Foster-Seeley discriminator

[technoguy]

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.