Signal level and frequency monitoring for telephone call tone circuit - feeding frequency information for call tone input signal to microprocessor via DC voltage converter providing galvanic separation

摘要

The monitoring method involves using a threshold comparator for monitoring a level of a call tone input signal (TRES). Its frequency is monitored by extracting frequency information transmitted to an evaluation microprocessor (MP) via a galvanic separation stage. A DC-DC voltage converter (WDL) controlled by a HF clock signal (TS) uses a pulse sequence containing the frequency information. Each of the pulses has a short duration relative to the half wave duration of the call time input signal to provide spaces in the HF clock signal, evaluated on the secondary side of the DC voltage converter to detect the frequency of the call time input signal. ADVANTAGE - Eliminates need for opto-coupler.