Signal send-receive circuit for telephone exchanges - has two adjustable thresholds for preventing receiver processing below-threshold receiving call signals

摘要

The appts. includes a transformer (U1) coupled telephone lines (a1,b1,a2,b2, with high frequency chokes (D) linked to an a-c generator comprising an operational amplifier (P1) and an a-c receiver with another operational amplifier (P2) via one and the same capacitor (C1). A control device (Z) applies control voltage to both the a-c sending circuit and to an a-c receiving sensitizing circuit. When the sending control voltage (Z1) is grounded a transistor (T1) enables the diode network (E1 to E4) to become conductive and to allow the transformer (U2) to transmit 25 cycle energy to the capacitors (C1,C2). These form not only a parallel resonance condition with the transformer secondary (U2,II) but also a series resonance condition with the choke coils (D,I,II). The a-c receiving amplifier (P2) is insensitive to these voltages in that the control voltage (ZZ) is set to produce a blocking voltage via feedback resistor (R) to the diodes (G1,G2). The variable voltage divider resistors (R8,R9) can be set so as to determine the minimum a-c signal level below which the receiver remains non-responsive to the pulsing rectified voltages applied at terminals (X,Y) of the operational amplifier (P2) thus producing no a-c output across resistor load (R7).