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Old 3rd Aug 2021, 7:49 pm   #6
G6Tanuki's Avatar
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 11,074
Default Re: FM tuned circuits, ratio detection etc

The classic Foster-Seeley and Ratio-detectors are sensitive to both frequency and phase: simplistically they use two tuned-circuits which are either magnetically-coupled or use a combination of magnetic- and capacitance-coupling.

When there's just an unmodulated carrier present the combination of magnetic-and-capacitance coupling produces no audio component [though it can provide a DC component for use in AGC and AFC].

This is essentially a state-of-balance.

When the frequency of the carrier is wobbled by FM modulation, this means it moves out of the peak of the discriminator's tuned-circuits and the result is an instantaneous imbalance, the extent of which follows the audio modulation of the carrier - so the discriminator produces an audio signal according to the extent of the imbalance.

AM signals - typified by impulse noise from lightning or unsuppressed car-ignitions - are limited in the IF stages of a FM receiver - and even if they make it past this to hit the FM discriminator they only serve to 'push up' the average level of the signal on each pulse, rather than having any either-side-of-the-response-curve component, so they don't present themselves at the audio output of the discriminator.
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