| 摘要 |
A telephone system is shown including subscriber loop interface circuits each for connecting a subscriber loop having a filter and a rectifier to a switching network of the system. The interface circuit includes a transformer having a primary winding, a secondary winding, and a sense winding. The primary winding is connected in series with a switching transistor whereby current pulses are fed into the primary winding. The secondary winding is connected to the filter and rectifier of the loop so that the current pulses are rectified to a DC current for the loop. An integrator including a current mirror feeding an integrating capacitor integrates each of the current pulses to give an integrated voltage. At the same time the voltage across the secondary winding is fed via a sense winding to a sample and hold circuit to generate a voltage which is compared against the voltage developed across the integrator. The output of the comparator is then used in conjunction with a clock pulse to turn on and off a flip-flop to control the width of the current pulses generated by the switching transistor. In addition thereto, circuitry is provided whereby voice signals from the switching network effectively modulate the current pulses fed through the primary winding of the transformer such that voice signals can be fed to the loop. Furthermore, the output of the sample and hold circuit is filtered to provide the transmission of voice signals from the loop to the switching network.
|
