Multi-channel or split-frequency, low frequency, telemetry circuit and method between transmitters and receivers

A digital telemetry system, comprising:

• a low frequency transmitter including an oscillator controlled synthesizer which is a software direct synthesis providing a 4-phase output based on a predetermined fixed frequency look up table;
• and
• a low frequency, narrow bandwidth, receiver, said receiver comprising:
• a buffer amplifier which receives an input signal and provides an amplified output signal;
• a bandpass filter which receives the output signal from the buffer amplifier and provides a filtered frequency signal, said bandpass filter being adjustable for selection of a center channel frequency based on programmed operation of a decoder;
• a squelch switch which receives the filtered frequency signal from the bandpass filter and provides an output signal having a high signal to noise ratio;
• a mixer circuit which receives inverted and non-inverted signals from the squelch switch, as well as 4-phase output signals from a synthesizer in order to perform coherent detection with an increased sensitivity and to provide an output signal which is the rectification of base band data;
• an adjustable lowpass filter which receives the output signal from the mixer and provides a filtered output signal;
• and
• a window detector circuit which receives the filtered output signal and provides a receiver output signal.