Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Patent
1998-02-25
1999-02-16
Hunter, Daniel S.
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
378399, H04M 100
Patent
active
058728430
DESCRIPTION:
BRIEF SUMMARY
This invention relates to telecommunications apparatus and in particular to a circuit for isolating a telephone or facsimile set from the telephone exchange.
Where equipment such as a call router is connected in series between the telephone set and the exchange, it is desirable to isolate the telephone set from the tone dialling process carried out by audio frequency information. At the same time a D.C. feed path must be provided from the exchange to the telephone set. Existing systems provide complete A.C. and D.C. isolation which is disadvantageous as it requires an external power supply at the telephone set.
The invention aims to provide an improved isolator circuit for A.C. isolation but to retain the D.C. path.
FIG. 1 is a circuit diagram of a first prior art A.C. isolator circuit;
FIG. 2 is a frequency response graph of the circuit of FIG. 1;
FIG. 2A is a circuit diagram of a second prior art filter:
FIG. 3 is a circuit diagram of an isolating circuit embodying the invention; and
FIG. 4 is a frequency response graph of the circuit of FIG. 3.
One way of providing A.C. isolation is illustrated in FIG. 1 in which the exchange is represented by voltage source E.sub.source which also includes a D.C. feed, and resistance source, R.sub.source and the telephone set is represented by R.sub.load. An inductor L is arranged in series between the load and source resistances. Provided that a suitably high inductance is chosen, the inductor will effectively isolate the source from the load at the frequencies within which it is required to operate, typically between about 300 Hz and 4 kHz whilst maintaining the D.C. feed path.
The frequency response of the circuit of FIG. 1 is shown in FIG. 2. The curve, labelled VdB (out) is the response measured at point B in FIG. 1, that is at the load resistor. At 300 Hz the response is -30 dB, whilst at 1000 Hz it is -40 dB representing an isolation of 24 dB and 34 dB allowing for a 6 dB loss due to the impedance matching of the system. Because of the symmetry of the circuit the performance is identical in both directions.
Whilst this method operates satisfactorily, a large inductance of at least 20 H is retuired which must be able to carry a current of at least 0.1 A. Such an inductor is physically large and expensive and is not a practical solution.
A second prior art approach to providing a low-pass filter is described in GB 2,002,193 and is known as a T filter as shown in FIG. 2A. A T filter comprises two resistive branches 30, 32 and a capacitive branch 34. In such a T filter the resistive branches must have a high resistance of the order 10,000 ohms, and the capacitor must have a small value. Since the impedance of the capacitor is inversely proportional to the frecuency, as the frequency is raised, the impedance of the capacitor 34 drops allowing the signal to earth. This circuit thus blocks high frequencies, but allows DC to pass. However, for the circuit to operate, resistive paths 30, 32 must be of high impedance compared to the capacitance 34. This leads to an unacceptable voltage drop for DC.
We have appreciated that a low pass filter for telecoms use should present a low resistance to D.C., whilst maintaining good A.C. blocking characteristics. Accordingly, there is provided an alternating current (A.C.) isolator circuit for isolating a telecommunications apparatus from a source comprising a resistive path between the telecommunications apparatus and the source having first and second branches, said resistive path permitting passage of direct current; and a capacitive path arranged between earth and a point intermediate the first and second branches, the capacitive path having an impedance less than that of either of the first and second resistive branches over the operating frequency range of the apparatus; wherein the first and second resistive branches each comprises an amplifier circuit arranged in parallel with a resistive element whereby the effective resistance of each resistive branch is dependent on the gain of the amplifier circuit, to present an apparent
REFERENCES:
patent: 4417099 (1983-11-01), Pierce
patent: 4578533 (1986-03-01), Pierce
patent: 5491402 (1996-02-01), Small
Appiah Charles N.
Betacom Consumer Electronics, Ltd.
Hunter Daniel S.
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