Circuit for the implementation of an impedance associated to a telephone speech circuit

Abstract: A circuit (1) for synthesizing an impedance associated with a telephone subscriber's circuit (2) connected to a two-wire telephone line (3) is a positive feedback configuration comprising: a single precision resistance (R) connected serially to the line (3); at least one low-pass filter; and an amplifier (7) between the filter (8) and the resistance (R). This circuit (1) allows both the termination impedance and the balance impedance to be synthesized through a single external precision component.

Description: This invention relates to a circuit for synthesizing an impedance associated with a telephone subscriber's circuit connected to a two-wire telephone line. The field of application of this invention relates, in particular but not solely, to telephone speech circuits incorporated to telephone subscriber's appliances, and the description which follows will make reference to said field of application for simplicity of illustration. There exists a demand in that specific field of application for good accuracy in the provision of the input impedance value, also called termination impedance, of the telephone circuit. In fact, each telephone company sets a maximum value of so-called return loss on the telephone band, and this value imposes a predetermined maximum admissible value on the termination impedance. It should be also noted that a telephone speech circuit provides for conversion from two to four wires, and that the accuracy of this conversion is tied, in turn, to the accuracy in value of another impedance, called balance impedance, which may be different from the termination impedance. The termination and balance impedances are currently provided by a complicated structure, external of the integrated telephone circuit, which employs, moreover, comparatively expensive precision components. In addition, the integrated circuit requires additional pins for making the various connections involved. Since the inception of the first monolithically integrated, telephone speech circuit, dating perhaps back to an article "A Programmable Speech Circuit Suitable for Telephone Transducers", IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 6, Dec. '82, continued efforts have been made to reduce the number of components external of the telephone circuit but incorporated to the telephone subscriber's appliance. It can be appreciated that this would both make for lower costs and improved reliability. To that problem, the prior art has already proposed a solution described, for instance, in an article "A software programmable CMOS telephone circuit", IEEE Journal of Solid-State Circuits, Vol. No. 7, July '91, wherein reference is made to a telephone circuit provided with no less than six external components. This prior art approach provides for the termination and balance impedances to be implemented basically by a precision external resistance. This resistance is transformed into a complex impedance by means of a control circuit, internal of the integrated speech circuit, which is operated in accordance with an appropriate transfer function. However, more external components are used additionally to the aforesaid resistance, namely: two coupling capacitors, two transistors, one diode, and a second resistance. It has been found, moreover, that the stability of the control loop is dependent on the impedance value of the telephone line, and that the circuit may become unstable at high values of this impedance. Other Examples of circuits for synthesizing an impedance associated with a telephone subscriber's circuit connected to a telephone line having a pair of terminals known from the prior art are given by US-A-4 558 185 and Patent Abstracts of Japan, vol. 14, no. 387 (E-967), 21.8.90 & JP-A-21 42 227. Another disadvantage of the prior approach is that for the impedance synthesis, integrated filters are required which must be calibrated individually to compensate for any variations originating from the production process. The underlying technical problem of this invention is to provide an impedance synthesizing circuit which has such structural and functional characteristics as to overcome the above-mentioned disadvantages of the prior art. This problem is solved by a circuit as indicated above and defined in the characterizing portion of the appended claims. The features and advantages of a circuit according to the invention will become apparent from the following detailed description of an embodiment thereof, to be taken by way of non-limitative example in conjunction with the accompanying drawings. In those drawings: Figure 1 is a diagramatic representation of a circuit for synthesizing a termination impedance, according to the invention; and Figure 2 is a diagramatic representation of a second circuit for synthesizing both a termination and a balance impedance, according to the invention. With reference to the drawing figures, generally and schematically shown at 1 is a circuit embodying this invention for synthesizing an impedance associated with a telephone speech circuit 2. More particularly, the circuit of this invention is adapted to synthesize a complex impedance which can function both as a termination impedance and a balance impedance. The telephone circuit 2 is a monolithically integrated type connected to a two-wire telephone subscriber's line having a pair of terminals L+ and L-. It will be assumed later in this description that the first of said terminals, designated L+, is the telephone signal receiving terminal, and the other terminal, L-, represents a signal reference. Conveniently, the circuit 1 of this invention is a positive feedback loop structure, at the first terminal L+ of the line 3, and comprises a single precision resistance R connected serially to the line and having one end connected to said first terminal L+. The circuit 1 further comprises a first filter 12 having a first input 11 which is connected directly to the line terminal L+ and receives the telephone signal present on the line 3. This filter 12 is essentially a capacitive coupler, and has a second input 10 connected to the other line terminal L-. Component 12 comprises an internal antialias filter effective to remove high-frequency signals and avoid intermodulation noise of the signals being input to the circuit 1. A second, zero-pole filter 8 is provided after the filter 12 which is connected centrally in the circuit 1 and has an output connected to the input of an amplifier 7 whose gain can be adjusted to suit the subscriber's demands. In the preferred embodiment being discussed, the gain g of amplifier 7 has been selected to be 1.5. The second filter 8 handles a sampled signal and is of a type known commercially as a switched capacitor. This filter type affords the required degree of accuracy for the impedance value, whilst involving no external calibration steps. In addition, that filter 8 affords best dynamic range for the power supply value (about 3 Volts) available to the telephone circuit 2. The structure of circuit 1 further includes a block 5, connected after the amplifier 7 and having an output 4 connected to the other end of the precision resistance R. This output 4 is characterized by a very low impedance. Said block 5 is essentially a unit gain amplifier of the type referred to as driver/smoothing, that is functioning as a low-pass filter as well as a driver for the resistance R. An input 6 of the filter/amplifier 5 is connected to the other line terminal L- to have the same signal reference as the first filter 12. Advantageously, the feedback loop formed by the circuit 1 exhibits high stability resulting from the selection of a positive feedback associated with a loop gain which is at all times less than unity. This makes for a stable circuit irrespective of the value of the line impedance across the terminals L+ and L-. Usually, in fact, stabilization of such a loop is made specially difficult by the presence of a highly variable line impedance. The circuit of this invention obviates this problem by having the line voltage-driven, that is at a low impedance. The operation of the inventive circuit will now be described. When a voltage V is applied to the terminal L+ of the line 3, the voltage Vr which appears across the resistance R is given by the following relation: Vr = V (1 - H(s)) where, H(s) is the transfer function from the input 11 of filter 12 to the output 4 of block 5. Accordingly, denoting by I=Vr/R the current draw of the resistance R, it can be shown from the above relation that: I = Vr/R = V[(1 - H(s)]/R Now, since the input 11 of circuit 1 has a high impedance, the current I can be taken with good approximation to be the overall current draw of the speech circuit 2 in response to a voltage signal V. It follows that the impedance Z1 of the circuit across the line 3 will be Z1 = V/I = R[1 - H(s)]. If the transfer function H(s) varies with the frequency of the telephone band, then it can be inferred that the resultant impedance across the terminals L+ and L- is a complex impedance, even if it has been generated by a single discrete component consisting of the resistance R. This complex impedance does meet the applicable standards from the administrations of telephone companies. In addition, the impedance value may be changed as required by acting on the form of the transfer function of filter 8 and/or the gain of amplifier 7. Where needed, the circuit 1 may be provided with an external read-only memory, e.g. of the EPROM type, wherein various transfer function forms of the filter 8 can be stored. Likewise, the filter 8 could be "programmed" through a serial bus under control by a microprocessor. Now, with reference to the example in Figure 2, a further embodiment of the inventive circuit will be explained wherein individual and co-operating parts which are similar in construction and operation to the previous embodiment parts are denoted by the same reference numerals. In this second embodiment, conversion from two to four wires is provided to suit specific requests on the received echo. Figure 2 shows diagramatically the circuitry that implements both the termination and the two-to-four wire conversion with balance and sidetone suppression. A summing node 9 is connected between the amplifier 7 and the filter/amplifier 5 to receive both the signal from the amplifier 7 and a second, outgoing signal from another amplifier 13, in this case a unit gain one. The last-named amplifier, 13, is connected after a second filter 15 of the switched capacitor type. The filter 15 and amplifier 13 are designed to supply a precise gain and a flat frequency response within the telephone band of interest and in the presence of a nominal line impedance across the terminals L+ and L-. A second circuit node 14 with subtractor function receives, at one end, a signal TX which is also input to the filter 15, and at the other end, the receive signal picked up after the antialias filter 12; TX being the signal to be transmitted following appropriate amplification. When, and only when, the nominal impedance is coincident with the balance impedance, the sidetone suppression can be effected quite simply by subtracting the receive signal from signal TX at the node 14. The receive signal might require scaling through an amplifier. The result of this subtraction is represented by a signal RX shown in Figure 2. Once again, the required termination and balance impedances for the two-to-four wire conversion are implemented by a single external precision component, still consisting of the resistance R. It is only where, during the transmission step, the balance impedance differs from the line impedance rating that an additional switched capacitor type of filter ought to be used between the filter 12 output and the subtract node 14 input. It should be just noted that the antialias filter 12, additionally to filtering out any noise from the line exceeding 80 kHz, will uncouple the impedance control loop at frequencies below 5 kHz. This uncoupling is necessary to hold the DC duty point fixed of the blocks which make up the control loop, regardless of the line length which defines said DC duty point. The circuit of this invention does solve, in its various embodiments, the technical problem and affords a number of advantages, outstanding among which is the use of a single external component of a discrete kind, i.e. the resistance R. Not to be neglected are then the advantages arising from stability, ensured irrespective of the line load value, and precision in synthesizing an impedance with no need for calibration. Finally, the high dynamic range afforded by the circuit adds to the beneficial features of the solution provided by this invention.

Claims: A circuit for synthesizing an impedance associated with a telephone subscriber's circuit connected to a telephone line, having a pair of terminals, comprising, within a positive feedback loop: a single precision resistance (R) connected serially to a terminal (L+) of the line (3); at least a switched capacitor sampling filter (8) having an input connected to said terminal (L+) of the line and receiving a telephone signal; and an amplifier (7) connected between said filter (8) and said resistance (R); characterized in that of comprising: a pair of additional filters (12, 5), a first (12) of the additional filters being connected in the feedback loop to receive a signal from said terminal (L+) and provide an output signal for said sampling filter (8), a second (5) of the additional filters being a low-pass filter receiving a signal from the switched capacitor filter (8) and being connected between said amplifier (7) and said resistance (R) with an output (4) connected to one end of said resistance (R); a second output (6) of the second (5) of the additional filters being connected in feedback relationship to an input (10) of the first filter (12) through a second terminal (L-) of said line (3) . A circuit according to Claim 1, characterized in that said amplifier has a gain (g) of 1.5. A circuit according to Claim 1, characterized in that it includes a summing node (9) connected after said amplifier (7) and receiving a signal from a second amplifier (13) cascade-connected to a second low-pass filter (15), said second filter (15) being input a signal (TX) for transmission. A circuit according to Claim 3, characterized in that it includes a subtracting node (14) receiving, on the one side, said signal (TX) for transmission, and on the other side, a signal picked up from the input of said zero-pole first filter (8) to suppress any presence of echo. A circuit for synthesizing a termination and balance impedance associated with a telephone subscriber's circuit connected to a telephone line having a pair of terminals, comprising, within a positive feedback loop: a single precision resistance (R) connected serially to one terminal (L+) of the line (3); at least a switched capacitor sampling filter (8) having an input connected to said terminal (L+) and receiving a telephone signal; an amplifier (7) connected between said filter (8) and said resistance (R); characterized in that of comprising: a second amplifier (13) serially-connected to a second low-pass filter (15); and a summing node (9) connected to the respective outputs of said amplifiers (7,13) and to one end of said resistance (R); a sidetone suppression, subtracting node (14) connected, on the one side, to the input of said second filter (15) receiving a signal (TX) for transmission, and on the other side, to the input of said first filter (8).

Inventors: CASTELLO RINALDO, ERRATICO PIETRO, TOMASINI LUCIANO, CASTELLO, RINALDO, ERRATICO, PIETRO, TOMASINI, LUCIANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H04M1
IPC codes: H04M1, H04B1