JPH0515118B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a central office line trunk circuit used in a private branch exchange or the like.

[Technical background of the invention]

As is well known, central office line trunk circuits used in private branch exchanges, etc., transmit and receive voice signals between the central office and the exchange, and also provide direct current loops to the central office current in order to respond to outgoing and incoming calls. It has the function of forming.

FIG. 1 is a diagram showing an example of a conventional office line trunk circuit. In the figure, the office line trunk circuit is connected to office 1 by two lines (chip line and ring line). The DC equivalent circuit of station 1 is a DC voltage source E _S (usually
(48V DC source) and internal resistance R _S , and the DC resistance of the line can be represented by R _L. 2 is the incoming signal from station 1 (generally 16
Hz, 100V _rns AC signal), 3 is a relay that forms a DC loop when responding to or transmitting an incoming signal, 4 shows low resistance for DC, and high resistance for AC. This is a DC shunt circuit shown in FIG. 1, and all the DC current from station 1 flows through this DC shunt circuit. 5 is an AC shunt circuit that exhibits low resistance in AC and high resistance in DC; only the AC component of the audio signal from station 1 on which DC current is superimposed passes through this AC shunt circuit 5. ,
It is transmitted to the switch frame 6 in the exchange.

Therefore, in such a configuration, when an incoming signal arrives from station 1 and the called party performs an off-hook operation in response, relay 3 closes and a DC loop is formed with station 1. At the same time, a communication path with the station 1 is formed by the AC shunt circuit 5 and the switch frame 6. This allows communication between station 1 and the extension telephone.

On the other hand, when a dial pulse is sent out, the relay 3 is closed in conjunction with the caller's off-hook, and a DC loop is formed with the station 1. When dialing is performed in this state, the relay 3 is turned on and off the number of times corresponding to the dialed number. That is,
For example, if you dial the number "2",
As shown in the waveform diagram of Fig. 2, relay ₃
It is intermittent twice in the interval T ₂ and T ₃ to T ₄ . As a result, a dial pulse is sent to station 1.

[Problems with background technology]

By the way, the DC shunt circuit 4 has a transistor TR ₂ through _{which the DC current from the station 1 flows, and a capacitor C 2 is} connected to the base of this transistor TR _{2 .}
By keeping the base potential constant, fluctuations in the base potential caused by the AC signal superimposed on the DC current from station 1 are suppressed, and the AC resistance between the collector and emitter of transistor TR ₂ is increased. It is configured so that only the components flow. However, this capacitor _C2 has the drawback that it deforms the dial pulse waveform when transmitting the dial pulse. That is, when the relay ₃ is turned off at time T1 in the waveform diagram shown in FIG. 2, no voltage is applied to the DC shunt circuit 4. For this reason, the capacitor
The voltage V _c2 after charging C ₂ passes through the path of the base of transistor TR ₂ → emitter → resistor R ₃ as shown in Figure 2b.
It discharges with the characteristics shown in . After this, time T ₂
When the relay 3 is turned on, voltage is again applied to the DC shunt circuit 4, and the capacitor _C2 is charged through the resistor _R2 .

However, transistors generally have the characteristic that collector current does not begin to flow unless the base-emitter voltage exceeds approximately 0.7V. Therefore, the collector current of transistor TR ₂ does not flow at all during ΔT until the charging voltage V _c2 of capacitor C ₂ reaches 0.7V, and even after that, the charging voltage
Since V _c2 rises slowly, the collector current does not rise suddenly. As a result, the local current I _L has a waveform shape with a slow rise as shown in Figure 2c.
Furthermore, since the rise characteristics of the local current I _L vary depending on the line resistance R _L , the rise characteristics change depending on the length of the line, resulting in a change in the break rate of the dial pulse.

[Purpose of the invention]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a central office line trunk circuit which eliminates the above-mentioned drawbacks, sends out an ideal dial pulse waveform, and does not change the dial pulse break rate due to line resistance.

[Summary of the invention]

Therefore, in the present invention, the capacitor connected between the base and emitter of the transistor forming the DC shunt circuit is disconnected from the DC shunt circuit during the period when the relay for forming the DC loop is off to generate dial pulses. This achieves the above objectives.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 3 is a diagram showing an embodiment of the present invention,
The only difference from the first circuit is that a relay K2 is added which operates on and off in synchronization with the on and off of relay 3 for forming a DC loop. This relay K2 is connected to a branch point between a resistor R2 connected to the base of a transistor TR2 and a capacitor C2,
It is provided between the capacitor C2 and the capacitor C2.

Therefore, in the configuration shown in FIG. 3, even if the relay 3 is turned off at time T1 in FIG. ₂ and no voltage is applied to the DC shunt circuit 4, the capacitor _C2 is connected to the relay K connected in series. When transistor TR ₂ is turned off, it is disconnected from the base of transistor TR ₂ . Therefore, the voltage V _c2 ' after charging the capacitor C ₂ continues from time T ₁ to time T ₂ as shown in FIG.
Hold the voltage at 1. Thereafter, at time T2 shown in FIG. 2, the relay 3 (relay K1) is turned on, and voltage is applied to the DC shunt circuit 4 again.
At the same time, relay K2 turns on and transistor
A predetermined base voltage is immediately applied to TR2, and a collector current immediately flows through transistor TR2.

As a result, the local current I _L ' becomes an ideal intermittent waveform with a steep rise characteristic without delay with respect to the intermittent connection of the relay 3, as shown in Fig. 2f, and moreover, the line resistance R _L The pulse break rate also no longer changes.

〔Effect of the invention〕

As explained above, the present invention enables the capacitor connected between the base and emitter of the transistor constituting the DC shunt circuit to be connected to the Since it is separated from the shunt circuit, the break rate does not change due to line resistance, and dial pulses with an extremely ideal waveform can be sent out.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional central office line trunk circuit, and FIG. 2 is a signal waveform diagram of each part of the conventional central office line trunk circuit and the central office line trunk circuit according to the present invention.
FIG. 3 is a diagram showing an embodiment of a central office line trunk circuit according to the present invention. 1... Station, 2... Incoming call detection circuit, 3... Relay for forming a DC loop, 4... DC shunt circuit, 5...
...AC shunt circuit, 6...switch frame.

Claims (1)

[Scope of Claims] 1. A first switching means forming a subscriber line; and a first circuit connected in series to the first switching means and passing only an alternating current component of an audio signal in the subscriber line. and a second circuit in which a transistor is connected in parallel with the first circuit, and a capacitor connected between the base and emitter of the transistor maintains the base potential of the transistor to pass only the DC component of the subscriber line. A central office line trunk circuit comprising: a circuit provided between the base of the transistor of the second circuit and the capacitor, and a break period of the dial pulse when the dial pulse is generated by the first switching means; A central office line trunk circuit comprising: second switching means for disconnecting the capacitor from the base of the transistor.