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

A circuit for synthesizing an impedance associated with a telephone subscriber's circuit connected to a two-wire telephone line is described. The circuit of the invention is adapted to synthesize a complex impedance which can function both as a termination impedance and a balance impedance. The termination impedance utilizes a positive feedback loop structure having a loop gain which is at all times less than unity. The circuit that implements both the termination and balance impedances with sidetone suppression is also described. Each of the embodiments is realizable with a single external component consisting of a resistor.

DESCRIPTION 
1. Field of the Invention 
This invention relates to a circuit for synthesizing an impedance 
associated with a telephone subscriber's circuit connected to a two-wire 
telephone line. 
2. Background of the Invention 
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 to 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, December 1982, continued efforts have been 
made to reduce the number of components external to the telephone circuit 
but incorporated in the telephone subscriber's handset. It can be 
appreciated that this would both make for lower costs and improved 
reliability. To that end, 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. 7, July 
1991, 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 resistor. This resistor 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, additional external components are used with the 
aforesaid resistor, namely: two coupling capacitors, two transistors, one 
diode, and a second resistor. 
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. 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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF 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 resistor 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-. Filter 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 
preferably 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, while involving no external calibration 
steps. In addition, such filters afford an adequate dynamic range for the 
power supply value (about 3 Volts) available to the telephone circuit 2. 
The structure of circuit 1 further includes a filter/amplifier 5, connected 
after the amplifier 7 and having an output 4 connected to the other end of 
the precision resistor R. This output 4 is characterized by a very low 
impedance. Said filter/amplifier 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 resistor 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-. This eliminates the common difficulty of 
stabilization of such feedback loops. In the prior art, the stabilization 
of such a loop is made especially 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. 
All circuit components of this invention can be integrated onto a 
semiconductor chip in one embodiment, if desired; however, in an 
alternatively and presently preferred embodiment, all components are 
integrated onto a single monolithic chip except the precision resistor R. 
Integrated circuits acceptable for use as the capacitive coupler filter 
12, the zero pole filter 8, amplifier 7 and unit gain amplifier 5, 
respectively, are well known in the art as individual circuits, but have 
not previously been assembled in this combination. Many different circuit 
configurations for the individual components are presently known; any 
acceptable individual circuit may be used for the respective components 
within the claimed invention. 
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: 
EQU Vr=V * (1-H(s)) 
where, H(s) is the transfer function from the input 11 of filter 12 to the 
output 4 of filter/amplifier 5. 
Accordingly, denoting by I=Vr/R the current draw of the resistor R, it can 
be shown from the above relation that: 
EQU 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 
termination impedance Z1 of the circuit across the line 3 will be 
EQU 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 resistor R. This complex 
impedance does meet the applicable standards from the administrations of 
telephone companies. In addition, the termination 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. The advantage over the prior art 
remains that all circuit elements may be implemented from known circuits 
on an integrated semiconductor chip; usually the precision resistor R is 
not on the chip, however. 
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 FIG. 2, a further embodiment of the 
inventive circuit will be explained wherein individual and cooperating 
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 requirements of the received echo. 
FIG. 2 shows diagrammatically 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 second 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 which performs a subtraction function receives, at 
one end, a signal TX which is also input to the second filter 15. The 
receive signal picked up after the filter 12 is also input to the second 
circuit node 14. TX is the signal to be transmitted following appropriate 
amplification. 
When, and only when, the nominal impedance is coincident with the balance 
impedance, 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. Circuits that can 
perform the individual functions of adder node 9, subtraction node 14, 
filter 15, and amplifier 13, respectively, are, again, known generally in 
the prior art. The result of this subtraction is represented by a signal 
RX shown in FIG. 2. 
As with the embodiment of FIG. 1, 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 resistor R. All 
other components are formed from circuits on an integrated semiconductor 
circuit, preferably monolithic but alternatively on different chips. 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 also be noted that the filter 12, in addition 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 resistor 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.