Telemetry digital subcarrier demodulator

Abstract

A telemetry receiver for demodulating subcarrier frequencies is disclosed. The flexibility of the receiver permits it to decode signals from a wide variety of transmitters. In the preferred embodiment, a biomedical digital telemetry subcarrier demodulator employs microprocessor control of a variable bandpass filter and cycle counter. The bandpass of the filter is set by the microprocessor to pass frequencies at and around the center frequency of the desired subcarrier frequency. The counter is preset for the desired number of cycles. In operation, the desired subcarrier frequency is passed by the filter, through a signal shaper, and to the counter. Upon receiving the first cycle, the counter activates a timer, which measure the period of the predetermined number of cycles. The desired signal is the percentage deviation of the subcarrier period from the average subcarrier period. The signal to noise ratio in a digital circuit may be improved by modulating the frequency of the clock supplied to the digital circuitry coupled with passing the signal through a bandpass filter whose passband is sufficiently narrow that interference signals caused by the clock will be within the passband a portion of the time and outside the passband a portion of the time.