GB2223379A

GB2223379A - Bell capacitance measuring circuit

Info

Publication number: GB2223379A
Application number: GB8921652A
Authority: GB
Inventors: Alexander J Binnie
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1988-09-28
Filing date: 1989-09-26
Publication date: 1990-04-04
Anticipated expiration: 2009-09-26
Also published as: GB2223379B; GB8822755D0; GB8921652D0

Abstract

Remote measurement of the bell capacitance in the line conductors at a telephone receiver is performed using a known capacitance 7 which is charged to a predetermined voltage. The connections of the charged capacitor are then switched to the line conductors 5 of the receiver to partially discharge the capacitor through the line conductors. High impedance monitoring means 8 are connected to the capacitance to produce, from the resulting voltage drop, a signal that is a measure of the capacitance in the receiver circuit. <IMAGE>

Description

BELL CAPACITANCE MEASURING CIRCUIT This invention relates to a circuit for measuring the bell capacitance of a telephone receiver.

In the rest state of a telephone handset, a bell or ringer is connected across the pair of conductors that form the telephone line and that connect the handset to the local exchange. In order to determine the presence of the handset at the end of a local line without entering the subscriber's premises, a technique has been developed in the telecommunications industry to detect the presence of the bell or ringer by sensing its capacitance, which is generally about 2 micro-farads through the conductors.

The technique involves the provision of a reversing switch at the input of an analog multi-meter designed specifically for the telecommunications industry.

By connecting the resistance range and rapidly depressing the switch, an indication can be obtained on the instrument meter if a capacitance of the right order of magnitude exists across the instrument input terminals. A linesman can, therefore, detect the presence of the bell capacitance and hence infer the presence of a telephone handset by performing this technique on a local telephone line. The reading obtained on the meter is dependent on the rate at which the reversing switch is actuated and is also affected by the dynamics of the analog indicator, which is usually a moving coil meter. The usefulness of this technique, therefore, is very dependent on the skill and experience of the operator.

With the introduction of receivers with electronic ringers, the technique has proved to be no longer reliable, as electronic ringers can present a resistive impedance of up to 100,000 ohms, which is much larger than the impedance of an electro-mechanical ringer.

Moreover, they generally have a diode bridge at their input which requires an AC signal of about 1 volt RMS before they will start to conduct. When the known technique is attempted with an electronic ringer, therefore, either the diode bridge will block the measuring signal, or the high impedance of the ringer will significantly affect the reading.

According to one aspect of the present invention, there is provided a method of remotely measuring the bell capacitance in the line conductors at a telephone receiver, by charging a capacitor with a known charge and then connecting the capacitor by a mechanical switch to a conductor of the line to the receiver through a measuring terminal of a voltage metering instrument, the resulting discharge of said capacitor being monitored by a high impedance device to produce an indicative signal value in the presence of a bell capacitance across the telephone line conductors.

In one form of the invention, the high impedance amplifier monitors the voltage across the capacitor and it is the final voltage which provides the meter reading of the bell capacitance. In another form of the invention, the change in voltage across the capacitor is monitored by the amplifier to provide the meter reading.

By way of example, Figs 1-3 of the accompanying drawings illustrate as schematic circuits three alternative forms of apparatus operating in accordance with the invention.

The apparatus shown in Fig. 1 has a DC power source 1, e.g. a 9 volt battery, connected to a DC/DC converter 2 which generates a stable high DC voltage, e.g.

about 100 volts. The power source 1 also provides power for operating the other electronic circuits of the apparatus. A two pole changeover switch 3 is biased to a position connecting the convertor to a capacitor 7 and the convertor, therefore, charges the capacitor to its high output voltage. It will be understood that at the end of this process the capacitor 7 carries a known charge.

When the switch 7 is changed over, the charged capacitor is connected across measuring terminals 4A, 4B of the apparatus and hence across the local telephone line 5 which is being tested to see whether there is a ringing circuit 6 connected to it. As the switch 3 is held in its new position, the voltage across the capacitor 7 will then decrease to a steady value in a specific relation to the relative values of the capacitor itself and the capacitance across the terminals 4A, 4B. A resistor 11 provides a discharge path from the bell capacitance when the switch 3 is returned to its illustrated biased state.

If the value of the capacitor 7 is Cs, the capacitance in the ringing circuit is Cx and if the effects of the local line and other components in the ringing circuit are assumed to be negligible, then for a voltage V across the capacitor 7 before switching, the voltage after changing over the switch 3 will be: CsV Cs+Cx The voltage across the capacitor 7, thus provides a measure of the value of the capacitance in the ringing circuit and to infer whether a bell is connected to the line. The voltage across the capacitor 7 is monitored by a high input impedance amplifier 8, the output from which is routed to a meter drive circuit 9 that produces the reading on analog meter 10. With this arrangement there is a zero scale indication when the bell capacitance is infinite.By selection of the components of the meter drive circuit a full scale indication can be obtained when the bell capacitance is zero, and a mid-scale indication when the bell capacitance is equal to the capacitor 7.

The alternative apparatus illustrated in Fig 2 is similar in many respects to the apparatus of Fig. 1, and parts already described are indicated by the same reference numbers. With the switch 3 in its biased state, both amplifier terminals are connected to the 0 volt line. When the switch changes over, the capacitor 7 and the bell capacitance 6 are connected in parallel to the high voltage terminal of the amplifier. Accordingly, the circuitry detects changes in the voltage across the capacitor 7 rather than the voltage value itself.

With the apparatus of Fig. 2, the voltage measured after the switch 3 is actuated is: CxV Cs+Cx In this case, when the bell capacitance is zero a zero scale reading is obtained. By selection of the components the apparatus can give a full scale indication when the bell circuit capacitance is infinite, and mid-scale indication when the bell circuit capacitance has the same value as the capacitor 7.

In both forms of apparatus described, the diode bridge of a present day electronic ringer has a negligible effect on accuracy since a relatively high measurement voltage, such as the 100 volts mentioned above, is large compared with the voltage required to make the diodes conduct. In addition, a static DC measurement is performed so that series resistance and inductance in the circuit will have little effect on the final reading. If the series resistance is very large, greater than lOOKohms for example, it will slow down the response time, but it will not affect the accuracy of the final reading. The local line will introduce an error due to line capacitance, which is some 50NF/km. However, local lines are usually less than 2.5km long and the error from this source will be small compared with a typical bell capacitance of about 2000NF.

Another feature of the invention is that it can be adapted to provide a reading on a conventional digital voltmeter. An example is illustrated in Fig. 3 which is similar in many respects to the apparatus described so that parts in common with the earlier example are indicated by the same reference numbers and only the operation of the additional parts will be described. For a voltage V across the capacitor 7 before switching to the state shown, then after switching the reference voltage Vref will be: CsV Cs+Cx and the voltage V. will be: in CxV Cs+Cx These voltages are connected to the input terminals and reference terminals of a digital meter and the reading presented on this meter, assuming it has a digital resolution of 3, will be:

Therefore, by making Cs equal to luF, the apparatus can read values of bell capacitance from 0.0001 to 1.999 uF with a resolution of 0.001of. Alternatively, if Cs is equal to lOF, values of bell capacitance from 0.01 to 10.99uF can be read with a resolution of 0.0luF.

Claims

1. A method of remotely measuring the bell capacitance in the line conductors at a telephone receiver, by charging a capacitor with a known charge and then connecting the capacitor by a mechanical switch to a conductor of the line to the receiver through a measuring terminal of a voltage metering instrument, the resulting discharge of said capacitor being monitored by a high impedance device to produce an indicative signal value in the presence of a bell capacitance across the telephone line conductors.

2. A method according to claim 1 wherein a high impedance amplifier obtains the signal value from the voltage across the capacitor when the charged capacitor is connected to said conductor.

3. A method according to claim 1 wherein a high impedance amplifier obtains the signal value from the change of voltage across the capacitor when the charged capacitor is connected to said conductor.

4. A method according to claim 1 wherein a high impedance amplifier obtains the signal value from the voltage across the capacitor when the charged capacitor is connected to said conductor, the capacitance measurement being presented on a digital volt meter.

5. Apparatus for remotely measuring the bell capacitance in the line conductors of a telephone receiver comprising a capacitor and means for charging said capacitor with a known charge and for connecting the charged capacitor to a line conductor to partially discharge it therethrough into the line capacitance, monitoring means for producing a signal value from said partial discharge to indicate the presence of the bell capacitance.

6. Apparatus according to claim 5 wherein said monitoring means comprises a high impedance amplifier.

7. Apparatus according to claim 6 wherein said amplifier is arranged to receive an input proportional to the capacitor voltage after said partial discharge.

8. Apparatus according to claim 6 wherein said amplifier is arranged to receive an input proportional to the charge of the capacitor voltage resulting from said discharge.

9. Apparatus according to claim 5 or claim 6 comprising a digital volt meter for displaying the bell capacitance value.

10. A method of remotely measuring the bell capacitance in the line conductors of a telephone receiver substantially as described herein.

11. Apparatus for remotely measuring the bell capacitance in the line conductors of a telephone receiver constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.