Foster-Seeley discriminator
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The Foster-Seeley discriminator is an FM detector circuit that works on the same principle as most commonly used FM detectors, which is through variations in frequency. It uses a twice-tuned RF transformer to convert frequency changes into amplitude changes.
This is accomplished by the use of resonance networks on the secondary of the transformer, tuned to the frequency of the carrier. If the input equals the carrier frequency, the inductive and capacitative responses should be equal in amplitude and opposite in phase which produces zero voltage on the output. Depending on whether the incoming FM signal varies below or above the carrier, the resultant inductive and capacitive vectors are summed and the result is a voltage proportional to the original phase shift from the carrier.
Foster-Seeley discriminators are sensitive to both frequency and amplitude variations, unlike some detectors. Therefore a limiter must be used which is basically a circuit tuned so that at lower amplitudes it acts as a Class-A amplifier and at higher amplitudes acts like a Class-C amplifier which clips off the peaks.