System for demodulating low-level frequency modulated signals utilizing a short term spectral analyzer

Claim 1

A DEMODULATING CIRCUIT FOR A FREQUENCY-MODULATED WAVE, COMPRISING A DELAY NETWORK HAVING AN INPUT AND AN OUTPUT, MEANS FOR APPLYING SAID WAVE TO THE INPUT OF SAID NETWORK, A FREQUENCY CHANGING MIXER HAVING FIRST AND SECOND INPUTS AND AN OUTPUT, CONNECTION MEANS BETWEEN THE OUTPUT OF SAID DELAY NETWORK AND SAID FIRST INPUT OF SAID FREQUENCY CHANGING MIXER, A LOCAL OSCILLATOR HAVING AN INPUT AND AN OUTPUT AND ADPATED TO BE FREQUENCY-MODULATED BY A CONTROL VOLTAGE, FURTHER CONNECTION MEANS BETWEEN THE OUTPUT OF SAID OSCILLATOR AND SAID SECOND INPUT OF SAID FREQUENCY CHANGING MIXER; A PASSBAND FILTER HAVING AN INPUT FED FROM THE OUTPUT OF SAID FREQUENCY CHANGING MIXER, AND AN INTERMEDIATE CIRCUIT FED FROM THE OUTPUT OF SAID FILTER AND DELIVERING DEMODULATED SIGNALS THROUGH A FURTHER CIRCUIT TO A UTILIZATION CIRCUIT; AN AUXILIARY CIRCUIT COMPRISING ANALYZER MEANS AND HAVING AN INPUT FED FROM SAID WAVE AND AN OUTPUT DELIVERING AN ESTIMATED SIGNAL PROPORTIONAL TO THE MEANS VALUE DURING A SMALL TIME INTERVAL OF THE INSTANTANEOUS FREQUENCY DEVIATION OF SAID WAVE FROM ITS ZERO-MODULATION FREQUENCY, SAID ESTIMATED SIGNAL CONSTITUTING SAID CONTROL VOLTAGE, AND MEANS FOR APPLYING SAID CONTROL VOLTAGE TO SAID LOCAL OSCILLATOR AND FOR ADJUSTING THE VALUE OF SAID CONTROL VOLTAGE SUCH THAT THE AVERAGE FREQUENCY DEVIATION OF SAID LOCAL OSCILLATOR IS SUBSTANTIALLY EQUAL TO THAT OF THE ABOVE-SAID WAVE; SAID FURTHER CIRCUIT BEING AN ADDITION CIRCUIT ADDITIVELY COMBINING SAID DEMODULATED SIGNALS WITH A FURTHER SIGNAL DERIVED FROM SAID CONTROL VOLTAGE.