Frequency modulation threshold extension demodulator utilizing frequency compression feedback with frequency drift correction

Claim 1

In a threshold extension receiver: a source of frequency modulated radio frequency signals having a frequency deviation therebetween; a mixer; a local oscillator supplying local oscillations; an intermediate frequency filter having a pass band substantially narrower than the frequency deviation of the said frequency modulated radio frequency signals; means for feeding said source of frequency modulated radio frequency signals and the local osacillations to said mixer, to thereby produce intermediate frequency oscillations in said mixer, said intermediate frequency oscillations having an average frequency within the pass band of said intermediate frequency filter; means for feeding the intermediate frequency oscillations of said mixer to said intermediate frequency filter, whereby the intermediate frequency filter will pass such components of said intermediate frequency oscillations as lie within the pass band; negative frequency feedback means, responsive to the signals which are passed by the pass band of said intermediate frequency filter, for modulating the frequency of said local oscillator such as to cause the intermediate frequency oscillations produced by said mixer to have a reduced frequency deviation, compared to the frequency deviation of said radio frequency signals; said negative feedback means having two independent feedback loops acting in parallel; said first feedback loop comprising means, responsive to the average frequency of said intermediate frequency oscillations, for slowly causing said local oscillator to change frequency in such a sense as to maintain said average frequency at a desired value; said second feedback loop comprising means, responsive to the instantaneous frequency of said intermediate frequency oscillations, for quickly causing said local oscillator to change frequency in such a sense as to drive the instantaneous frequency of said intermediate frequency oscillations towards said desired value; wherein said first feedback loop comprises a band pass filter; and said second feedback loop comprises a low pass filter; and wherein the pass bands of the low pass filter and the band pass filter have no pass band in common; wherein said first feedback loop comprises: a resonant tank circuit tuned to said desired value; means to reactively and loosely couple said resonant tank circuit to the signals passed by said intermediate frequency filter; whereby said resonant circuit will sympathetically resonate approximately in quadrature to said signals when the instantaneous frequency of said signals is at said desired value; and whereby as the instantaneous frequency of said signals departs from said value, the phase relationship between said signals and the resulting sympathetic oscillations varies rapidly and monotonically with the departure; a phase detector having a first and second input and whose output is the vector cross product of two inputs; means to feed the voltage across said resonant circuit to the first input of said phase detector; means to feed the signals passed by said intermediate frequency filter to the second input of said phase detector; and means to feed the output of said phase detector to said local oscillator.