FM Feedback demodulator having threshold extension circuit with two-pole crystal filters

A method of demodulating a single channel per carrier FM signal comprising the steps of:

• (a) combining said single channel per carrier FM signal at a prescribed intermediate frequency in a first mixer circuit with a local oscillator signal so as to convert said FM signal to a first frequency converted IF FM signal,
• (b) selectively filtering and amplifying said first frequency-converted IF FM signal within a first prescribed narrow bandwidth,
• (c) further selectively filtering the signal obtained from step (b) within a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth while controlling the phase of said obtained signal in accordance with the frequency thereof,
• (d) combining the signals obtained from steps (b) and (c) in a second mixer circuit to provide an FM discriminator output signal, and
• (e) controlling said local oscillator signal in accordance with the level of said FM discriminator output signal, and wherein
• each of steps (b) and (c) includes the selective filtering of said first frequency-converted IF FM signal and the signal obtained from step (b), respectively, in a respective two-pole crystal bandpass filter having said first and second prescribed narrow bandwidths, respectively.

A frequency modulation feedback demodulator for demodulating single channel per carrier FM input signals comprising:

• a first mixer circuit having a first input port to which an intermediate frequency FM input signal is applied, a second input port to which a local oscillator signal is supplied, and an output port from which a first frequency-inverted IF FM signal is derived;
• a tuned IF amplifier circuit connected to the output port of said first mixer circuit and having an automatic gain control loop for controlling the output amplitude thereof;
• an IF amplifier circuit coupled to the output of said tuned IF amplifier;
• a predetection filter comprised of a two-pole crystal bandpass filter having a first prescribed narrow bandwidth coupled to the output of said IF amplifier circuit;
• an amplifier driver circuit connected to the output of said predetection filter;
• a frequency/discriminator network including
• a series-connected attenuator/frequency detection filter circuit, including a two-pole crystal filter having a second prescribed narrow bandwidth greater than said first prescribed narrow bandwidth, coupled to the output of said amplifier driver circuit, and
• a second mixer circuit having a first input port connected to the output of said amplifier driver circuit, a second input port coupled to said two-pole crystal filter having said second prescribed bandwidth, and an output port;
• a loop compensation filter connected to the output port of said mixer circuit;
• a voltage controlled oscillator having an input coupled to said loop compensation filter and an output connected to the said second input port of said first mixer circuit for supplying said local oscillation signal thereto;
• and
• an output amplifier circuit coupled to the output of said loop compensation filter from which a baseband demodulated FM signal is derived.