Methods of testing multifrequency dialing signals

摘要

A predetermined sequence of multifrequency signals are fed to a multifrequency dialing board (24) and are separated into their component signals by high and low filter circuits (50 and 52). The component signals are converted into (1) square wave signals by zero crossing detectors (54 and 56) and (2) d.c. signals having amplitudes proportional to their respective frequencies by converters (72 and 74). The d.c. signals are then converted by analog-to-digital converters (80 and 82) into digital signals having values equivalent to the amplitude of the d.c. signals. A microprocessor circuit (60) monitors the digital signals to determine the amplitude of the signals. The microprocessor circuit (60) determines the frequency of the component signals by monitoring the time required for eight cycles of the respective square wave signals. The microprocessor circuit (60) also facilitates the self-calibration of the multifrequency dialing board (24) which ensures the correct operation thereof in the event of an error due to temperature or aging components.