Foster–Seeley discriminator

The Foster–Seeley discriminator[1][2] is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster[3] and Stuart William Seeley.

The Foster–Seeley discriminator was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.

The phase of the voltage at the secondary coil depends on whether the carrier is below or above the resonance, resulting in a positive or negative shift, respectively.

phase difference occurring between the voltages in two loosely coupled[4] resonant circuits at the peak frequency.

, the primary voltage is applied to the center tap of the secondary, producing a sum and a difference

, the amplitude of the sum/difference at the upper or lower half, respectively, will be higher, which results in a change of the voltage across the output capacitors.

Foster–Seeley discriminators are sensitive to both frequency and amplitude variations, unlike some detectors.

Therefore a limiter amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as noise.