Foster–Seeley discriminator

Lore & Background

The Foster–Seeley discriminator is a type of FM detector circuit that was invented in 1936 by Dudley E. Foster and Stuart William Seeley. It was originally designed for automatic frequency control in receivers, but it also became useful for demodulating FM signals. The circuit works by using a tuned RF transformer to turn changes in frequency into changes in amplitude. This transformer, tuned to the carrier frequency, connects to two rectifier diodes in a configuration that resembles a full bridge rectifier.

The voltage phase at the secondary coil shifts depending on whether the carrier frequency is above or below the resonance point, producing either a positive or negative shift. The circuit relies on the nearly 90-degree phase difference that occurs between the voltages in two loosely coupled resonant circuits at the peak frequency. Through a coupling capacitor, the primary voltage reaches the center tap of the secondary coil, creating a sum voltage on the top terminal and a difference voltage on the bottom terminal. As the phase relationship of the input signal changes, the amplitude of the sum or difference at the upper or lower half becomes higher, which alters the voltage across the output capacitors. A choke, sometimes replaced by a resistor, provides a DC path through the rectifier diodes.

The demodulator's bandwidth depends on the Q factor of the resonant circuit, and the phase response of the secondary—and thus the circuit’s voltage response—to frequency changes forms an S-curve. Unlike some detectors, Foster–Seeley discriminators are sensitive to both frequency and amplitude variations. Because of this, a limiter amplifier stage must be placed before the detector to remove amplitude variations in the signal, which would otherwise be detected as noise. The limiter operates as a class-A amplifier at lower amplitudes and becomes a saturated amplifier at higher amplitudes, clipping the peaks and limiting the amplitude.

Reader's Guide

The Foster–Seeley discriminator is notable for its sensitivity to both frequency and amplitude variations, unlike some detectors. Because of this sensitivity, a limiter amplifier stage must be used before the detector to remove amplitude variations in the signal that would otherwise be detected as noise. The limiter acts as a class-A amplifier at lower amplitudes and becomes a saturated amplifier at higher amplitudes, clipping off peaks and limiting the amplitude. The demodulator's bandwidth depends on the Q factor of the resonant circuit, and the phase response of the secondary to frequency deviation is an S-curve. This circuit was invented in 1936 by Dudley E. Foster and Stuart William Seeley, originally for automatic frequency control of receivers, but it also found application in FM demodulation. Its design uses a tuned RF transformer and rectifier diodes in a configuration resembling a full bridge rectifier.

Did You Know?

• It uses a tuned RF transformer to convert frequency changes into amplitude changes.
• The circuit requires a limiter amplifier stage before it to remove amplitude variations.