JPS5923665B2 - Subscriber line test circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a subscriber line test circuit that can accurately measure line-to-line insulation resistance in subscriber lines.

In a telephone switching system, the line from the exchange to the subscriber line where telephone subscribers are connected via terminal equipment.

Subscriber line tests are conducted to determine whether there are any abnormalities in the network.
Subscriber line tests usually test the subscriber line's AB wires, the inter-line insulation resistance ZAB, and the insulation resistance Z between the A wire or B wire and grounding.
This is done targeting A, ZB and capacitances CA, CB.
FIG. 1 is a diagram showing a subscriber line and its test status.

In the figure, 1 is a test circuit, 2 is a telephone, ZAB, ZA
, ZB is insulation resistance, and Co is line capacitance. Among such subscriber line tests, the method shown in FIG. 2 has conventionally been used to measure the line-to-line insulation resistance ZAB.

As shown in the figure, the power supply +Vin is connected through the series resistor Zo.
to one line, e.g. A line, and the other line B
ZAB was measured by grounding the line and measuring the voltage Vout between the A line and ground. second
As is clear from the figure, what is measured by this connection method is the parallel insulation resistance of the line-to-line insulation resistance ZAB and the insulation resistance ZA between the A line and the ground. That is, in FIG. 2, the following relationship holds: V0ut=Vin×(1) ZA+ZAB.

Therefore, the measuring method shown in FIG. 2 has the disadvantage that the line-to-line insulation resistance ZAB cannot be measured correctly.

The present invention aims to eliminate such drawbacks of the prior art, and its purpose is to reduce the influence of the insulation resistance between one wire and ground by using a balance circuit, and to accurately measure the wire and ground. An object of the present invention is to provide a subscriber line test circuit capable of measuring insulation resistance between cables. In order to achieve this object, the subscriber line test circuit of the present invention inputs the potentials of the two wires that are generated when the subscriber lines each consisting of two wires are connected to a positive power source and a negative power source through series resistors. It comprises a circuit consisting of one emitter follower and a differential amplifier which receives both outputs of the circuit as inputs, and the line-to-line insulation resistance of the subscriber line can be measured by the output voltage of the differential amplifier. It is characterized by Examples will be described below.

FIG. 3 is a circuit diagram showing the configuration of an embodiment of the subscriber line test circuit of the present invention.

In the figure, Q1, Q2, Q3, and Q4 are transistors, and ZD is a Zener diode ~ R1? R2? R3,R
4, R5, R6, R7, and R8 are resistors, and Zl and Z2 are series resistors. Now, as shown in Figure 2, the series resistance Zl,
positive power supply +1 and negative power supply −V2 respectively through Z2
Connect to A and B wires. The voltages on both lines are input to the bases of transistors Ql and Q2 that constitute the emitter follower, and the outputs obtained at the respective emitters are further amplified by differential amplifiers Q3 and Q4, resulting in an output voltage of 0.
produce ut. Since both transistors Q, , Q2 are connected to the collector ground plane, their input impedance is sufficiently high and therefore does not affect the insulation resistances ZAB, ZA, ZB.

When line insulation resistance ZAB is large, both transistors Ql
, Q2 is large, and therefore the current of transistor Q3 is large and the current of transistor Q4 is small. Therefore, the output voltage VOut taken out from the collector of transistor Q3 becomes smaller. Conversely, when the line insulation resistance ZAB is small, the output voltage VOut becomes large. The value of series resistance Z, , Z2 and transistor Q,
, Q4 are constant, there is a constant relationship between the output voltage VOut and the line insulation resistance ZAB, and the line insulation resistance can be determined by measuring the output voltage 0ut. In this way, by measuring with a balanced circuit, the line-to-line insulation resistance ZAB can be measured more accurately.

However, in this case as well, the measured line insulation resistance ZAB is
It goes without saying that the series circuit of the one-line-to-ground insulation resistance ZAZB is connected in parallel.

In FIG. 3, the Zener diode Zd is used to keep the emitter bias of the transistors Q3 and Q4 constant. As described above, in the subscriber line test circuit of the present invention, since a balance circuit is used to measure the line-to-line insulation resistance, the influence of the 1-wire-to-ground insulation resistance ZA, ZB is small, and the subscriber line test circuit of the present invention is different from the conventional subscriber line test circuit. Compared to this, there is an advantage that the line-to-line insulation resistance ZAB can be measured more accurately.

Note that FIG. 4 shows terminals 3- and 3-2 to which the present invention is applied.
,...3-.

, a telephone switching system consisting of a terminal device 4, a master device 5, and a switch 6. As explained above, according to the subscriber line test circuit of the present invention, the potentials of the two lines that are generated when the subscriber lines each consisting of two wires are connected to the positive power supply and the negative power supply through the series resistors are respectively input. The system is equipped with a circuit consisting of two emitter followers and a differential amplifier which receives both outputs of this circuit as inputs, and the line-to-line insulation resistance of the subscriber line is measured by the output voltage of this differential amplifier. , it is less affected by the insulation resistance between one line and ground, and the value of the insulation resistance between the subscriber lines can be measured more accurately.

[Brief explanation of drawings]

Fig. 1 shows a subscriber line and its test status, Fig. 2 shows a conventional line-to-line insulation resistance measuring circuit, and Fig. 3 shows the configuration of an embodiment of the subscriber line test circuit of the present invention. Circuit diagram shown, 4th
The figure is a block diagram showing a telephone switching system to which the present invention is applied. 1...Test circuit, 2...Telephone, 3-
,,3-2,...3-.

Claims (1)

[Claims] 1. A circuit consisting of two emitter followers each receiving the potential of the two wires generated when the subscriber line consisting of two wires is connected to a positive power source and a negative power source through series resistors, and both outputs of the circuit, respectively. A subscriber line test circuit comprising a differential amplifier as an input, and capable of measuring line-to-line insulation resistance of the subscriber line based on the output voltage of the differential amplifier.