GB2146201A

GB2146201A - Loop detectors for telephone line circuits

Info

Publication number: GB2146201A
Application number: GB08421481A
Authority: GB
Inventors: Peter David Hargreaves; Neil Carrol
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1983-09-06
Filing date: 1984-08-24
Publication date: 1985-04-11
Also published as: GB2146201B; GB8323875D0; GB8421481D0

Abstract

To provide loop detection without affecting line voltage an oscillator 10 is connected in parallel with a telephone instrument 1 on a telephone line 2. A 30 volt zener diode 8 in series with the oscillator causes the oscillator to switch off when the line is looped. The oscillator is coupled by a transformer 12, 14 to an electronic detection circuit 15, 16 which monitors the off-hook and dialling states of the telephone instruments. To prevent double detection of dial pulses due to "line ringing" on dialling a capacitor across the oscillator supply is charged by the overvoltage of the start of a break pulse to maintain the supply voltage above the zener threshold during the period of line undervoltage. <IMAGE>

Description

SPECIFICATION Loop detectors for telephone line circuits The present invention relates to loop detectors for telephone line circuits.

It is often required to monitor the condition of a telephone line at or near a subscriber's instrument, for example to indicate when the instrument goes "off hook" or to monitor loop-break impulsing signals originating from the instrument. In the case of telephone lines energised from a public exchange strict limitations may be imposed on the current drain or voltage drop occasioned by line monitoring circuits.

According to the present invention a loop detection arrangement which is responsive to looping and/or loop-break impulsing on a telephone line comprises an oscillator circuit, means to connect said oscillator for energisation between the conductors of said telephone line, and electronic circuit means arranged to receive an output signal from said oscillator by way of a transformer coupling for the purpose of detecting and/or monitoring the loop condition of the telephone line.

Preferably the oscillator circuit is connected between the conductors of the telephone line by way of a bridge rectifier. The oscillator circuit may include a resistor/capacitor network arranged substantially to prevent false detection of loop-make condition during loopbreak periods due to "line ringing" voltage swings.

A loop detector arrangement in accordance with the present invention will now be descried by way of example with reference to the accompanying drawing, of which: Figure 1 shows diagrammatically a part of the electric circuit of the loop detector arrangement, and Figure 2 shows an electric waveform illustrating the operation of part of the circuit shown in Fig. 1.

Referring first to Fig. 1 a telephone subscriber's instrument 1 is connected to public exchange equipment (not shown) by way of a telephone line pair 2, which at the exchange end may be connected to a voltage source such as a 50 volt battery or power supply (not shown).

When the subscriber's instrument is quiescent, or "on-hook", therefore there is a voltage of some 50 volts between the conductors 2, which is applied by way of a rectifier bridge 3 and a resistor 4 to charge a small capacitor 5. A resistor 6 in parallel with the capacitor 5 provides a slow discharge path for that capacitor.

The voltage developed across the capacitor 5 is applied by way of a high value resistor 7 across two zener diodes 8 and 9 in series, and the proportion of that voltage which is set up across the zener diode 9 forms the energising voltage for a transformer-coupled oscillator circuit 10. Two windings 11 and 1 2 of a transformer in the oscillator circuit 10 provide the necessary positive feedback between the output circuit and the input circuit of a fieldeffect transistor 13, while a third winding 14 of the transformer applies an oscillatory signal across the base-emitter junction of a transistor 1 5. This transistor 1 5 effectively rectifies the oscillatory signal, and a smoothed output from the collector electrode of this transistor 1 5 forms a logic-type signal having a level of some 5 volts positive when the oscillator 10 in un-energised and close to zero volts positive, both with respect to a "ground" line 16, whe the oscillator 10 is energised. The oscillator 10 may operate for example at a frequency of 1 6 KHz.

Referring now to Fig. 2, when the subscriber's instrument 1 is taken into use, that is, whe the handset (not shown) is taken "offhook" the voltage between the conductors of the line pair 2, adjacent the terminals of the instrument 1 will fall to a much lower value, typically 9 volts. When the dial (not shown) of the instrument 1 is moved off-normal, or when an electronic impulsing circuit of the instrument 1 takes up the corresponding condition, at the commencement of dialling, the voltage between the conductors falls virtually to zero. When the impulsing contacts or circuit subsequently open the voltage between the conductors of the line pair 2 rises to the full 50 Volts, although due to line inductance "ringing" the voltage may rise considerably above 50 Volts, and may subsequently fall below 50 Volts, before settling down.The capacitor 5 serves to smooth out this "ring ing", as indicated by the dashed line 1 7 in Fig. 2, although it will also have the effect of slowing the voltage fall at the commencement of the following "make" period, as indicated by the dashed line 18 in Fig. 2.

During those periods when the line voltage is above a value of between 30 and 40 Volts, that is when the instrument is idle and during impulsing "break" periods, the oscillator 10 is energised and the output voltage at the collector electrode of the transistor 1 5 is low.

At other times when the line is looped, except during impulse "break" periods, the oscillator 10 is un-energised and the output voltage at the collector electrode of the transistor 1 5 is high, that is about 5 Volts.

In order to avoid pulse distortion due to the "slow-make" effect of the capacitor 5 (as indicated by the dashed line 18) a capacitor 1 9 is provided in the oscillator circuit 10 to slow the start-up of oscillations by a similar extent, as indicated by a dashed line 20 in Fig. 2.

The resistor 4 serves to reduce the shunting effect across the line pair 2 of the capacitor 5 to voice-frequency signals. The current drain on the line pair 2 due to the circuit of Fig. 1, during idle or quiescent conditions is arranged to be less than 80yA.

The loop detector arrangement shown in Fig. 1 may be arranged to monitor up to eight lines of a private electronic branch exchange (PEBX) by using a line concentrator.

Claims

1. A loop detection arrangement which is responsive to looping and/or loop-break impulsing on a telephone line comprising an oscillator circuit, means to connect said oscillator circuit for energisation between the conductors of said telephone line, the electronic circuit means arranged to receive an output signal from said oscillator circuit by way of a transformer coupling for the purpose of detecting and/or monitoring the loop condition of the telephone line.

2. A loop detection arrangement in accordance with Claim 1 wherein the oscillator circuit is connected between the conductors of the telephone line by way of a bridge rectifier.

3. A loop detection arrangement in accordance with Claim 1 or Claim 2 wherein the oscillator circuit includes a resistor/capacitor network arranged substantially to prevent false detection of loop-make during loop-break periods due to line ringing voltage swings.

4. A loop detection arrangement substantially as hereinbefore described with reference to the accompanying drawing.