GB2130851A - Hybrid junction circuits - Google Patents

Abstract

A hybrid junction circuit which is of application to a security telephone installation connected to a public telephone system comprises an adjustable balancing network BN. Impedance components of the network BN are switched by electronic switches S1 and S2; to adjust the balancing impedance, in response to control signals CS1 and CS2 from a microprocessor control circuit C. Before balanced operation commences the circuit C selectively operates the switches S1 and S2 to adjust the balance impedance to provide a reasonable match to the external impedance presented to the bi-directional port A of the hybrid. <IMAGE>

Description

SPECIFICATION Hybrid junction circuits This invention relates to hybrid junctions, that is to say to hybrid junction circuits or devices which have four pairs of terminals so arranged that a signal entering at one terminal pair divides and emerges from the two adjacent terminal pairs, but is unable to reach the opposite terminal pair. Such a circuit or device is alternatively referred to as a birdge hybrid, a hybrid coupler, a directional coupler or most simply as "a hybrid" which simplified term will generally be used hereinafter.

The invention is more particularly concerned with hybrids which, when balanced, couple two uni-directional ports to a bi-directional port, without incident power on one of the unidirectional ports being transferred to the other. A generalised block diagram of such a hybrid device is shown in Figure 1 of the accompanying drawings.

In that figure there is shown a hybrid H having a uni-directional input or GO port G, a unidirectional output or RETURN port R, a bidirectional port A and a fourth terminal pair 1 which is connected to a balancing network BN.

Power which is supplied to the port G is not transferred to port R but is divided between the balancing network BN and the bi-directional port A. Similarly, power incident on the port A is shared between the network BN and the port R. In general the circuit operates in the manner of a balanced bridge, and performs perfectly only when the network BN has a balance impedance which exactly matches the impedance presented to the port A by the external circuit or equipment to which that port is connected. This impedance may be purely resistive or a complex impedance with a reactive component.

Simple forms of such hybrid circuits are used, for example, in telephone circuits to couple a handset to the telephone line so that signals from the transmitter are coupled to the line but are not transferred to the receiver. In this application perfect matching is not essential, and a balance impedance can be chosen which provides an adequate match with all the external equipment to which the telephone installation is likely to be connected.

The invention is of particular, but by no means exclusive, application to security telephone systems such as "scramblers" where accurate routing of the incoming and outgoing scrambled messages through a hybrid is important. Thus it is necessary for the balancing network to be more accurately adjusted to the impedance presented by the external telephone line at the time connected, which presents problems with a security system which enables a telephone installation to enjoy dialled security connection with other telephones. The object of the invention is to provide a hybrid circuit which can be used to overcome these probiems.

To this end a hybrid circuit, according to the invention, has an adjustable balancing network, means for automatically adjusting the impedance of the network at the start of every transmission with which the circuit is used, and a control circuit which before normal transmission commences controls said means to adjust the balance impedance to provide a reasonable match to the external impedance presented to the bi-directional port of the hybrid.

Thus the normal fixed impedance network is replaced with a variable impedance network capable of matching a range of external impedances, and the hybrid is associated with means to adjust the network at the start of every transmission in which the circuit is used. Thus the hybrid is rendered adaptive to external impedance.

Preferably the GO and RETURN ports of the hybrid are coupled to the bi-directional port thereof through a transformer, with the balancing network connected to a tapping of the transformer primary winding and in parallel with a fixed resistor (which is connected across the RETURN port) and one side of the primary winding. The network in a simple embodiment may be a switched variable resistor network, conveniently employing electronic FET (Field Effect Transistor) switching as the means to adjust the impedance. However, if necessary the network can employ reactive components.

In some circumstances the network may in practice provide a compromise between a fully complex impedance adjustment and a resistive only adjustment. In such an embodiment the network may comprise a switched resistive network section in conjunction with a section having a small number, for example two, complex impedance component networks. In practice in telephony applications only a small number of classes of networks are likely to be required, and within each class the switched resistive network section will usually provide sufficient variability to match a wide range of practical circumstances.

The circuit may be embodied in a security telephone installation usable with similar installations and connectable thereto through the public telephone system, each installation having a similar hybrid circuit at the telephone line interface. In this case the installation at each end of the telephone line is able to ensure that the incoming signal from the remote end is silent while it checks the external impedance presented at the telephone line and adjusts its hybrid impedance network to suit. This is desirably achieved by sending out a signal on the telephone line, measuring the reflected signal and adjusting the latter to a minimum value.

Alternatively, both end installations can adjust their hybrid impedances at the same time by simultaneously transmitting different known signals, the difference between these signals being such that there is no possibility of one being mistaken for the other. Each end installation then adjusts its hybrid to minimise imperfections in the signal received from the other end installation.

This method saves time, as the two end hybrids are adjusted simultaneously instead of in turn, but it results in somewhat less accurate adjustment.

With such a simultaneous transmission from each end of recognisably different signals the adjustment may be achieved in a different manner with the transmitted and received signals being compared at each end. With this method, wnich in some circumstances is less easy to apply, each hybrid is adjusted to minimse the correlation between the transmitted and received signals. The two methods of adjustment, employing simultaneous transmission of different signals, are equivalent if the two signals transmitted are orthogonal.

The invention will now be further described with reference to the accompanying drawings which illustrate, by way of example, two preferred embodiments of the invention. In the drawings: Figure 1 is a block diagram óf a fundamental hybrid circuit, as already described; Figure 2 is a basic block circuit diagram of the hybrids of both embodiments; Figure 3 diagrammatically illustrates a balancing network of one of the embodiments and an associated control circuit, and Figure 4 is a similar diagrammatic illustration of the other embodiment.

In the circuit diagram shown in Figure 2 a low impedance voltage source G provides a unidirectional a.c. signal connected to the GO port of the hybrid circuit illustrated, the latter including a coupling transformer T. The transformer T has a split primary, with two winding sections W1 and W2, and a secondary winding W3 connected to the uni-directional port A. The source G is connected to the free ends of the windings W1 and W2 through fixed resistors R1 and R2, respectively, the resistor R2 being connected across the RETURN port R. the balancing network BN is connected between the common ends of the windings W1 and W2 and the remote side of the resistor R2, hence being in parallel with R2 and W2 connected in series.

Thus the source G couples to the load at port A via windings W1 and W2, the component R1 and the balancing network BN. It can readily be shown mathematically, using conventional circuit theory, that in the balanced condition with BN correctly chosen the voltage across BN is equal and in phase with that induced across W2 by the current flowing in W1 and W3. Hence the voltage developed across R2 is zero. However, a voltage can be developed across R2 as a result of an incident signal at the port A, via R1, W1, W2 and the low impedance source G.

The balancing network BN is of adjustable impedance in accordance with the invention, and in the embodiment of Figure 3 comprises a switched variable resistor network. This network comprises a resistor R3 in parallel with which resistors R4, R5... R11 can selectively be switched by electronic switches S having FET switching elements. The switching control signals CS are provided from a microprocessor of a control circuit C, in the form of a latched 8 bit word. The values of the individual resistors R4, R5. ..R1 1 are chosen so as to form a best fit approximation to 32 logarithmically spaced steps to match a 4:1 range of external load impedance.

The other embodiment illustrated in Figure 4 differs in respect of the variable impedance network which provides a compromise between a fully complex impedance adjustment and resistive only adjustment as with the first embodiment. In this other embodiment the network BN comprises a switched resistive network section having resistors R3, R4. . .R11 as before in parallel with a section having two switched complex impedance component networks Z1 and Z2, which are selectively switched by electronic switches S1 independently of switching of the resistive section the switches of which are now designated S2.

Separate control signals CS1 and CS2 from the control circuit C indpendently control the switches S1 and S2.

During adjustment of the network BN of the second embodiment the switches S2 enable the network to be adjusted to provide the right reactive class to suit the circumstances. With both Z1 and Z2 out of circuit the resistive section R3,R4 ... R1 1 provides a resistive impedance class corresponding to the Figure 3 embodiment. With Z1 in circuit the section R3, R4. . .R1 1 provides resistive adjustment in a reactive impedance class with Z1 containing both inductance and capacitance but mainly capacitive; and with Z2 in circuit the section R3, R4, .R1 1 provides resistive adjustment in a reactive impedance class with Z2 being mainly inductive.

Thus three impedance classes can be covered with this embodiment, which will allow adequate matching with the majority of telephone installations. Within each impedance class a switched resistive network section such as R3, R4...R1 1 will normally provide an adequate range of adjustment.

The embodiments illustrated were developed for use in a security telephone system employing a scarambler unit associated with a normal telephone and which can communicate, through the public telephone network, with other telephones each associated with a similar unit.

Each unit employs the described hybrid circuit, the hybrids being provided at each end of the telephone link and at the telephone interface of each unit. Thus, for the most accurate method of hybrid adjustment each end of the system can take control of the other to ensure that the incoming signal from the remote end is silent, while the control circuit C sends out a test signal TS through its own hybrid H. The control circuit C measures the reflected signal RS at the RETURN port of the hybrid, and operates to adjust the impedance value of the network BN to minimise the reflected signal, this providing the best obtainable balance.

The above described impedance balancing procedure takes up a short time at the start of each security telephone call, and is incorporated in a call set up procedure during which conversation or other messages are in any case blocked.

However, it does involve that the two end installations adjust their hybrid circuits in turn and if somewhat less accurate adjustment is acceptable an alternative method can be employed which allows the simultaneous adjustment of hybrid impedance at the two installations. This involves that the two control circuits C should transmit recognisably different signals between which these circuits are capable of distinguishing. In this case each circuit C can then adjust its own hybrid to minimise imperfections in the signal received from the other circuit C. Alternatively, in a method which can be employed less generally each control circuit adjusts its hybrid to minimise the correlation between the signal which it transmits and the signal which it receives.

The security telephone system itself forms no part of the invention and hence is not described herein, and it is merely one example of the way in which hybrid circuits in accordance with the invention can be utilised. It is a particularly suitable application as the control circuit of one unit is able to co-operate with the remote connected unit while the balancing procedure is carried out, the invention requiring that the external impedance connected to the bidirectional port of the hybrid can be measured before power transmission through the hybrid in the optimum balanced condition commences.

Claims (12)

1. A hybrid junction circuit comprising an adjustable balancing network, means for automatically adjusting the impedance of the network at the start of every transmission with which the circuit is used, and a control circuit which before balanced operation commences controls said means to adjust the balance impedance to provide a reasonable match to the external impedance presented to the bi-directional port of the hybrid.

2. A hybrid circuit according to claim 1, wherein GO and RETURN ports thereof are coupled to a bidirectional port of the circuit through a transformer.

3. A hybrid circuit according to claim 2, wherein the balancing network is connected to a tapping of a primary winding of the transformer and in parallel with a fixed resistor and one side of said primary winding, the fixed resistor being connected across the RETURN port and in series with said one side of the primary winding.

4. A hybrid circuit according to any one of the preceding claims, wherein the balancing network comprises a switched variable resistor network.

5. A hybrid circuit according to claim 4, wherein the balancing network comprises said switched resistive network section in conjunction with a section having a number of complex impedance component networks, the resistive section and the complex impedance section being independently switched by said control circuit.

6. A hybrid circuit according to claim 5, wherein said complex impedance component networks are two in number, one being of a capacitive character and the other of an inductive character.

7. A hybrid circuit according to any one of claims 4 to 6, wherein switching of the balancing network employs electronic FET switches.

8. A hybrid circuit according to any one of the preceding claims, wherein the circuit is embodied in a security telephone installation for connection to a similar installatin'through the public telephone system.

9. A hybrid circuit according to claim 8, wherein said control circuit comprises a microprocessor programmed to effect the adjustment of the balancing network during a call set-up procedure, the control circuit sending a test signal down the connected telephone line whilst the incoming signal from said similar installation connected to the other end of the telephone line is silent, and whilst transmitting the test signal automatically adjusting the balancing network to minimise the reflected signal.

10. A hybrid circuit according to claim 8, wherein said control circuit comprises a microprocessor programmed to effect the adjustment of the balancing network during a call set-up procedure, the control circuit sending a test signal down the connected telephone line whilst receiving a recognisably different signal from said similar connected installation, and adjusting the balancing network to minimise imperfections in the received signal.

11. A hybrid circuit according to claim 8, wherein said control circuit comprises a microprocessor programmed to effect the adjustment of the balancing network during a call set-up procedure, the control circuit sending a test signal down the connected telephone line whilst receiving a recognisably different signal from said similar connected installation, and adjusting the balancing network to minimise the correlation between the transmitted and received signals.

12. A hybrid junction circuit substantially as herein particularly described with reference to Figures 2 and 3, or Figures 2 and 4, of the accompanying drawings.