JPH0378357A - Dc termination circuit - Google Patents

Abstract

PURPOSE:To obtain a circuit whose side tone level does not vary even if a loop current varies by preventing the loop current from flowing in a transformer. CONSTITUTION:The transformer TO is constituted so that its primary side is of two windings, and a capacitor C2 is connected between the primary windings. A base current is supplied to a transistor Q1 or Q2 through a base resistor RB according to the polarity of the loop current I, and the polarity of the loop current can be detected by detecting the loop current and detecting which is ON between the photocouplers PC1 and PC2. Thus, the suppression of the side tone against the variation of the loop current can be made possible, and the DC termination circuit not requiring the addition of a circuit element, for instance, the insertion of the resistor or the like in the case of short line length according to the length of an office line can be constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a DC termination circuit for an interface with a central office line, which improves sidetone characteristics in a telephone device.

(Prior Art) A direct current flows simultaneously with a signal current in an interface circuit with a central office line in a telephone device. In this case, when direct current flows through the transformer, the sidetone level changes significantly due to changes in the loop current value.

FIG. 4 is a diagram showing a conventional DC termination circuit.

In the figure, direct current and signal current are connected to terminal Ll from the station line.
,L! is input. The signal is 2 from the primary side of the transformer T.
It is converted to the next side and output to seven output terminals Tt. Transistors Q and Q constitute complementary transistors, which are turned on alternately depending on the polarity of their collector-emitter voltages. SS is a transistor Q,,Q,
The base current is supplied by a bidirectional diode array connected in series with the base of the Also Ro and transistor Q.

+Qz common emitter resistance, PC, and PC.

are photocouplers connected to the respective collectors of the transistors Q, Q-, which are connected to a light-receiving element (not shown) and output signals corresponding to each.

That is, the photocouplers PCi and Pct output a DC current detection signal and a DC polarity detection signal depending on which photocoupler PC1 or PC8 is on.

(Problems to be Solved by the Invention) The DC termination circuit having the above configuration has the drawback that since the DC loop current I flows through the transformer T, the sidetone level changes greatly due to a change in the loop current ■. This change will be discussed later.

An object of the present invention is to provide a DC termination circuit that suppresses changes in sidetone level.

(Means for Solving the Problems) In order to achieve the above object, the present invention connects two sets of series circuits of phototransistors and transistors in parallel between station line terminals, and the two sets of transistors are mutually complementary transistors. Construct a circuit, connect both transistors in parallel through a common emitter resistor, and connect in parallel a series circuit of an anti-parallel diode, a base resistor, and a capacitor, and connect both transistors at the connection point between the base resistor and the capacitor. The gist is a DC termination circuit characterized in that the bases of the transformer are connected to each other, and a capacitor is connected between the windings of a transformer having two windings on the primary side, and is connected between the station line terminals.

(Function) With the above configuration, the present invention provides a circuit in which no loop current is passed through the transformer, and the sidetone level does not change even if the loop current changes.

(Example) Hereinafter, an example of the present invention will be described along with the drawings. Note that the embodiments are merely illustrative, and it goes without saying that various changes and improvements can be made without departing from the spirit of the present invention.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In the figure, L+, Lg are terminals to which the office line is connected, photocoupler PC, ) transistor Q1. Series circuit of emitter resistor R7 and photocoupler PC, ) transistor Q2. A series circuit of emitter resistors Rt is connected in parallel to terminals L+ and L8, and diodes D, , Dt are connected in antiparallel.
, a series circuit of a base resistor R8° and a capacitor C1 is also connected in parallel to the terminals and L8, and the transistor Q
. The photocoupler PCI, PC is composed of complementary transistors, one of which becomes conductive depending on the polarity of the DC loop current (2), and outputs a signal via a light receiving element (not shown). In addition, the diodes D, , D, are photocoupler p
This is to compensate for the voltage drop of c, , pct.

Further, the transformer T0 has two windings on the primary side, and a capacitor C8 is connected between the primary windings.

Thus, a base current is supplied to the transistor Q and Q via the base resistor R1 depending on the polarity of the loop current I, and current detection of the loop current ■ and which photocoupler P
The polarity of the loop current can be detected depending on whether C or PC8 is on.

Note that if current detection and polarity detection of the loop current are not required, the photocouplers Pct and Pct can be replaced with diodes.

Furthermore, FIG. 2 is a circuit diagram showing an example in which the DC termination circuit of the present invention is applied to an interface circuit with a central office line. DS is a dial switch, Is is a hook switch, C
is an incoming call detection circuit operated by hook switch H3,
HYB is a 2-wire/4-wire conversion circuit. By configuring such an interface circuit with the station line, the transformer T0 is not affected by the loop current ■, and can be used for 2-wire/4-wire
The sidetone level of the speech circuit including the line conversion circuit can be suppressed.

Next, FIG. 3 is a diagram showing the connections on the light receiving element side for current detection and polarity detection, in which the signal S from the photocouplers pc, pc,
, ,S, is output.

Next, the characteristics of a conventional DC termination circuit and a DC termination circuit according to an embodiment of the present invention will be explained. This comparison of characteristics was performed on an equivalent basis.

Figure 5 is a block diagram of the communication circuit, and the sidetone level is V @
/ Vt. However, vo is the transmitting system level, vi is the receiving system level, and ■ is the DC loop current. Note that DC is a direct current termination circuit, and FIGS. 6 and 7 are the conventional and inventive measuring circuits, respectively, and FIGS. 8 and 9 are
The figures show the characteristic diagrams of FIG. 6 and FIG. 7, respectively, and the measurements are of resistance RL.

The input frequency of the receiving system level Vi was changed using the current IL as a parameter, and the transmitting system level V was measured with respect to this change.

In Fig. 7, RL=OΩ, Rt -10Ω,
There is a 15dbmO difference at maximum 100Hz for It=125ΩA, LL L-15mA, whereas in Fig. 8, Rt-0Ω, RL-10Ω, R
L -22Ω, It -15mA, I
For L-125A, RL bulk OΩ, IL = 1
Except for the case of 25-A, it is within Bdb-, and furthermore,
If RL-0Ω and TL = 15mA are excluded, it can be suppressed to within 2db.

(Effects of the Invention) As explained above, according to the present invention, two sets of series circuits of a phototransistor and a transistor are connected in parallel between the station line terminals, and the two sets of transistors mutually constitute a complementary transistor circuit. Both transistors are connected in parallel through a common emitter resistance, and a series circuit of an antiparallel diode, a base resistor, and a capacitor are connected in parallel, and the bases of both transistors are connected to the connection point between the base resistor and the capacitor. By connecting a capacitor between the windings of a transformer whose primary side has two windings and connecting it between the station line terminals, it is possible to suppress the sidetone level against changes in the loop current. For example, depending on the length of the central office line, it is possible to provide a DC termination circuit that does not require the addition of circuit elements such as insertion of a resistor when the line length is short.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example in which the embodiment of the present invention is applied to an interface circuit with a central office line, and Fig. 3 is a photocoupler light receiving element circuit. Figure 4 is a circuit diagram showing an example of a conventional DC termination circuit.
FIG. 5 is a diagram for explaining the sidetone level, FIGS. 6 and 7 are characteristic measuring circuit diagrams of the conventional and the present invention, and FIGS. 8 and 9 are diagrams showing the characteristics of the conventional and the present invention. Lt, Lt ・ ・ PC,, pct Q,, Q, ・ ・ R, ・ ・ ・ ・ D+, Dt ・ ・ R8 ・ ・ ・ ・ cl, Cz ・ ・ To ・ ・ ・ ・ ・ T+, Tt ・ ・Office line terminal Photocoupler Transistor Emitter Resistor Diode Base Resistor Capacitor Transformer Output terminal

Claims (1)

[Claims] Two sets of series circuits of a phototransistor and a transistor are connected in parallel between the station line terminals, and the two sets of transistors mutually constitute a complementary transistor circuit, and are connected in parallel through a common emitter resistance of both transistors. ,
A series circuit of an anti-parallel diode, a base resistor, and a capacitor is connected in parallel, and the bases of both transistors are connected to the connection point of the base resistor and the capacitor, and the primary side is connected between the windings of a transformer consisting of two windings. A DC termination circuit characterized in that a capacitor is connected between the station line terminals and the station line terminals.