IL43517A - Direct current supply arrangement for subscriber telephone terminal - Google Patents

Description

DIREGT CURRENT SUPPLY ARRANGEMENT FOR SUBSCRIBER TELEPHONE TERMINAL present invention relates to a subscriber unit for use a telephone The subscriber unit of the invention is particularly useful with respect to the The present invention is therefore described below particularly with respect to that but it will be appreciated that or certain of its could be advantageously used in other systems as One of the most important components of electronic telephone the line circuit interface unit which interfaces the electronic part of the exchange with the The line circuit supplies the direct current needed for the set for signalling and also for the In the line circuit provides the relatively high ringing and also protects against surge Further included in the line circuit are the and dialling detector and the transfer The circuit is usually connected to the exchange through a line transformer whose on the line side is split into two equal the direct current voltage being connected through or a between the taps of the two parts of the transformer Such a circuit isolates the power thereby enabling the connection to it of many line In the circuit enables the detection of the In such a the line wires can be easily unbalanced by any unbalance of the transformer relay or resulting in increased the transformer has to without raA direct current in accordance with the This requires a transformer of substantial size to prevent even though the power delivered by the guard the microphone of the set particularly on An object of the present invention is to provide a line circuit which provides advantages in one or more of the above According broad aspect of the present there is provided a line circuit for connecting a subscriber terminal to a signal transmission network including a direct current supply for driving the subscriber terminal and a line transformer connecting the subscriber terminal to the signal transmission characterized in that the direct current supply a constant direct current circuit including a capacitor blocking the passage of the constant direct current through the line According to another feature o the the constant direct current supply is of large impedance and supplies a constant current of about 30 when the line resistance is less than 1400 A number of important advantages are provided by the foregoing small line transformers may be used since they pass only the alternating microphone currents and not the direct current for driving the In the microphone alternating current outputs are independent of line there is no need for ballasts to guard the microphone of the set on short the use of the constant current sources enables all the subscriber units to be connected to a common power supply with minimum between the being reduced by the large impedance of the constant current In it has been found that the when driven by a constant current produces a cleaner than in the conventional According to another feature of the the constant direct current supply includes a pair of constant direct current sources and a feedback each wire of the line being connected to In the preferred embodiment o the invention described the constant direct current sources comprises a pnp and an transistor connected together in series with DC said feedback loop comprising a first pair of resistors connected between and in series with two and a second pair of resistors each connected at one side to the juncture of resistors of the first and at the other side to the base of one of the According to a further feature of the the line circuit further includes an amplifier in the feedback for regulating the current through the transistors towards According to a still the line circuit also includes a transfer order when one of the wires is shorted to to sense the unbalance of symmetry and produce a order According to another aspect of there is provided a unit a line circuit as and a link receiving output of the line for connecting to the signal transmission the described the link includes a hybrid circuit comprising an aasymmetrical resistor bridge smaller loss of the signal transmitted to the transmission network than received from the transmission As mentioned the invention is particularly in a system in which the signal transmission network is a common transmission the subscriber further including an and an for accessing the hybrid circuit to the common according to specific Further features and advantages of the invention will be apparent from the description The invention is herein by way of example with r fe e efe ed em me l ed Incorporating the present 2 is a block diagram illustrating the standard unit of the system of 3 is a circuit diagram illustrating one form of line circuit accordance with the invention use in the unit of 4 is a circuit diagram illustrating a modification of line circuit Sf and 5 illustrates a specific subscriber unit corresponding to but including further this subscriber unit being use in the system of The system illustrated the drawings is a private automatic including a unidireotional switching It uses a single highway for transmitting all the analog information and signalling within the A single highway is used in this case because of the small number of terminals within the At one the system may include the possibility for connecting 2 central battery subocrlber terminals 2 the operator terminal plus four and four tie lines 8 to similar private automatic branch The line circuit corfnects tha terminals 2 to a switching network 12 hybrid circuit 14 which effects a The hybrid circuit and the switching network the standard unit are commonly called standard this link being indicated by the box 16 in T subscribers are tied by connecting a loop of hybrid The line circuit 10 supplies the direct current needed in the set for signalling and for driving the In it connects the set to the ringer supplied also contains the signalling detectors and which detect the foregoing conditions and produce a signal is fad to the 22 via tha LHW bus to control the tone generators It further Includes the transfer orde detector which detects a order subscriber shorting one of tha line wiras to which transfer order is also fad to 22 via bus The 22 supplies all the timing information to the Switching network and also functionally supervises every terminal and every link of telephone The system of 1 is more particularly described in the Patent Application therefore further details 6f tha system not essential for an understanding of the present indention are not included the present The of which the present invention is primarily relates to the line circuit and the standard 16 portions of tha A standard subscribers unit incorporating the present invention is more particularly As shown in tha line circuit in includes a direct current supply for the the direct current supply comprising a pair of constant currant sources to ba described more particularly line circuit further includes a signalling detector 104 for and dialling signals which are fed to 22 of the system via bus LHW a signalling detector 106 for producing transfer order signals which are fed to the bus THW The line circuit further includes a ringer source 108 for the ringing voltage to the this source being controlled by a turn controlled by a ringing integrator 114 receiving a voltage from ring bus which is enabled by a signal on line the latter signal being the same one applied to the of the switching network of the respective line The line circuit in 2 further includes a and a circuit the latter circuit being known per se and therefore not further described The link includes a hybrid circuit 124 to 14 in which is coupled to the line circuit by line transformer 120 and which transmits audio as as the signalling to and from the common transmission highway The input side of the link includes 126 which couples the hybrid circuit 124 to the common transmission highway via an amplifier and a The output side of the link includes 132 which couples the common transmission highway to circuit 124 via a circuit and amplifier The constant current sources 102 are each connected to one of the wires of the direct current needed for the set without loading or unbalancing the line They are of large impedance and supply a constant current of 30 provided the line resistance is less than 1400 the line resistance is 600 A capacitor C1 is provided in series with the line and the primary winding of line transformer The provision of constant current sources 102 for driving the microphone of the subscriber set assures that the microphone alternating current output will be independent the line and therefore there is no need for ballasts to guard the microphone of a set having a short since the constant current eources are of large this enables the connection of all subscriber to a common current supply without substantial capacitor C1 blocks the passage the direct current to the line and therefore the line transformer can be a small substantially smaller than the conventional without danger of In it has also been found that the when driven by a constant current produces a cleaner form than in the conventional 3 illustrates one form of constant direct current suppl which may be used for the constant durrent of The constant direct current supply illustrated in 3 includes a pair of symmetrical constant direct current sources and a feedback One constant current source includes a pnp transistor having its emitter connected via resistor R1 to a V bus and collector connected via resistor R2 to subscriber line wire The other constant currant source includes an npn transistor Q2 having its emitter connected via resistor to a V bus and collector connected via resistor R4 to the other subscriber line wire A further pair of resistors R6 are in series between the two transistors and across the subscriber line wires LS1 Each transistor Q2 includes feedback resistor R8 connected at one to the juncture of the two resistors and at the other side to the base of the respective transistor a diode 01 is connected via a resistor between the base of transistor and its emitter resistor a second diode 02 is connected via resistor R10 between the base of transistor 02 and its emitter resistor a capacitor C2 is connected across the V bus and the base of transistor and another capacitor C3 is connected across the V bus and the base of transistor One side of the primary winding of line transformer 120 is via blocking capacitor to the of transistor and the other side of the transformer winding is connected to the collector of transistor It will be seen that the including resistors R7 and current in the two transistors provide a constant current output the subscriber line wires LS1 and The diodes D2 maintain constant current with temperature since they are poled so that the voltage across tracks the base emitter thereby maintaining a substantially constant voltage across the emitter resistor of the respective and a substantially constant The capacitors C3 in the system of 3 isolate connection between the voltage sources and the A great identical circuits can therefore be connected to the same voltage As a practical each capacitor C3 illustrated in 3 would preferably be composed of electrolytic and capacitors in Resistors R5 and R6 are needed for the feedback Their value high enough to the as an each may be 47 As noted the current sources have When the line is open and dialling or the line loop resistance is greater than a definite value the two transistors will Resistors R2 and R4 do influence the balance of the current their value is than that of resistors R5 and for example resistors R2 R4 may each about 100 They are needed to provide some resistance to the line in the open line which is an important requirement the hybrid circuit The transfer order is effected by the subscriber shorting to aarth one of the line it makes difference which As a practical the connection through the earth between the connection earth and the connection to is usually zero the resistance being in the order of up to 100 the transfer order can be discriminated because of the unbalance of symmetry achieved by the which unbalance changes the voltage of the middle Junction of the feedback the circuit illustrated in 3 will not always give e good and reliable indication of the especially for long For there may be an unbalance voltage produced because of asymmetrical values of components of the two current and it may always be possible to reliably discriminate between that unbalance voltage and the unbalance caused by the transfer order short to 4 a modified line circuit in which the sensitivity to discriminate between a transfer order unbalance and a spurlus unbalance is increased by improving the feedback The modified circuit illustrated in 4 basically the same as that of 3 therefore correspondingly numbered except that an amplifier 140 is in the feedback loop between the junction of resistors R6 and the middle junction Vm of the feedback Amplifier 140 regulates the current through the transistor towards Shorting one of the line wires to earth changes the control voltage and therefore the output of the This voltage can be brought near to zero on normal operation so that voltage deviations effected by the transfer order can be reliably The transfer order detector 106 2 and is connected to the middle junction Vm of feedback A suitable detector is illustrated in A suitable signalling detector which detects and dialling is also described below with respect to the standard subscriber unit illustrated in The two transistors Q2 of the constant direct current souroes in 4 are protected by a diode Dz is connected between the collectors of the two A reverse voltage will be shorted by the Zener whereas a voltage with the same polarity the voltage sources will be limited by the Zener 5 illustrates a standard subscriber unit particularly for use in the system of the copending patent The subscriber unit of includes the circuit illustrated in 4 for providing constant direct current to the includes transistors and the feedback loop containing amplifier 140 for improving the sensitivity of the system to discriminate an unbalance caused by a transfer The system of includes an additional capacitor C4 to control amplifier 140 in order to the time response of the feedback This of the feedback loop time response reduces the possibility of error as a result of transient iffects when The signalling detector in detects the and dialling These line are detected by using an integrator 150 whose time constant is particularly to distinguish between noise and bouncing effects and theabove line Since the signalling detector 104 also has to pass the dialling capacitor connected between line and the line must be limited so that the dialling voltage will not be unduely These distortions will capacitor C1 will influence the frequency response of the subscribe As one capacitor C1 be The transfer order detector 106 includes two comparators 154 in the unit illustrated in These comparators sense the sign of frame definite voltage in order to discriminate a voltage unbalance in the feedback loop of the constant direct current The hybrid circuit 124 uses an resistor bridge so that there is a smaller ioss of the signal going from the common transmission highway HW to the This effected by enlarging the loss from the line to the common transmission this manner of reducing the loss in the information transmitted by the highway HW to the enables the maintenance of the required undistorted voltage swing supplied to the Reducing such a loss a hybrid transformer requires assymmetry between the incoming and outgoing windings of the Two subscribers are tied by a connecting a loop of two hybrid Each subscriber circuit is a Unbalance of the hybrid bridge causes leakage from the to Each hybrid circuit contains amplifier to compensate for no more than the lasses from transmitter to the and from the line to the The loop is thus made free of oscillation for all values line The 136 are preferably operational Each filter has about a 50 slope outside the needed and a special dip in the sampling The circuit 13 is provided for improving the ratio and for facilitating the reconstruction of the sampled This circuit is indluded only in the outgoing part of the for transmitting the information from the common Each subscriber is called by the activation of its iical bell on a The high voltage ringing beats are supplied by the ringer of the The rigning signal is of a frequency of and provides one duration beats every 4 The beat voltage is formed from the 50 Hz line by using a suitable increasing the period of each alternation by three over that of the The high voltage ringing beats are supplied to the line through contacts As described with reference to this relay is operated by relay driver 112 and ring integrator circuit the pulses supplied on line to enable the ring bus The arrangement is such that ring bus 116 is enabled to the ring in of the subscriber to whom ringing voltages are The system tests between the ring beats when the current sources are connected to the While the invention has been described with respect to a it will be appreciated that or features could be used in other for Another possible application is in transmission switching networks constructed metallic Many other modifications and applications of the illustrated embodiment will be insufficientOCRQuality

Claims (6)

P.A. 43517/2 WHAT IS CLAIMED IS:

1. A line circuit for connecting a subscribed terminal to a signal transmission network including a direct current supply for driving the subscriber terminal and a line transformer connecting the subscriber terminal to the signal transmission network with the line between the line circuit and the subscriber terminal having two wires; characterized in that the direct current supply is a constant direct current supply, including a pair of symmetrical constant direct current sources, each coupled to one of said two wires and a feedback loop; and a capacitor in the line circuit for blocking the passage of the constant direct current through the line transformer.

2. line circuit according to Claim 1, wherein the constant direct current supply has a large output impedance and supplies a constant current of about 30 ma when the line resistance is less than 1400 ohms.

3. A circuit according to Claim 1, wherein said constant direct ee¾H? £s) supply further comprises first and second DC voltage sources for supplying driving voltage for said current sources; and wherein said pair of constant current sources comprises a pnp and an npn transistor connected together in series with respective ones of said first and said second DC voltage sources, and said feedback loop (comprising) ^i&L ed a first paid of resistors connected between and in series with the two transistors, and a second pair of resistors each connected between the juncture of the two resistors of the first pair, and the base of one of the transistors. f P.A. 43517/2

4. A circuit according to Claim 3, wherein each constant current source further includes am (emitter) resistor coupled between the emitter of the transistor and the respective one of said first and second DC voltage sources and a diode and resistor connected i series from the base of the transistor to the respective DC voltage source, said diodes being biased sb that the voltage thereacross follows the base emitter voltage to operativel •maintain (lag) a substantially constant voltage across the emitter resistor and a substantially constant emitter current.

5. A circuit according to Claim 4, further including a Zener diode connected (aesHHH*) between the collector of the two transistors.

6. A line circuit for connecting a subscriber terminal to a signal transmission network substantially as described with reference to and as illustrated in the accompanying drawing. DANIEL FREIMANN, Advocate ',· . . . P. O. B. 9099, T E L-AV IV 61 090 \ 'Κ': '