| 摘要 |
<p>A subscriber's telephone set, provided with a ringer connected in series with a capacitor across a line loop, is provided with a unit for the suppression of spurious ringing currents including an antiparallel connection of a diode and a transistor in series with the ringer/capacitor combination. The transistor has a base/emitter circuit connected across the line loop by way of a Zener diode and an R/C network. The Zener diode breaks down in response to oscillations whose frequency and amplitude are characteristic of genuine ringing currents from the associated central office, thereby driving the transistor into conduction and enabling the ringer to operate; other oscillations or pulse trains, such as those due to dialing pulses generated by another telephone set connected across the same line loop, remain ineffectual with the transistor cut off.</p> |
