JP3175056B2 - Switching circuit for billing signal creation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a simplification of a switching circuit for generating a billing signal in a public subscriber circuit of a digital exchange. A charge for a public telephone, that is, a signal for receiving a charge at regular time intervals determined by the distance to the called party or the like is created and supplied by a charge signal creation circuit in a subscriber circuit in the telephone exchange system.

FIG. 3 shows a connection configuration diagram of a subscriber circuit of a telephone switching system. In the figure, 41 is a subscriber circuit, 42 is a billing signal creation circuit, 43 is a public telephone, 44 is a network,
45 indicates a trunk circuit, 46 indicates a central control unit (CC), 47 indicates a signal receiver driver (SRD), and 48 indicates a system clock (SCLK). When a billing timing signal output by the signal receiver driver 47 is input under the control of the central controller 46, the billing signal creation circuit 42 of the subscriber circuit 41 creates a billing signal from the system clock 48 and outputs it to the public telephone 43. Since this billing signal is transmitted while being superimposed on the subscriber line during a call, it is necessary to reduce noise generated by turning on / off the billing signal.

FIG. 4 is a block diagram of a billing signal creation circuit.
Shown in In the figure, 51 is a subscriber circuit, 52 is a billing signal creation circuit, 53 is a power supply monitoring circuit, 54 is a frequency dividing circuit, 55 is a low-pass filter, 56 is a switching circuit, and 57 is a subscriber terminal. The charging signal creation circuit 52 inputs the system clock to the frequency dividing circuit 54, and converts the charging timing signal into a switching circuit.
The charging pulse is sent to the subscriber terminal 57 via the power supply monitoring circuit 53, and charging processing is performed.

The system clock is a high-frequency clock signal of 2.048 MHz.
The frequency is divided into 16KHz 50% duty continuous pulses. This signal is transmitted as a sine wave by removing the odd-order harmonic component by passing through a low-pass filter 55. The sine wave is transmitted as a billing signal by the switching circuit 56 when the billing timing signal is input.

In order to reduce the influence of noise on the subscriber terminal 57 at the time of outputting the charging signal, the switching circuit has a characteristic that the signal gradually increases at the start of one charging signal output and gradually decreases at the end of the output. It needs to be realized. FIG. 5 shows a waveform diagram of the charging signal creation circuit 42. In the figure, indicates the output of the frequency dividing circuit 54, the output of the low-pass filter 55, the charging timing signal, and the output of the switching circuit 56. As shown in the figure, the switching circuit 56 is configured so that the waveform gradually increases when the charging timing signal is input and gradually decreases when the output ends.

[0006]

2. Description of the Related Art FIG. 6 shows a circuit diagram of a conventional switching circuit. In the figure, 20 is a diode, 21 and 22 are resistors, 23 and 24 are transistors, 25 is a level converter, 2
6, 27 are operational amplifiers, 28 is a resistor, 29 is a capacitor, 30 is a resistor, 31 is a diode, and 32, 33 and 34 are resistors. Other resistors and capacitors are not directly related to the circuit description,
Shown by R and C. The power supply uses + 5V and -5V.

This circuit outputs a continuous sine wave obtained by a low-pass filter as a billing signal through a diode 20 and an operational amplifier 27. By changing the potential on the cathode side of the diode 20, the signal of the diode 20 is gradually increased.
It is used as a switch having a gradually decreasing characteristic. Here, the anode of the diode 20 is raised to a certain positive level by two resistors 21 and 22. A sine wave output through the diode 20 is possible, and the potential of the cathode is changed by the switching of the transistor 23. Is done.

On the other hand, the switching of the transistor 23 is controlled by turning on and off the other transistor 24. For example, 2.5 V and a charging timing signal from a signal receiver driver (SRD) are input to the base of the transistor 24. Assuming that the charging timing signal is, for example, a TTL level pulse, the output of the level converter 25 takes either +5 V or -5 V. At the charging timing, the base of the transistor 24 is set to +5 V, and this can be turned on.

When the transistor 24 is turned on, the base potential of the transistor 23 is gradually increased by the integration circuit composed of the resistor 28 and the capacitor 29. As the transistor 23 gradually turns on, its collector potential, that is, the cathode potential of the diode 20 gradually decreases,
20 turns on. However, when the transistor is first turned on, the cathode potential is small and the current is small, so the diode 20
Has a large equivalent resistance, and the equivalent resistance decreases as the current increases.

That is, in the conventional example, a charging signal having a gradual increase characteristic can be generated by gradually turning on the transistor 23 and decreasing the equivalent resistance of the diode 20.
Further, at the end of the charging signal output, the transistor 23 is turned off and the equivalent resistance of the diode 10 is gradually increased, so that a gradually decreasing characteristic is realized.

In the figure, the operational amplifier 26 is supplied with an appropriate divided voltage input by two resistors 33 and 34,
This is for raising the bias voltage of the resistor 32 by the output. Further, a series circuit of a resistor 30 and a diode 31 is connected between the base of the transistor 23 and the input of the operational amplifier 26, and a connection point between the two is connected to the non-inverting input of the operational amplifier 27.

[0012]

A switching circuit in a conventional charging signal generating circuit uses a diode as a switch for gradually increasing the charging signal when a charging signal is generated by turning on / off a continuous sine wave signal. Was. Since this diode switch cannot completely cut off the current, a transistor switch is used in the subsequent stage as shown in FIG. 6, and there is a problem that the circuit scale becomes large due to a two-stage switch configuration.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a charging signal with a gradual increase characteristic by a simple configuration including a combination of a field effect transistor and an operational amplifier, and to reduce noise generated when a charging signal is transmitted.

[0014]

FIG. 1 is a block diagram showing the principle of the present invention. In the figure, 1 is an on / off switch circuit,
2 denotes a voltage follower circuit, 3 denotes an integration circuit, and 4 denotes an amplification circuit. The on / off switch circuit 1 comprises a field effect transistor (FET), the voltage follower circuit 2 comprises a capacitor and an operational amplifier, the integrating circuit 3 comprises an operational amplifier, a resistor and a capacitor, and the amplifier circuit 4 comprises an operational amplifier and a resistor. Consists of

The sine wave input to the switching circuit is frequency-divided from the clock of the digital switching system to 12 KHz or 16 KHz by a frequency dividing circuit and input as a sine wave signal through a low-pass filter, as in the conventional example. The drain terminal of the field-effect transistor FET of the on / off switch circuit 1 is connected to the output of the low-pass filter via the voltage follower circuit 2, the source terminal is connected to the charge signal sending circuit via the amplifier circuit 4, and the gate terminal is connected to the integrating circuit 3. Are connected to the billing timing signal output via the.

[0016]

In the switching circuit, a continuous sine wave signal is turned on / off by a billing timing signal, and is transmitted as a billing signal to a public telephone connected to the digital exchange. The continuous sine wave output from the low-pass filter has a gradually increasing characteristic when the charging timing signal is on and a gradually decreasing characteristic when the charging timing signal is off by an on / off control and a variable amplifier circuit composed of a field effect transistor and an operational amplifier in the switching circuit. The billing signal is output in the form of a burst.

As described above, according to the present invention, with a simple circuit configuration including a field effect transistor and three operational amplifiers,
A billing signal having a gradually increasing characteristic can be created.

[0018]

FIG. 2 is a circuit diagram showing a switching circuit according to an embodiment of the present invention. In the figure, 11 is a field effect transistor, 12, 13, 14 are operational amplifiers, 15, 16, 17, 18 are resistors, and 19, 10 are capacitors. The switching circuit is 1
Field-effect transistors 11 and three operational amplifiers 12, 1
3, 14 and 4 resistors 15, 16, 17, 18 and 2 capacitors 1
Consists of 9, 10.

The drain terminal D of the field effect transistor FET11 receives the charge sine wave signal from the low-pass filter, and is connected to the output of a voltage follower circuit composed of an operational amplifier 12 provided to eliminate the influence from other circuits. The source terminal S is connected to the other end of the input side resistor 17 of the inverting amplifier composed of the operational amplifier 14 and the resistors 17 and 18, and the gate terminal G is used to moderate the change at the time of on / off of the charging timing signal. Operational amplifier 13 and resistors 15, 16
And an output of an integrating circuit constituted by a capacitor 19.

When the billing timing signal is off, the output of the integrating circuit is lower than the cut-off voltage of the field effect transistor FET11, and the billing signal is not transmitted because the billing sine wave signal is not input to the inverting amplifier. . Next, when the charging timing signal is turned on, the output of the integrating circuit gradually rises to a constant voltage by the time constant of the resistor 15 and the capacitor 19.

[0021] By nature of the source-drain resistance R _DG of the field effect transistor 11 is changed by the gate voltage, the source-drain resistance R _DG is gradually reduced, the amplification degree R18 / inverting amplifier (R _DG + R17) Becomes a billing signal having a so-called gradually increasing characteristic. When the billing timing signal changes from on to off, the operation reverses to that of the billing signal and gradually decreases, and eventually turns off completely, and is output as a billing signal having a gradual increase and a gradual decrease.

[0022]

As described above, according to the present invention, the public telephone
Generation of billing signal for receiving coins at regular intervals
Of the switching circuits required in the road
To completely cut off the current that the field effect transistor has
Characteristics and gate voltage control for source / drain
Paying attention to the characteristic that the resistance between
By using the register, a billing signal with little noise can be transmitted by a small-scale circuit configuration, and it is possible to maintain good communication quality.

[Brief description of the drawings]

FIG. 1 is a diagram showing the principle configuration of the present invention.

FIG. 2 is a circuit diagram of an embodiment.

FIG. 3 is a connection configuration diagram of a subscriber circuit.

FIG. 4 is a block diagram of a billing signal creation circuit.

FIG. 5 is a waveform diagram of a billing signal creation circuit.

FIG. 6 is a circuit diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 On / off switch circuit 2 Voltage follower circuit 3 Integrating circuit 4 Amplifying circuit 11 Field effect transistor 12, 13, 14, 26, 27 Operational amplifier 15, 16, 17, 18, 21, 22, 28, 30, 32,33 , 34 Resistance 19, 10, 29 Capacitor 20, 31 Diode 23, 24 Transistor 25 Level converter 41, 51 Subscriber circuit 42, 52 Billing signal creation circuit 43 Public telephone 44 Network 45 Trunk circuit 46 Central controller 47 Signal receiver driver 48 System clock 53 Power supply monitoring circuit 54 Divider circuit 55 Low-pass filter 56 Switching circuit 57 Subscriber terminal

Continuation of front page (72) Inventor Kiyoshi Shibuya 3-18-18 Shin-Yokohama, Kohoku-ku, Yokohama-shi, Kanagawa Prefecture Inside Fujitsu Communication Systems Co., Ltd. (56) References JP-A-2-92066 (JP, A) JP-A-2-266647 (JP, A) JP-A-63-232607 (JP, A) JP-A-4-22263 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04M 15 / 00

Claims (1)

(57) [Claims] 1. A sine wave obtained by dividing a system clock by a frequency dividing circuit gradually increases and decreases when a charging timing signal is turned on.
At the time of a burst, it is a burst-like billing signal with a gradually decreasing characteristic.
The charging signal is transmitted to the public
In a charging signal generation circuit of a public subscriber circuit to be transmitted to a talker , a field effect transistor FET is provided in an on / off switch circuit of the switching circuit, and the drain terminal is connected to an output of a low-pass filter via a voltage follower circuit. the source terminal to the billing signal transmission circuit via the amplifier circuit, the charging signal generated for the switching circuit, characterized in that connected to the charging timing signal output of the gate terminal via the integrating circuit.