3 and LED4. The power supplies which provide potentials Vi, Vj, Vo and Vp are electrically separate from the power supplles that supply potentials Vk, Vl, Vm and Vn.
It is thus apparent that an input signal applied to terminal 30 or 54 will be transmitted from one terminal to the other but will not return to the terminal at which the signal originates. Circuit 28 provides both physical and electrical isolation between terminals 30 and 54 and provides for the linear transfer of information between the respective terminals. The gain of amplifiers 38 and 44 can be varied by adjusting Zol and Zo2. Thus signals can be linearly transmitted between terminals 30 and 54 with gain introduced and relatively high electrical isolation. The output light signals of LED3 and LED4 can be coupled to LDD6 and LDD7, respectively, through optical fibers. The use of low-loss high-linearity optical fibers permits system 28 to be used as an optical transmission system.
Now referring to FIG. 4, there is illustrated an optically coupled two-wire to two-wire bilateral communication system 55. System 55 has been designed as a replacement for the transmission path with voice frequency gain that 106282~
is illustrated by reference No. 115 in FIG. 4 of U.S. Patent No. 3,671,676 in which the assignee is the same as in this present application. The input terminals denoted as T and R of input and power circuitry 57 are connected thrbugh a central telephone office (not illustrated) to a first telephone set (not illustrated). The input terminals denoted as Tl and Rl are connected to a second telephone set (not illustrated) through the same central telephone office (not illustrated). These two telephone sets are thus coupled together through system 55. The input and power supply circuitry 57 and 59, which are illustrated in block form, will be described later in detail.
The output of 57 is intermediate input/output terminal 56 and the output of 59 is intermediate input/
output terminal 61. Terminal 56 is coupled through the series combination of a capacitor C12, a resistor R50, and a resistor R57 to an input terminal 60 of a differential two-input operational amplifier 62. Resistors R58 and R59 are connected to the common connection of R50 and RS7. A
resistor R60 is connected between terminal 60 and the anode of a light-detecting diode 64. The cathode of 64 is coupled to a potential +Vx. R59 is connected to a potential -Vx.
Output terminal 76 of amplifier 62 is connected to the anode of light-emitting diode 78. The cathode of 78 is coupled to a potential -Vx through the parallel combination of a resistor R64 and a capacitor C16. Terminal 56 is further connected through the series combination of a capacitor Cll and a resistor R49 to output terminal 66 of a two-input differential operational amplifier 68.
Terminal 66 is coupled to the series combination of a capacitor C13 and a resistor R54 to resistor R55 and a -"` 1062824 capacitor C15. C15 and a resistor R56 are both coupled to input terminal 70 of operational amplifier 62. R55 and R56 are both coupled to a potential Vx ref. Terminal 66 is also connected to the series combination of resistors R51, R53 and R98. R98 is coupled to Vx ref and to a switch Sl. Sl is a two-position switch which shorts R53 to Vx ref or connects R53 to Vx ref through R98. The common node of R53 and R51 is coupled through resistor R52 to capacitor C14 and to an input ~erminal 72 of operational amplifier 68.
Terminal 72 is coupled through a resistor R63 to the anode of light-detecting diode 74. C14 is also coupled to terminal 66. Resistor R61 is coupled to a resistor R62, and a capacitor C28 to input terminal 74 of operational amplifier 68. C28 is coupled to a potential Vy ref. R62 is coupled to Vx ref, and R61 is coupled to +Vx.
` A light-detecting diode 80 is coupled by the anode through a resistor R67 to an input terminal 82 of an operational amplifier 84. The cathode of 80 is coupled to a potential +Vy. A resistor R66 is coupled between +Vy and an input terminal 88 of amplifier 84. A capacitor C29 is coupled between Vx ref and terminal 88. A resistor R75 is coupled between Vy ref and terminal- 88. Terminal 82 is connected through the series combination of resistors R78 and R80 to the output terminal 86 of operational amplifier 84. A capacitor C23 is connected between terminals 82 and 86. R78 and R80 are connected to the series combination of a resistor R79 and a resistor R99. R99 is coupled to a potential Vy ref and a two-position switch S2. In one position S2 shorts R79 directly to Vy ref. In the other position R79 is coupled to Vy ref through R99.
Terminal 86 is connected through the series combination of resistor R77 and capacitor C22 to intermediate input/output terminal 61 and to an input terminal 90 of an operational amplifier 92 through the series combination of a capacitor C21 and a resistor R69.
Terminal 86 is also connected to input terminal 94 of 92 through the series combination of a capacitor C20, a resistor R76, and a capacitor C18. A capacitor Cl9 and a resistor R74 are both connected between C18 and Vy ref. A
resistor R73 is connected between terminal 94 and Vy ref.
Resistors R70 and R71 are both connected to R69. R70 and R71 are connected to -Vy and Vy ref, respectively.
Terminal 90 is connected through resistor R68 to the anode of a light-detecting diode 96. The cathode of 96 is coupled to +Vy. Output terminal 98 of amplifier 92 is coupled to the anode of a light-emitting diode 100. The cathode of 100 is connected to a potential-Vy through the parallel combination of a resistor R65 and a capacitor C17.
The combination of Cll and R49 and C22 and R77 (contained within dashed-line rectangles Zl' and Z4', respectively) act as line terminations. The combination of C13, C15, R54 and R55 (contained within dashed-line rectangle Z2') acts as a first hybrid substraction or cancellation impedance. The combination of C18, Cl9, C20, R74 and R76 (contained within dashed-line rectangle Z3') acts as a second hybrid subtraction or cancellation impedance.
The combination of C14, R51, R52, R53, R98 and Sl (contained within dashed-line rectangle Zol') serves as a gain control feedback impedance across input terminal 72 and output terminal 66 of operational amplifier 68. The combination of C23, R78, R79, R80, R99 and Sl (contained within dashed-line rectangle Zo2') serves as a gain control feedback impedance across input terminal 82 and output terminal 86 of operational amplifier 84.
C12 and C21 serve to block DC input signals from reaching input terminals 60 and 90 of operational amplifiers 62 and 92, respectively.
R50 serves to attenuate input signals coupled to terminal S6. R58 and R59 serve as a voltage divider network. R70 and R71 likewise serve as a voltage divider network. R57 and R69 serve as signal summing resistors.
Csl represents the inherent parasitic capacitance between light-emitting diode 78 and light detector diode 80.
Cs2 represents the inherent parasitic capacitance between light-emitting diode 100 and light-detecting diode 74. As has already been described, C28 is coupled between input terminal 74 of operational amplifier 68 and potential Vy ref. In addition, C29 is coupled between input terminal 88 of operational amplifier 84 and potential Vx ref. C28 and C29 serve to inject common mode signals into the respective first input terminals of the operational amplifiers 68 and 84 so as to cancel common mode signals injected into the second input terminals by the two parasitic capacitances Csl and Cs2. This effectively prevents common mode signals from propagating through the system 55.
LED78 is positioned with respect to ~DD64 and LDD80 such that light emitted by LED78 impinges on the photo-sensitive areas of LDD64 and LDD80. This incident light causes a photo-induced feedback current through LDD64 and an output signal current through LDD80.
` 10628Z4 LED100 is positioned with respect to LDD96 and LDD74 such that light emitted by LED100 impinges on the photosensitive areas of LDD96 and LDD74. This incident light causes a photo-induced feedback current through LDD96 and an output signal current through LDD74.
Potentials +Vx and +Vy are of such amplitude and polarity that LDD64, LDD80, LDD96 and LDD74 are operated in reverse bias. R60, R63, R67 and R68 are included to insure that if by mistake LDD64, LDD80, LDD96 or LDD74 are forward-biased, that the current flow is limited.
Potentials -Vx and -Vy are of such amplitude and polarity that LED78 and LED100 can be forward-biased and thus emit light. R56 and R73 serve to insure that input terminals 70 and 94 do not float in potential to limit the pickup of noise signals.
+Vx and -Vx are supplied to amplifiers 62 and 68.
+Vy and -Vy are supplied to amplifiers 84 and 92.
The series combination of R66 and R75 set up a dc votage level at terminal 88. The series combination of R61 and R62 set up a dc voltage level at terminal 74.
Undesirable common mode signals in the power supply which generates potential Vx ref are coupled to 88 through C29. Csl couples these same common mode signals to terminal 82 through R67. The value of C29 is selected to equalize the undesirable common mode signals which reach 88 and 82. These equal input signals effectively cancel each other and the output signal of operational amplifier 84 is essentially free of these undesirable signals.
Undesirable common mode signals in the power supply which generates Vy ref are coupled to 74 through C28. Cs2 couples these same common-mode signals to 72 through R63.
The value of C28 is selected to equalize the undesirable common mode signals which reach 72 and 74. These equalized signals effectively cancel each other and the output signal of operational amplifier 64 is essentially free of these undesirable signals.
An input voice signal applied to the T and R input terminals of 57 propagates to terminal 56 and then to terminal 60 through C12, R50 and R57. The applied signal also propagates through the series combination of Cll and R49 to termlnal 66 which acts as a virtual ground because it i9 the output terminal of operational amplifier 68. The impedance within Zl' is that impedance which a central telephone office line and telephone set is nominally terminated in. The input signal applied to T and R does not reach input terminal 70 because terminal 66 is a virtual ground.
The combination of amplifier 62, LED 78, LDD64 and LDD80 functions in essentially the same manner as amplifier 24, LED2, LDD3 and LDD4 of FIG. 2. Thus an ac input signal applied to input terminal 60 is optically coupled to terminal 82 and the inherent nonlineari~y of LED78 is effectively cancelled due to the optical feedback to LDD64.
The signal appearing at terminal 82 is amplified by amplifier 84 and is coupled from output terminal 46 to intermediate input/output terminal 61 through R77 and C22, and then is coupled through C21 and R69 to input terminal 90 of amplifier 92. In addition, the signal appearing at terminal 86 iS coupled through the combination of C18, Cl9, C20 and R74 and R76 to input terminal 94 of amplifier 92.
Z3' and Z4' are designed, taking into account the impedance characteristics of a transmission line and telephone set coupled to Tl and Rl such that substantially equal signals from terminal 86 are coupled to 90 and 94. secause equal input signals appear at the two inputs of 92, there is no output signal and the current level in LEDlO0 does not change. Consequently, the input signal applied to 56 cannot propagate through LED100 to LDD74 and then through 68 back to 56 where it originated.
An input voice signal applied to the Tl and Rl terminals of 59 propagates to terminal 61 and then to terminal 90 through C21 and R69. The signal also propagates through the series combination of C22 and R77 to terminal 86 which acts as a virtual ground since it is the output terminal of operational amplifier 84. Z4' is that impedance which a transmission line (subscriber loop) and telephone set is nominally terminated in. The input signal at 61 does not reach input terminal 94 since terminal 86 is a virtual ground.
The combination of amplifier 92, LED100, LDD96 and LDD74 ,functions in essentially the same manner as amplifier 62, LED78, LDD64 and LD~80. Thus an input signal applied to terminal 90 is optically coupled to terminal 72 and the inherent nonlinearity of LEDlO0 is effectively cancelled due to the optical feedback through LDD96.
The signal appearing at terminal 72 iS amplified by amplifier 68 and is coupled from output terminal 66 to terminal 56 through Zl' and then through C12 and R50 to input terminal 60 of amplifier 62. In addition, the signal at 66 iS coupled through Z2' to input terminal 70 of amplifier 62. Zl' and Z2'` are designed, taking into account -lg--~062824 the impedance characteristics of the transmission line (subs¢riber loop) and telephone set coupled to T and R, such that substantially equal signals from terminal 66 are coupled to 90 and 94. Because equal input signals appear at the two inputs of 62, there is no output signal change and the current level in LED78 does not change. Consequently, the input signal applied to 61 cannot propagate through LED78 to LDD80 and then through 84 back to 61 where it originated.
An LED can be coupled to an LDD through an optical fiber. This allows the LED to be separated from the LDD by substantial distances which reduces the parasitic capacitance betweén the LED and LDD. The use of low loss high linearity optical fibers permits system 55 to be used as an optical transmission system.
R64 and C16 serve as a gain and frequency shaping feedback impedance with respect to amplifier 62. R65 and C17 serve the same basic function with respect to amplifier 92.
With Sl positioned to short out R98 the gain of signals being amplified by 68 is higher than when R78 is n~t shorted. With S2 positioned to short out R99 the gain of signals being amplified by 84 is higher than when R99 is not shorted. The length of a subscriber telephone loop determines which of the two positions Sl and S2 are set to.
Mispositioning of LEDs 78 and 100 with respect to - LDD64 and 80 and LDD74 and 96 respectively, causes the amplitude of signals which reach 72 and 82, respectively, to vary. This in turn leads to variations in gain between terminals 56 and 61. Resistors R53 and R79 can be varied in value to compensate for gain variations due to any mispositioning.
It is thus apparent that system 55 is a bilateral communication system in that an input signal applied to T
and R or Tl and Rl is transmitted with the desired gain from one input/output to the other, but does not return from where it originated. System 55 allows for the linear transfer of voice signal information with gain and provides relatively high electrical isolation between two sending/
receiving units.
Referring now to FIG. 5 there is illustrated a circuit embodiment of the input and power circuitry 57 of FIG. 4. The T and R input terminals are coupled through battery feed inductor coils (not illustrated) to a potential Vz and a reference potential, which are both typically available in a central telephone office. Vz is typically +
or - 48 volts and the reference potential is typically ground potential. Input terminals T and R are connected through resistors R43 and R44, respectively, to a polarity guard which is shown within dashed-line rectangle 102. The polarity guard consists essentially of diodes Dl, D2, D3 and D4, which are interconnected as illustrated. The cathode of Dl is coupled to intermediate input/output terminal 56, one terminal of a resistor R45, the collector of a transistor Q3, the collector of a transistor Q4, and the cathode of a zener diode Dzl. The second terminal of R45 is coupled to the base of Q3 through a resistor R46 and is coupled to one terminal of a capacitor C8 and a resistor R93. The second terminals of C8 and R93 are connected to the first terminals of a resistor R92 and a capacitor C9. This common terminal is connected to the second terminals of resistors R47 and R48 and the anode of Dzl. This terminal supplies the 1062~324 potential +Vx.
The first terminal of R47 iS connected to the emitter of Q3 and the base of Q4. The second terminals of R92 and C9 are connected together to the first terminal of a resistor R94 and a capacitor C10. This terminal supplies the Vx ref potential. The second terminals of R94 and C10 are coupled together to the anode of D4. This terminal supplies the potential -Vx.
One of the requirements of a central telephone office is that a specified essentially constant current flow between the T and R terminals when a telephone set coupled to the T and R terminals is off-hook. The dc current serves as the carrier for voice signals. Because the battery feed inductors have a certain finite dc resistance, it is necessary that the effective resistive loading between the cathode of Dl and the anode of D4 be equivalent to a value -which results in the desired current flow through T and R.
A simple resistor across these terminals would meet this particular electrical requirement.
The impedance of the telephone set and transmission line coupling it to the T and R input terminal is typically 900 ohms in series with 2~F. A relatively low resistance coupled between terminal 56 and the anode of D4 would greatly attenuate any voice input or output signals.
The circuitry which comprises R45, R46, R47, R48, R92, R94, C8, C9, C10, DZ1 and Q3 and Q4 acts as an essentially high impedance constant current sink and ac filter combination. R43 and R44 are utilized in order to limit excessive power dissipation. Dzl normally operates in reverse bias. Avalanche breakdown operation occurs only if the potential of 56 becomes excessive. The three potentials -1~)6Z8Z4 +Vx, +Vx ref, and -Vx are all DC potentials which are utilized by the bidirectional communication system 55 of FIG. 4.
Now referring to FIG. 6 there is illustrated a circuit schematic embodiment of the input and power circuitry circuit 59 of FIG. 4. Input terminal Tl is connected to intermediate input/output terminal 61 and the first terminal of a resistor R84. The second terminal of R84 is coupled to the first terminal of a thermistor Thl and the first terminal of a capacitor C24. The second terminal of Thl is coupled to a reference potential which exists in the central telephone office. The second terminal of C24 is coupled to the first terminal of a second thermistor Th2 and -to the first terminal of a resistor R85. The second terminal of Th2 is coupled to a potential Vv which exists in the central office. Vv is typically - 78 volts and the reference potential is typically the ground potential of a central telephone office.
The second terminal of R85 is connected to input terminal Rl and to the second terminals of a capacitor C26 and a resistor R85, and to the first terminals of a resistor R97~and a capacitor C27. This terminal supplies the Vy ref potential which is utilized by system 55 of FIG. 4.
The second terminals of R97 and C27 are connected to the second terminal of a capacitor C6 and an output terminal of a full-wave rectifier A. This terminal suppli~es the -Vy potential which is utilized by the system 55 of FIG. 4. The first terminals of R83 and C26 are connected together to the second terminal of a resistor R82. This terminal supplies the +Vy potential utilized by the system 55 of FIG. 4. The first terminal of R82 is connected to an output of full-wave rectifier A.
The output terminals of a transformer Tl are coupled to the inputs of full-wave rectifier A. A first input terminal of Tl is connected through a thermistor Th3 to a potential Vw which is available in a central telephone office. The second input terminal of Tl is connected to a switch S3. An oscillator circuit Osc is coupled to and controls switch S3.
The oscillator circuit causes S3 to open and close such that an alternating current is established through the primary of Tl. This gives rise to an alternating current in the secondary of Tl. This alternating current is rectified by full-wave rectifier A and then filtered by C6 so as to create a DC voltage at the first terminal of R82, R83 and R97 are selected to be of substantially equal values such that the potential drop at their common junction is one-half the potential value applied across both resistors. C26 and C27 serve to filter the DC voltage appearing across R83 and R97.
An input voice signal from a telephone-set coupled between Tl and Rl causes a voice signal to be transmitted to intermediate input/output terminal 61. Power supply Vv supplies DC bias current for the telephone set connected to input terminals Tl and Rl. Capacitor C24 serves as an effective AC short. Voice input signals coupled to intermediate input/output terminal 61 effectively bypass the potential Vv.
The embodiment of the invention illustrated in FIGS. 4, 5 and 6 has been constructed and tested in a working telephone system. Western Electric 502AR dual input differential operational amplifiers were used for amplifiers 62, 68, 84 and 92. Below are listed values of resistors and capacitors utilized. All resistor values are in ohms and all capacitive values are in microfarads except as indicated:
R43 - 10 R52 - 147k R44 - 10 R53 - 7.5k R45 - 22k R54 - 18k R46 - 1.78k R55 - 3.9k R47 - 21.5k R56 - lOOk R48 - 147 R57 - lOOk R49 - 909 R58 - 17.8k R50 - 27k R59 - 147k R51 - 56.2k R60 - lOk R61 - 21.5k R76 - lOk R62 - 348k R77 - 464 R63 - lOk R78 - 147k R64 - 147 R79 - 5.62k R65 - 147 R80 - 56.2k R66 - 38.3k R82 - 287 R67 - lOk R83 - 133k R68 - lOk R84 - 215 R69 - lOOk R85 - 215 R70 - 237k R92 - 511 R71 - 17.8k R93 - 17.8k R73 - lOOk R94 - 511 R74 - 8.2k R97 - 133k R75 - 4.64k R99 - 1.62k C6 - .1 C18 - .047 C8 - .22 - Cl9 - .001 C9 - 40 C20 - .047 10628~4 C10 - 40 C21 - .05 Cll - 2.0 C22 - 2.0 C12 - .011 C23 - 18pF
C13 - .022 C24 - 4 C14 - 18pF C26 - 40 C15 - .022 C27 - 40 C16 - .015 C2~ - 4.75pF
C17 - .015 C29 - 51pF
The LEDs used in system 55 were GaP diodes and the LDDs were silicon diodes.
With a standard -10dsm telephone voice signal applied to the T & R inputs, the measured full harmonic distortion level at the Tl and Rl outputs was 60 to 65 dB
down from the input signal level. The third harmonic distortion level was significantly smaller than the second.
Undesirable common mode signals, for example the "60-cycle hum" of AC power lines, are coupled through stray capacitances or inductances into the T & R or the Tl or Rl terminals. It is desirable that these stray signals not be coupled between T & R and Tl and Rl. One test measure of a communication system is the rejection or attenuation of such undesirabie common mode signals. With a 20dBm-3000Hz common mode input signal applied to input terminals T & R, the measured output signal appearing at Tl and Rl is 77dB down from the level of the input signal. When the input signal frequency is 100Hz, the corresponding output signal is 95dB
down.
While the system of FIG. 4 has been specifically designed for use within the telephone voice frequency range, it is relatively easy to redesign component values for other frequency ranges including dc operation signal.
` 106Z~Z4 The embodiments described herein are intended to be illustrative of the general embodiments of the invention.
Various modifications are possible consistent with the spirit of the invention. For example, a three-transistor-type Darlington amplifier can be substituted for the Darlington pair amplifier or the operational amplifier. In a system which uses the triple Darlington pair amplifier, the cancellation impedances are coupled between the outputs of the emitters of the tripl~e Darlington which replace amplifiers 68 and 84 and the input base terminals of the triple Darlingtons which replace amplifiers 62 and 92, respectively. A resistor is coupled between a potential and the collectors of the three transistors of the triple Darlington amplifiers which replace amplifiers 68 and 84.
The collectors of the triple Darlingtons which replace 68 and 84 are coupled to terminals 56 and 61, respectively, through appropriate line termination impedances. Still further, LEDl of FIG. 1 could be coupled in series with the collector of Q2 instead of the emitter.
Claims (4)
1. A bidirectional communication system comprising:
first and second amplifiers, each having two input terminals and an output terminal;
third and fourth amplifiers, each having at least one input terminal and an output terminal;
first and second light-emitting diodes (LEDs), the first and second LEDs being coupled to the output terminals of the first and second amplifiers, respectively;
first, second, third and fourth light-detecting diodes (LDDs), the first LDD being coupled to a first input terminal of the first amplifier and the third LDD being coupled to a first input terminal of the second amplifier;
the first LED being positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the first and second LDDs;
the second LED being positioned with respect to the third and fourth LDDs such that light emitted by the second LED impinges on the third and fourth LDDs;
the second and fourth LDDs being coupled to the input terminals of the third and fourth amplifiers, respectively;
a first input/output terminal being coupled to the first input terminal of the first amplifier through a first summing impedance and to the output of the fourth amplifier through a first line matching impedance;
a second input/output terminal being coupled to the first input of the second amplifier through a second summing impedance means and to the output of the third amplifier through a second line matching impedance means;
the output terminal of the fourth amplifier being coupled to the second input terminal of the first amplifier through a first cancelling impedance means; and the output terminal of the third amplifier being coupled to the second input terminal of the second amplifier through a second cancelling impedance means.
first and second amplifiers, each having two input terminals and an output terminal;
third and fourth amplifiers, each having at least one input terminal and an output terminal;
first and second light-emitting diodes (LEDs), the first and second LEDs being coupled to the output terminals of the first and second amplifiers, respectively;
first, second, third and fourth light-detecting diodes (LDDs), the first LDD being coupled to a first input terminal of the first amplifier and the third LDD being coupled to a first input terminal of the second amplifier;
the first LED being positioned with respect to the first and second LDDs such that light emitted by the first LED impinges on the first and second LDDs;
the second LED being positioned with respect to the third and fourth LDDs such that light emitted by the second LED impinges on the third and fourth LDDs;
the second and fourth LDDs being coupled to the input terminals of the third and fourth amplifiers, respectively;
a first input/output terminal being coupled to the first input terminal of the first amplifier through a first summing impedance and to the output of the fourth amplifier through a first line matching impedance;
a second input/output terminal being coupled to the first input of the second amplifier through a second summing impedance means and to the output of the third amplifier through a second line matching impedance means;
the output terminal of the fourth amplifier being coupled to the second input terminal of the first amplifier through a first cancelling impedance means; and the output terminal of the third amplifier being coupled to the second input terminal of the second amplifier through a second cancelling impedance means.
2. The apparatus of claim 1 wherein:
the third and fourth amplifiers each have a second input terminal;
a first parasitic capacitance couples the first LED to the second LDD; and a second parasitic capacitance couples the second LED
to the fourth LDD.
the third and fourth amplifiers each have a second input terminal;
a first parasitic capacitance couples the first LED to the second LDD; and a second parasitic capacitance couples the second LED
to the fourth LDD.
3. The apparatus of claim 2 further comprising:
a first capacitance coupled between the first amplifier and the second input of the third amplifier; and a second capacitance coupled between the second amplifier and the second input terminal of the fourth amplifer.
a first capacitance coupled between the first amplifier and the second input of the third amplifier; and a second capacitance coupled between the second amplifier and the second input terminal of the fourth amplifer.
4. An optically coupled bidirectional communication system comprising:
a first amplifier circuit means having an output and an input;
a first light-emitting means coupled to the output of the first amplifier circuit means;
first and second light detector means, the first and second light detector means being positioned so as to receive light from the first light-emitting means;
the first and second light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
the first light detector means being coupled to the input of the first amplifier;
second amplifier circuit means having an output and an input;
the second light detector means being coupled to the input of the second amplifier circuit means;
third amplifier circuit means having an input and an output;
a second light-emitting means coupled to the output of the third amplifier circuit means;
third and fourth light detector means, the third and fourth light detector means being positioned so as to receive light from the second light-emitting means;
the third and fourth light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
the third light detector means being coupled to the input of the third amplifier circuit means;
fourth amplifier circuit means having an input and an output;
the fourth light detector means being coupled to the input of the fourth amplifier circuit means;
a first summing impedance and a first cancellation impedance;
a first input/output terminal being coupled to the input of the third amplifier circuit means through the summing impedance and being coupled to the output of the second amplifier circuit means; and the first cancellation impedance being coupled to the output of the second amplifier circuit means and to the input of the third amplifier circuit means ;
a second summing impedance and a second cancellation impedance;
a second input/output terminal being coupled to the input of the first amplifier circuit means through the second summing impedance and being coupled to the output of the fourth amplifier circuit means;
the second cancellation impedance being coupled to the output of the fourth amplifier circuit means and to the input of the first amplifier circuit means.
a first amplifier circuit means having an output and an input;
a first light-emitting means coupled to the output of the first amplifier circuit means;
first and second light detector means, the first and second light detector means being positioned so as to receive light from the first light-emitting means;
the first and second light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
the first light detector means being coupled to the input of the first amplifier;
second amplifier circuit means having an output and an input;
the second light detector means being coupled to the input of the second amplifier circuit means;
third amplifier circuit means having an input and an output;
a second light-emitting means coupled to the output of the third amplifier circuit means;
third and fourth light detector means, the third and fourth light detector means being positioned so as to receive light from the second light-emitting means;
the third and fourth light detector means being characterized in that incident light thereon is proportional to the current induced therethrough and the ratio of the induced currents is essentially constant;
the third light detector means being coupled to the input of the third amplifier circuit means;
fourth amplifier circuit means having an input and an output;
the fourth light detector means being coupled to the input of the fourth amplifier circuit means;
a first summing impedance and a first cancellation impedance;
a first input/output terminal being coupled to the input of the third amplifier circuit means through the summing impedance and being coupled to the output of the second amplifier circuit means; and the first cancellation impedance being coupled to the output of the second amplifier circuit means and to the input of the third amplifier circuit means ;
a second summing impedance and a second cancellation impedance;
a second input/output terminal being coupled to the input of the first amplifier circuit means through the second summing impedance and being coupled to the output of the fourth amplifier circuit means;
the second cancellation impedance being coupled to the output of the fourth amplifier circuit means and to the input of the first amplifier circuit means.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US54840475A | 1975-02-10 | 1975-02-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1062824A true CA1062824A (en) | 1979-09-18 |
Family
ID=24188718
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA244,517A Expired CA1062824A (en) | 1975-02-10 | 1976-01-29 | Two-way telephone transmission system utilizing opto-couplers |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS51105701A (en) |
| CA (1) | CA1062824A (en) |
| DE (1) | DE2604805A1 (en) |
| GB (1) | GB1522903A (en) |
| NL (1) | NL7601288A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3308987A1 (en) * | 1983-03-14 | 1984-09-20 | Siemens AG, 1000 Berlin und 8000 München | Circuit arrangement with an optical transmitter |
| DE3325420A1 (en) * | 1983-07-14 | 1985-01-31 | Telefunken electronic GmbH, 7100 Heilbronn | CIRCUIT ARRANGEMENT FOR CONTROLLING SEMICONDUCTOR LASERS |
-
1976
- 1976-01-29 CA CA244,517A patent/CA1062824A/en not_active Expired
- 1976-02-04 GB GB447576A patent/GB1522903A/en not_active Expired
- 1976-02-07 DE DE19762604805 patent/DE2604805A1/en not_active Withdrawn
- 1976-02-09 NL NL7601288A patent/NL7601288A/en not_active Application Discontinuation
- 1976-02-10 JP JP51012922A patent/JPS51105701A/ja active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| GB1522903A (en) | 1978-08-31 |
| NL7601288A (en) | 1976-08-12 |
| JPS51105701A (en) | 1976-09-18 |
| DE2604805A1 (en) | 1976-08-19 |
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