FosterSeeley

The Foster–Seeley discriminator is a type of frequency discriminator used in FM demodulation, commonly found in mid-20th century radio receivers. It converts frequency deviations of an FM signal into a corresponding dc output voltage, which can then be amplified and heard as audio. The circuit is typically fed by the limiter stage of an FM receiver, which preserves constant amplitude while preserving frequency information.

The core of the Foster–Seeley discriminator is a center-tapped transformer secondary connected to two diodes. The ends of the secondary feed the diodes, whose outputs are rectified and then combined through a load network consisting of resistors and often a filter capacitor. The DC output voltage depends on the instantaneous phase relationship between the two half-wave rectified signals, which varies with the input frequency deviation from the carrier. By balancing the transformer and choosing appropriate component values, the average output becomes roughly proportional to frequency deviation over a usable range, providing the audio signal after further processing.

Advantages of the Foster–Seeley design include relative simplicity and the use of readily available components, making it a popular choice in early FM receivers. Its performance, however, can be sensitive to amplitude variations, transformer coupling, and temperature drift, requiring careful alignment and stabilization. The discriminator is closely related to other limiter-discriminator approaches and is often discussed alongside ratio detectors and slope-type discriminators as part of historical FM demodulation methods.

See also: FM demodulation, ratio detector, slope detector.