US2386515A - Two-way carrier wave signal transmission system - Google Patents

Description

L. K. SWART 2,386,515

SIGNAL TRANSMISSION SYSTEM TWO-WAY CARRIER WAVE 1944 2 Sheets-Sheet 1 Filed Dec. 50

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70 SOURCE 0F 0. C. CON TROL CURRENT TO SOURCE 0F D.C. CONTROL CURRENT INVENTOR LKSWART 0, C. CONTROL CURRENT A TTORNE Y vOct. 9, 1945.

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TWO-WAY CARRIER WAVE SIGNAL TRANSMISSION SYSTEM Filed Dec. 30, 1944 2 Sheets-Sheet 2 OF RECT/F/ED SPEECH TO .rouncs 0F g f nacolvmoL CURRENT '4 T0 CONTROL saunas C 0F GROUNDED A. c. 0!? ac.

. MORMALLY 514.550

OPPOS/NG co/voucmva DIRECT/0N or VAR/570R! we. ,wo wu INVENTOIR I By LKSWART ATTORNEY Patented Oct. 9,1945

UNITED STATES PAT ENT OFFICE."

SIGNAL TRANSMISSION SYSTEM Leland K. Swart, Mountain Lakes, N. J., assignor I to Bell Telephone Laboratories, Incorporated,

New York, N. Y., av corp oration of New York Application December 30, 1944, Serial No. 570,591

' r 11 Claims.

' distortion due to'the switching operations required to change the direction of signal transmission.

Another object is to effectively silence the signal receiver during the operation of the signal transmitter at a subscribers station of a two-way carrier telephone system. I

Other objects are to switch the frequency of an oscillator from one value to another, to enable an inoperative oscillator or to disable an operative one-"under control of signaling currents in such manner as to minimize switching transients.

These objects are attained in accordance with one embodiment of the invention in a two-way carrier telephone system employing at each of the subscribers stations connected by a two-way transmission line, which may be a high voltage power line, a signal receiver of the superheterodyne type and an oscillator circuit adapted for producing carrier waves of different frequencies utilized respectively for transmitting outgoing telephone signals over the line, and for aiding in the detection of incoming telephone signals from i the signal-modulated carrier waves received over the'line.

Each station is normally conditioned for listening, and in that condition the oscillator circuit is arranged to supply a wave of suitable frequency to beat in the first detector of the superheterodyne receiver with the signal-modulated carrier received over the line, so as to produce a combination wave of the frequency to which the following intermediate frequency receiving amplifier is tuned, and to detect the incoming-telephone signals from the amplified intermediate frequency wave and to amplify these signals in the following second detector and audio amplifier in the usual manner. A voice operated switching device is responsive to outgoing telephonic signals generated at the station to change the frequency of the carrier oscillator to that desired for the transmitting condition, the resulting carrier frequency, which is selected to be outside the range of amplification .of the receiver so as to silence the latter during signal transmitting intervals, being combined in the modulator of thesignal trans- :mitter with the outgoing signals to provide a signal-modulated carrier which is transmitted over the line.

Among the features of the invention are various signal-controlled switching arrangements including vacuum tubes, copper oxide varistors orv gaseous tubes, for enabling or disabling a vacuum tube oscillator, or for changing its oscillation frequency from one value to another, without the production of objectionable switching transients.

The various objects and features of the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings in which:

Fig. 1 shows schematically a subscriber's station of a two-way carrier telephone system, embodying the invention; and

Figs. 2 to 9 show schematically alternative sigrial-controlled switching arrangements forcontrolling an oscillator which may be employed in the system of the invention shownin Fig. 1.

Fig. 1 shows one of a plurality of similar subscriber's stations of a-two-way carrier telephone system, including a signal transmitter TC and a' signal receiver RC of the superheterodyne type, associated with a two-way transmission line TL connecting these stations. The transmitter TC includes a telephone transmitter I with its associatedenergizing battery 2, and the amplifying vacuum tubes 3 and 4 operating respectively as a transmitting amplifier and a plate circuit transmitting modulator. The grid circuit of the amplifier tube 3 is coupled to the telephone transmitter I through transformer 5, and its plate circuit is coupled through transformer 6 to theplate circuit of modulator tube 5. The plate circuit of the modulator tube 4 is coupled by output transformer l to the two-way transmission line TL leading to other subscribers stations. A single tube, for example, an RCA 6F8o tube, may be substituted for the separate amplifier and modulator tubes 3 and 4 shown.

The receiver RC is of conventional superheterodyne or double detection type including a first vacuum tube detector stage D1 having its grid circuit connected through the resistance-condenser coupling circuit 8 across the line TI. on;

the output side of transformer 1; an intermediate frequency amplifier'Ar, tuned to an frequency amplifier Ar; a vacuum tube audio amintermediate -y frequency C, which may be, for example, 1'15 kilo- I its own oscillating circuit. v frequency B from OB is applied through the conpressed by coupling 1 the second detector stage the modulator tube 4,

fier AA'may be included in a single tube l3 as indicated; for example, an RCA 6F8c may be used.

A carrier wave of frequency A, for example, 300 kilocycles, for the outgoing speech wave is supplied by the oscillator A comprising a single vacuum tube having inductively coupled plate and grid circuits and a coil-condenser frequency determining circuit IS in its plate circuit, as indicated. The output of the oscillator 0A is connected through the series condensers I6 and I! to the grid circuit of the modulator tube 4 in the transmitter TC. A similar vacuum tube oscillator Os having inductively coupled plate and grid circuits and a coil-condenser frequency determining circuit l8 in its plate circuit, provides a carrier wave of the frequency B, for example, 475 kilocycles.

The carrier oscillators Oil and 0e are controlled to provide two-way operation of the station under control of the oscillator control vacuum tube 0C and the amplifier-rectifier vacuum tube AR, which may be an RCA 75-type tube, in the following manner:

In the normal receiving condition of the station, when no outgoing speechenergy is bein applied to the transmitter TC, the oscillator On is normally oscillating at the frequency B (475 kilocycles) since its grid is not biased to prevent such operation, the only bias being received from The carrier wave of denser l9 and the condenser of coupling circuit 8 in series to the control grid circuit of the first detector stage D1 of the superheterodyne receiver RC, and beats in that detector with the incoming speech modulated carrier of frequency A (300 kilocycles) received over the line TL from a talking subscriber at-another station, and also imcircuit 8 on that detectors grid circuit, to provide a combination wave in the detector output of the intermediate frequency C (175 kilocycles) to which the following intermediate frequency amplifier A: is tuned. This intermediate frequency wave is amplified by the amplifier Ar, and the modulating speech frequencies are demodulated from the amplified wave in following audio amplifier AA and are heard in the associated telephone receiver l2. The oscillator 'Os continues to oscillate at the frequency B until the subscriber at the station starts to talk,

and until this occurs the oscillator 0A is maintained in a non-oscillating condition by the biasing voltage drop across the large (100,000-ohm) resistor 20 in series with its cathode provided by the plate current of the oscillatorcontrol tube OC flowing through that resistor.

When the subscriber at the station starts to talk, his speech current generated in telephone transmitter l is impressed by transformer on the amplifier 3, A portion of the amplified speech energy in the output of amplifier 3 is applied through transformer 6 to the plate circuit of and another portion is impressed through condenser 2| and potentiometer 22 on the control grid circuit of the three-electrode amplifier portion of the tube AR. The amplified speech waves in the plate circuit of the amplifier of tube AR. passes through the primary winding Pl of transformer 23 and are induced D2, are amplified by the 'tively shunted by the condensers 26 and 21, associated with these respective windings and the respective diode plates of the tube AR, making the upper terminals of these resistors have a negative potential with respect to ground, which may be adjusted to a desired value by proper adjustment of the potentiometer 22 in the input of the tube AR. The negative potential of the resistor 25 associated with the secondary winding S2 of transformer 23 is applied directly to the grid circuit of the oscillator ,control tube 0C, biasing that tube to cut-off. The resultant reduction to zero of the plate current of that tube, flowing through the cathode resistor 20 of the oscillator tube 0A, will remove the normal negative bias on the latter tube provided by the voltage drop across that resistor, alowing On to oscillate at the frequency A (300 cycles). At the same time, the negative potential from the resistor 24 associated with the secondary winding SI of transformer 23 is applied directly to the grid of the oscillator tube 0a to bias that tube to cut-off, preventing the latter from oscillating. Thus, the frequency of the carrier oscillator arrangement at the station is switched from the frequency B to the frequency A as soon as voice energy is applied to the transmitter TC, andi in the absence of applied voice energy, the reverse process takes place, the carrier frequency being switched from A to B. The selected values of the condensers 26 and 21 associated with the resistors 24 and 25 will determine the speed at which the switching takes place, as well as the degree of hangover of the switchin circuit between speech syllables. The values of these condensers would be made such that there is sufficient holdover of frequency A so that normal pauses between syllables of a word will not result in the reversion of the carrier produced at the station to the frequency B, such as will occur at the station when the subscriber thereat stops talking.

While speech energy continues to be applied to the telephone transmitter I of the transmitter TC, the oscillator 0A continues to oscillate to supply the carrier of frequency A (300 cycles) to the grid circuit of the modulator 4, which carrier will be modulated in that modulator by the speech energy applied to the plate circuit of the tube to provide a speech modulated carrier of the frequency A (300 kilocycles) which is impressed by transformer 1 on the line TL, and is transmitted thereover to the other subscribers stations. When the station has been changed from a listening to a talking condition in the manner which has been described, the frequency A (300 cycles) of the signal-modulated carrier wave apearing on the line at theinput to the receiver RC is so far away from the intermediate frequency C cycles) to which the receiver of the station is tuned that it will not be amplified in the receiver, and the telephone receiver I2 of the station will be silenced.

When the subscribers at two connected stations both talk at once, each will cause the oscillator at his station to beswitched to frequency A and ,The oscillator of eac coming signal-modulated carrier. vThus, the

- of undesirable large starting" and stopping" transients tending to produce clicks or. other distortion in the transmitted speech; Such switching transients would be attained in systems in which the required switching involves thejntroduction in or removal of considerable losses from thesignal transmission paths, or making changes ,in the gain of these paths by biasing the grids of vacuum tubes negatively so as to switch substantial amounts of energy in the anode circuits of these tubes, especially when the system is operating at a frequency not greatly removed from the voice transmission; The switching arrangement of the invention has certai-, other advantages, such as a selectively tuned high gain receiver and non-critical switching circuits.

To cut down'the numberpf tubes employ d in the system of the invention, it may be desirable to user a single double triode tube in place of the separate oscillator tubes A and OB], forexample, an RCA 6C8 tube; and also a single double triode tube to replace the detector tube D1 in the receiver RC and the oscillator control tube 00, for example, an RCA 6E8c tube.

..Figs. 2 to 9 show alternative arrangements which may be employed for switching the frequency of a vacuum tube oscillator from one value to another, or for enabling or disabling such an oscillator, without changing the static currents in the vacuum tube, so as to avoid the production of objectionable switching transients.

of these figures comprises a single vacuum tube 0 having its plate circuit inductively or c-apacitively coupled to its grid circuit, and having a parallel inductance coilcondenser arrangement associated with the coupled circuits for determining the frequency of oscillation.

In the circuit of Fig. 2 the coil of an inductance L1 in the plate circuit of the oscillator tube 0 is inductively coupled to a coil of inductance L2 .in the grid circuit of the tube, and a third inductance coil of inductance L3, which may or may not be inductively coupled to L1 or L2, is connected in a closed series circuit with the inductance coil L2 and.the condenser C1 to form a frequency-determining or oscillating circuit for the oscillator tube. The varistors VRI and VR2, which are preferably of the copper oxide rectifier type, are connected in series across the inductance coil La, and arerelatively poled in such a direction that they in effect constitute an open circuit across that coil. A source of direct current, which may befrectified speech; is adapted to be connected by the switch S across the midpoint of the coil L3 and a point between the varistors VRI and V'R2. With the switch S open, the frequency of oscillation of the oscillator is determined by the selected values of the inductances L2 and In in series and the shunt capacity C1. When the switch S is closed to apply a direct current biasing potential poled as indicated across the inductance Le, the impedance characteristics of the varistors VRI and VR2 will be so changed that they effectively placea low resistance shunt across that inductance, and

since this materially reduces the inductance of the oscillating circuit, th period of the oscilla coils L1 and L2. In the circuit of Fig. 4 the capacity C1 is also connected in shunt with the inductance L2 in the grid circuit of the oscillator tube, but the inductance L3 is connected in series with the normal high impedance varistors VB! and VR2 across the coil L2 and shunt capacity C1, so that the inductance L2 is eifectively removed from the oscillating circuit in the normal condition of that circuit; and the source of direct control current is adapted to be connected by the closing of the switch S directly across the varistor VRI, and across the 'varistor VR2 in series with theradio frequency coil R. F. C. provided to prevent shunting of oscillations from L1 and L2. The closing of the switch S to connect the source of direct control current across the ivaristors in the circuit of Fig. 4 will change the impedance of the varistors VB! and VR2 so oscillating circuit and thus increase the period of oscillation of the oscillator.

- Figs. 5 b0 show various arrangements employing cold cathode gas tubes for changing the operation characteristics of a single vacuum tube osci1lator.- In the arrangement of Fig. 5, with the switch S opened, the oscillator will generate a wave of a frequency determined by the inductance L1 in series with the inductance le and the shunt capacity C! in the oscillation circuit connected in series with the condenser C2 in the feedback path between the plate and the control grid circuit of the oscillator tube 0. The cold cathode gas tubes GTI and GT2, shunted re-' spectively by the resistors R1 and R2, are connected eifectively in series with each other across this oscillating circuit, and are normally deionized so as not to interfere with the oscillation of the oscillator. When the switch S is closed to apply a direct current voltage from the associated direct current source, which may be a rectified speech source, as indicated, across the mid-point between the coils L1 and L2 and the mid-point between tubes GTI and GT2, of suificient value, say, 70 volts, to cause ionization in these tubes, a relatively low resistance shunt will be placed on the oscillating circuit effectively disabling the oscillator.

In the circuit arrangement of Fig. 6 the vacuum tube oscillator O normally oscillates at a period determined by the inductance L1 in series with the inductance L2 and the shunt" capacitance C1 in the oscillation circuit connected in series the gas in that tube to provide a relatively low resistance shunt through'that tube and the ca-- pacity C3 across the inductance L1. Thi will reduce the total inductance of the oscillating circuit and increase the period of oscillation of the oscillator.

In the circuit of Fig. '7 the plate of the oscillator tube is connected to the grid thereof through the series circuit comprising the condenser C2 and a circuit comprising the capacity C1 shunted by the inductances L1 and L2 and the intermediate cold cathode gas tube GT4. With the switch S open, the gas tube GT4 i deionized so that it effectively provides an open circuit between the inductances L1 and la in the oscillating circuit and therefore the oscillator normally will not oscillate. When the switch S is closed, a source of direct current voltage, which may be rectified speech, of sufficient value, say, '70 volts, to cause ionization in the tube GT4 is applied across that tube, the resulting ionization in that tube causing the connection between the inductances Ll and L2 to be effectively closed. This will cause the oscillator to oscillate at a period determined by the values of the series inductances L1. and Lo and the shunt capacity C1.

The oscillator circuit of Fig. 8 normally oscillates at a period determined by the values of the inductance L1 in series with the inductance L2 and the shunt capacity C1 in the frequency-determining circuit connected between the plate and the grid of the oscillator tube. The copperoxide varistors VR3 and VR4 are connected in series across the inductance L1 in this oscillating circuit andare so poled that they provide a very high impedance path in shunt with that inductance and thus do not normally prevent oscillation of the oscillator. When the switch S is closed to connect a direct current source of biasing potential poled as indicated, through the series resistance R3 and shunt capacity C3 across a point between the two varistors VR3 and VR4 and the mid-point of the inductance coil L1, these varistors will become a low impedance and will effectively short out the inductance coil L1, thus decreasing the inductance in the oscillating circuit and causing the oscillator to oscillate at a. higher frequency. The radio frequency coil R. F. C. in series with the biasing circuit is provided to preventshunting out or suppressing oscillation by virtue of the connection of C5; or the direct current control circuit to the mid-point of L1.

In the oscillator circuit of Fig. 9, the two copper-oxide varistors VH3 and VR4 are connected in shunt with the oscillating circuit comprising the inductances L1 and Le shunted by the capacity C1, connected in series with condenser C2 between the plate and grid of the oscillator 2, instead of in shunt with the inductance L1 alone as in the circuit of Fig. 8, and a source of control voltage, which may be a grounded alternating current or a direct current source poled as shown is adapted to be connected by switch S across the mid-point between the two varistors and the mid-point between the inductances L1 and In. The rectifying arrangement comprising the series copper-oxide varistor VR5 and the shunt capacity C4 in the biasing circuit makes it possible to impress a direct current bias on the varistor when either an alternating current or direct current potential is applied at the input of the control circuit. Let us consider that an alternating current potential is impressed on this control circuit on the positive half of the cycle. The varistor VR5 will pass current and charge the condenser G4 at a positive potential with respect to ground. This positive potential will materially reduce the impedance of the varistors VR3 and VR4 thus effectively short-circuiting the oscillating circuit and stopping the operation of the oscillator. This direct current bias will also be present during each. negative half cycle of the impressed voltage because of the energy fed from the condenser C4 during this period.

The rectifying arrangement of Fig. 9 may be applied also to any of the oscillator circuits of Figs. 2 to 8 to provide a direct current source of control potential from an alternating current source.

The variable frequency oscillator of Figs. 2, 3, 4, 6 or 8 maybe substituted for the oscillator arrangement comprising the oscillator tubes 0A and 0a, the control tube 00 and the associated grid biasing arrangements in the system of Fig.1, in which case the direct current biasing potential for switching the frequency of the former oscillator from one value to another would be obtained from the output of the amplifier-rectifier AR during the period in which speech energy is applied to the transmitter TC. Also, the normally operative oscillator and associated disabling arrangement of Fig. 5 or Fig. 9 may be substituted for the normally operative oscillator 0A and associated disabling arrangement in the system of Fig. l, and the normally disabled oscillator and associated enabling arrangement of Fig. 7 may be substituted for the normally disabled oscillator On, control tube OC and the associated oscillator enabling arrangement in the system of Fig. l, the direct current control for the substituted arrangement in each case being supplied from the output of the amplifier-rectifier AR.

Various modifications of the circuits illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In combination at a station of a two-way carrier wave signaling system, a signal transmitter including a signal modulator, a signal receiver including a signal demodulator, a common oscillator for supplying carrier waves of difswitching device responsive to outgoing signals enerated at the station to change the produced frequency of said oscillator to that used for transmitting said outgoing signals.

2. The combination of claim 1, in which said oscillator comprises one portion normally in oscillating condition to supply to said demodulator the carrier wave of the frequency used during said receiving intervals, and another normallydisabled oscillator portion, which, when operative, oscillates to supply to said modulator the carrier wave of the frequency used for transmitting outgoing signals, said signal-operated switching device being responsive to the-outgoing signals generated at the station to disable the first oscillator portion and to enable said other oscillator portion during signal transmitting intervals.

3. The combination of claim 1, in which said common oscillator comprises two vacuum tube oscillator portions each having a control grid, the grids of said oscillator portions being noranally biased so that one portion is in a state of oscillation to supply a carrier of the desired irequency to said demodulator, and the other portion is disabled, said signal-operated switching device being responsive to outgoing signals generated in said transmitter to reverse the bias on the grids of said oscillator portions 50 that said one portion is disabled, and said other oscillator portion is enabled to supply to said modulator a carrier wave of the frequency used for transmitting said outgoing signals.

4. In combination in a two-way carrier wave telephone system, a two-way transmission line and a subscribers station comprising a signal transmitter including a wave modulator feeding said line, a signal receiver of the superheterodyne type, including a wave detector fed from said line and a following wave amplifier tuned to a given intermediate frequency, a carrier wave source comprising one oscillator portion normally operative to transmit a carrier wave of suitable frequency to said detector to beat therein with an incoming signal-modulated carrier wave of another frequency received by said receiver fromv said line, so as to produce a, combination wave of said intermediate frequency, and a second normally disabled oscillator portion, which, when operative, transmits to said modulator a carrier wave of a third frequency to combine therein with outgoing voice signals generated in said transmitter to produce a signal-modulated carrier wave for transmission over said line, a voiceoperated switching device responsive to voice signals generated in said transmitter to disable said one oscillator portion and enable said second oscillator portion andresponsive to cessation in the generation of said signals to return said station to the normal condition with said one oscillator portion operative and said second oscillator portion disabled, said third carrier frequency being outside the amplification range of said tuned intermediate frequency amplifier. I 5. The combination of claim 4. in which said VQice operated switching device comprises means connected to said transmitter for amplifying and detecting a portion of the voice signals generated therein, and means responsive to the detected signals'to produce biasing voltages providing the desired reversal of the operating condition of said two oscillator portions.

6. The combination of claim 4, in which each of said oscillator portions comprises a vacuum' tube oscillator having a control grid, the normal disabling of said second oscillator portion is provided by a control vacuum tube having a control grid, normally operative to bias that oscillator portion against operation, said voice-operated switching device comprises an amplifier-rectifier supplied with a portion of the voice signals generated in said transmitter, and means responsive to the rectified voice output of said amplifierrectifier to apply a disabling bias to the grid of said one oscillator portion, and a disabling bias to the grid of said control tube so as to effectively remove the disabling bias produced thereby on said second oscillator portion.

7. In combination, a vacuum tube oscillator having two difierent frequency determining circuit arrangements, means normally making only one of said frequency-determining arrangements effective so that said oscillator normally produces a wave output at one frequency, a source of direct current, and means responsive to the application of waves from said source to said oscillator to cause said other frequency-determining circuit arrangement only to be eflective so that said oscillator produces a wave output of a different frequency.

8. In combination, a vacuum tube oscillator having a frequency-determining circuit including inductance and capacitance, variable impedance means connected across at least a portion of said inductance, the shunt impedance provided by said variable impedance means being normally so high as to constitute an effective open circuit across said inductance portion making the latter effective in said frequency-determining circuit to cause oscillation of said oscillator at a frequency determined by the total inductance and capitance in said circuit, a direct current source, and means responsive to the current from said source to change the shunting impedance of said variable impedance means to a very low value, thereby effectively removing said inductance portion from said frequency-determining circuit and causing said oscillator to oscillate at a dillerent frequency.

9. The combination of claim 8, in which said variable impedance means across said inductance portion comprises two copper oxide varistors poled in oposition providing such a high shunt impedance that said inductance portion is effective in said frequency determining circuit, and said source applies a direct current bias to said varistor of such value and with such poling as to reduce the impedance thereof to such a low value as to effectively short-circuit said inductance portion and thus increase the frequency of oscillation of said oscillator.

10. The combination of claim 8, in which said variable impedance means comprises one or more gaseous discharge devices which are normally deionized to provide a very high shunt impedance across said inductance portion so that the latter is effective in said circuit, and are ionized in response to the direct current bias applied by said source to effectively short-circuit said inductance portion and thus change the frequency of oscillation of said oscillator.

11. The combination of claim 8, in which said variable impedance device comprises tWo oppositely poled copper oxide rectifiers shunting the whole inductance in said frequency-determining circuit, said rectifiers normally providing an ef- "fective open circuit across said Whole inductance and thereby allowing oscillation of said oscillator at a frequency determined by the values of said inductance and capacitance, and being biased by the current from said source to reduce the impedance of said rectiflers to a very low value so as to effectively short-circuit said whole inductance and thus to stop oscillation of said oscillator.

LELAND K. SWART.

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