GB2159021A - Voice frequency repeater - Google Patents

Abstract

A voice frequency repeater for a telephone circuit between two distant stations for amplifying voice signals in both directions, which comprises amplifiers for the voice signals arranged to prevent oscillation of the loop formed by amplifiers the amplifiers being arranged, in operation, to be indistinguishable from hybrid circuits and including operational amplifiers. For each direction, a hybrid circuit 3, 4 couples a balanced input through the repeater and also to the output of the opposite direction amplifier as a common mode signal so as to effect cancellation and to prevent oscillation. <IMAGE>

Description

SPECIFICATION Voice frequency repeater The present invention refers to a voice frequency repeater as used in telephone circuits to amplify voice signals in telephone lines.

Telephone system trunk lines between exchanges separated by appreciable distances are usually equipped with speech amplifiers or repeaters which compensate for speech energy losses caused by line attenuation. Such speech amplifiers or repeaters must not only amplify voice communication signals in both directions within the frequency band 300-3400Hz, but must permit the passage of the loop current, the call signal, and the dialling signal, the call signal being conventionally a 25Hz 80V sine wave and the dialling signal conventionally a rectangular form signal of 48 volts peak to peak at a period of 100 milliseconds.Further, the speech amplifiers or repeaters, referred to, are required to tolerate reversal of the telephone line connections and to improve the characteristics of the telephone line, as regards distortion of the voice signal, reflection of the signal, stationary waves in the line, and noise, with energy consumption not exceeding the limits permitted in normal systems (100mA).

The requirement for simultaneous bidirectional amplification gives rise to serious problems of stability, which, if not dealt with, will result in the provision of an oscillation system rather than an amplification system.

Another difficulty arises in matching the speech amplifier or repeater to the telephone line which behaves as a transmission line on which (i) the signal is attenuated increasingly as its frequency increases, (ii) phase distortion of the signal arises because the propagation velocity of the signal varies as a function of the frequency and (iii) standing waves are created by reflection of the signal caused by impedance mismatch. A voice frequency repeater inserted in the line must take account of the characteristics of the line.

Various design philosophies have been developed in the prior art for voice frequency repeaters, the more familiar among these being voice frequency repeaters with balanced differential transformers sometimes referred to as hybrid coils, and negative impedance voice frequency repeaters.

In the voice frequency repeater with balanced differential transformers, two undirectional amplifiers are used, each with a transformer of four windings, which has the property of allowing signals to pass in only one direction.

It is necessary that the windings of each transformer be arranged in pairs and have impedances equal to that of the line.

A difficulty in design lies in the fact that the voice signal is a broad band signal (the highest frequency is 11 times that of the lowest), requiring that the transformers maintain their performance over a broad band of frequencies.

A further difficulty in the design of the transformers arises because, as mentioned above, a telephone line is a transmission line and its impedance depends on the frequency. Thus, the impedance of the transformers must change with frequency in a manner corresponding to the varying impedance of the line. Not surprisingly, the solution is usually a compromise, and an approximate impedance is selected for use over the entire frequency range (300 to 3400Hz).

The choice of impedance and the desired degree of balance in the transformers are subject also to the constraints of production on a large scale.

In the negative impedance voice frequency repeater, a circuit which behaves as a negative resistance is inserted in the telephone line to cancel out the line losses.

The negative resistance voice frequency repeater requires the presence of a tansformer in a two-wire telephone iine, the primary winding of the transformer being in series with the line, and the secondary winding including a resistance in series with an active circuit, there being reversal of the polarity of the line current. Thus the line resistance is refiected with reversal at the primary winding of the transformer, and an apparent negative resistance is introduced into the line.

To preserve the input and output impedances, another circuit of two transformers, an active circuit and a resistor must be introduced inserted in parallel with the line.

Although, in theory, a negative resistance VFR is reasonably efficient, the construction of the transformers is not straight forward. For example, the transformers are required to accommodate the telephone loop current (direct current feeding the telephones) in the primary windings.

The presence of various transformers of differing critical characteristics degrades the performance of the circuit, and, in large scale production,the problems are the same or greater than those encountered for voice frequency repeaters with balanced differential transformers.

In accordance with the present invention, a voice frequency repeater, for use in a telephone circuit, includes amplifier means arranged in a loop to amplify voice signals in both directions without oscillation, characterised by the fact that the amplifier means consists of a network of operational amplifiers.

A voice frequency repeater in accordance with the present invention will now be described, by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a schematic representation of a prior art circuit for a voice frequency repeater (VFR) using differential transformers, Figure 2 is a simplified block diagram of a VFR in accordance with the present invention, Figure 3 is a representation of an operational amplifier such as is used in the present invention, Figure 4 is a graphic representation of the differential mode and ordinary mode signals at the inputs of each operational amplifier such as is shown in Figure 3, and, Figure 5 shows the arrangement of a VFR in accordance with the invention, where the negative inputs of the amplifiers (inverting inputs of the operational amplifier) are used as the inputs ports from the telephone line.

Figure 1 represents the prior art, wherein transformers having fourwindings are used to connect two amplifiers into a line between two telephones to provide bi-directional amplification without forming an oscillatory loop.

Figure 2 shows a block diagram of a VFR in accordance with the invention, each of the blocks 3 and 4 being made up of a network of operational amplifiers connected as shown in Figure 5, described below.

Figure 3 illustrates the form of the operational amplifier forming the building blocks of the VFR in accordance with the present invention. As is known in the art, making R1 equal to R2 and R3 equal to R4, the operational amplifier circuit has the following characteristics: a) amplification of a differential mode signal as represented by S1 of Figure 4, and, b) rejection to a high degree (as a function of the quality of the operational amplifier and the accuracy of the resistances) of a common mode signal as represented by S2 of Figure 4.

In the operation of the arrangement represented by Figure 2, the signal S1 is to be allowed to travel from the position B through the block 4, the amplifier A2, and the block 3, to the position A, but not allowed to return from the position A two the position B in a manner which would modify the signal level S1 at the position B. The situation is the same for the signal S2 which is going from the position Ato the position B.

In the block diagram of Figure 2, the signal S1 appears in the input leads of the circuit block 4 in the differential mode and passes freely to the line side A, but appears in common mode in the input leads of the circuit block 3, and does not pass through the circuit block 3. The signal S2 appears in the input leads of the circuit block 3 in the differential mode and passes freely to the line side B, but appears in common mode on the input leads of the circuit block 4 and does not pass through the circuit block 4.

The passage of simultaneous bidirectional signals is therefore achieved without forming a loop in the VFR for the signals S1 and S2.

The circuit blocks 3 and 4 are identical and include only differential amplifiers interconnected to achieve bi-directional signal transmission as described above, avoiding the need for the large and relatively heavy inductors or transformers that are required in the audio frequency range.

Referring now to Figure 5, there is shown a VFR circuit in accordance with the invention, using inverting inputs of the amplifiers, marked conventionally by a negative sign. It should be understood that the non-inverting inputs may be used without altering the operation or principles of the present invention.

In the circuit of Figure 5, the amplifiers Z1A, Z2A and Z5A on one side of the line and Z1 B, Z2B and Z5B on the other side form the circuit blocks 3 and 4, respectively, shown in Figure 2. A telephone line is a balanced conductor and the balance is preserved by the three operational amplifiers connected on each side to the line.

This configuration of three operational amplifiers accepts balanced signals with a functional performance markedly superior to the simple circuit illustrated in Figure 3. In particular it shows good performance in relation to common mode signal rejection.

The two amplifiers Z1A and Z2A are identical, as they form part of the same integrated circuit. Z1B and Z2B similarly belong to an integrated circuit.

The values of the resistors are such that the circuit is symmetrical. The required relationships between the values of the resistors are set out below: R1A=R2A=R10A=R1 lA R3A = R4A= R9A= R12A R5A=R6A=R13A=R14A The above equalities should preferably be maintained within a tolerance of +1%.

The telephonic signal coming from side "A" of the line passes through resistors R1A and R11A and appears at the inverting inputs Z1A and Z2A in the differential mode. The resistors R1A and R1 1A have the same high value (much greater than the impedance of the line) and do not influence the input impedance. This impedance is defined by the resistors R6A and R13A, whose sum is equal to Zo which is the line impedance. The pairing of the impedances between the line and the VFR maintains the symmetry of the system.

The signal is amplified by operational amplifiers Z1A and Z2A. Since R3A and R12A are equal, and R4A and R9A are equal, and the amplification factors are R3A/R1A and R12AIR1 1A, symmetry is maintained.

The amplified signal appears in the differential mode at the inputs of Z5A and is amplified by the factor R18A R17A R18A being equal to R20A. The signal leaves the amplifier Z5A on a signal line and is again divided into a differential signal by the amplifier block Z6A which incorporates various circuits necessary for the efficient performance of the equipment, the circuits being variable gain amplifiers, low and high pass filters, phase equalizers and stabilizing circuits.

All these circuit elements are functionally the same as those used in conventional circuits and, in the VFR, are constructed with operational amplifiers, resistors and capacitors.

The output of the amplifier block Z6A is a balanced signal which is passed to two operational amplifiers Z3B and Z4B. To preserve the symmetry of the signal, R8B and R16B are equal, and R7B and R15B are equal.

The output signals of Z3B and Z4B are conducted to the line side "B" by the resistors R6B and R13B respectively, which represent an output impedance for these signals combined. R6B is equal to R13B (Symmetry Effect) and R6B + R13B = Zo. Thus the output impedance is equal to the line impedance.

The output signal from Z3B is applied simultaneously via the resistors R5B and R6B (R5B being equal to R6B) to the operational amplifier Zi B (via resistors R1 B and R2B). This signal therefore reaches Zi B in the common mode, and is not transmitted back by Z1 B. The impedance Zo which is equal to the nominal line impedance, makes the output signal from Z3B appear in phase with equal amplitude in the leads of Z1 B, preventing its return.

The output signal from Z4B is applied simultaneously via the resistors R13B and R14B (equal in value to the operational amplifier Z2B (via resistors R1 0B and R1 1 B). This signal reaches Z2B in common mode, and is not transmitted back by Z2B.

The impedance Zo makes the phase and amplitude of this signal the same in the leads of Z2B, preventing its return.

The signal, coming from side "B" of,the line appears in leads of Ri B and R2B in the differential mode, being amplified and conducted by Z1 B, Z2B and Z5B to the VFR in a way identical to that described above for side "A".

The amplifier circuit Z6B is identical to Z5A.

It will be appreciated from the above description that the voice frequency repeater includes a network of differential directly coupled amplifiers having characteristics determined by associated resistor networks, in which the amplifiers form respective first and second balanced paths, having opposite directions, which accept differential mode signals and reject common mode signals, the input to the first balanced path being connected to the output of the second balanced path and vice-versa, the network arrangement being such that a differential mode signal arriving at the input to the first balanced path is maintained as a differential mode signal along the first balanced path but reaches the input to the second balanced path as a common mode signal, and vice-versa.

The substitution of the hybrid coil of the prior art by an active element (operational amplifier) is a completely new technique for the elimination of oscillations in a bidirectional telephone circuit, and provides voice repeaters having several advantages over VFRs of the prior art. These advantages include: 1. Reduction in the weight and size of the equipment: 2. Reduction in cost; 3. Simpler fabrication of the equipment; and 4. The avoidance of critical components, there being significant uniformity in operational amplifier integrated circuits.

The VFR in accordance with the present invention is indistinguishable in operation from VFRs of the prior art employing transformers having 4 windings or hybrid coils 1 (see Figure 1) which facilitates the replacement of prior art devices by those of the present invention with minimal disturbance to the system.

Claims (5)

1. A voice frequency repeater for a telephone circuit between two distant stations for amplifying the voice signal in both directions, which comprises amplifier means arranged to amplify voice signals in both directions without the formation of an oscillatory loop, wherein the amplifier means consists of a network of operational amplifiers.

2. A voice frequency repeater in accordance with claim 1,wherein each amplifier means includes a group of three of the said operational amplifiers arranged as a differential amplifying means.

3. A voice frequency repeater in accordance with claim 2, wherein at least two of the said operational amplifiers belong to a single integrated circuit.

4. A voice frequency repeater as claimed in claim 1, including a network of differential directly coupled amplifiers having characteristics determined by associated resistor networks, in which the amplifiers form respective first and second balanced paths, having opposite directions, which accept differential mode signals and reject common mode signals, the input of the first balanced path being connected to the output of the second balanced path, and vice-versa, the network arrangement being such that a differential mode signal arriving at the input to the first balanced path is maintained as a differential mode signal along the first balanced path but reaches the input to the second balanced path as a common mode signal, and vice-versa.

5. A voice frequency repeater substantially as herein described with reference to, and as illustrated by Figure 5 of the accompanying drawings.