Active damping circuit for four wire to two wire changeover - has auxiliary tone generator for constant adjustment of variable damping resistor

摘要

The appts. is for use in telephone systems at points where four wire circuits are connected to two wire circuits. The performance of an active damping circuit comprising two amplifiers (V1,V2) can be improved by adding a tone signal to the first amplifier. A pulse generator (G1) controls an alternating current generator (G2) feeding a signal (U8), typically at 1000Hz, to an input of the first amplifier (V1). This signal is retrieved from the output (U2) of the second amplifier (V2) by a band pass filter (BP1). It is rectified and applied to an operational circuit (NW) which determines the value of the variable damping resistor (R4). The circuit (NW) receives a timing impulse from the pulse generator (G1) and updates the resistor (R4) value at each pulse such that the rectified voltage (U9) from the filter (BP1) is a minimum. In the intervening period, the resistor value is held at the last setting.