FM Feedback demodulator having threshold extension circuit with

Details FM Feedback demodulator having threshold extension circuit with FM Feedback demodulator having threshold extension circuit with

1977-06-14

1978-05-02

Grimm, Siegfried H.

Demodulators

Amplitude modulation demodulator

Having specific distortion, noise or other interference...

325346, 325348, 325349, 325423, 325438, 329117, 329122, 333 72, H03D 306, H04B 126

Patent

active

040877566

ABSTRACT:
Demodulation of FM signals employing an FM feedback loop of the present invention includes a first mixer circuit for mixing an IF input signal with a voltage controlled oscillator signal to down-convert the IF input signal to a predescribed center frequency IF signal. This signal is coupled through a tuned amplifier having an automatic gain control loop and is then filtered in a predetection filter and amplified. The predetection filter is composed of a two-pole crystal bandpass filter having a very narrow bandwidth. The filtered signal is then coupled to a frequency discriminator circuit comprised of a frequency detection second mixer and filter circuit, the frequency detection filter circuit being composed of a second two-pole crystal bandpass filter, the bandwidth of which is an order of magnitude larger than that of the predetection filter. The output of the frequency discriminator is coupled through a loop compensation filter to the voltage controlled oscillator so that changes in the output frequency of the voltage controlled oscillator reduce the deviation of the FM signal in the intermediate frequency and frequency discriminator stages. The use of two-pole crystal bandpass filters improves the frequency stability of the system and significantly reduces post detection noise levels, as compared with conventional limiter/discriminator techniques. The two-pole crystal bandpass filters have a Butterworth configuration. In addition to the FM feedback loop, a squelch circuit is also coupled to the output of the frequency discriminator.

REFERENCES:
patent: 3231822 (1966-01-01), Tillotson
patent: 3517268 (1970-06-01), Webb
patent: 3742361 (1973-06-01), Wason

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