The FosterâSeeley discriminator [1] [2] is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster [3] and Stuart William Seeley. The FosterâSeeley discriminator was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.
The FosterâSeeley discriminator uses a tuned RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency, is connected to two rectifier diodes. The circuit resembles a full bridge rectifier.
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.
Through the coupling capacitor , which introduces its own phase shift (ideally presumed to be ), the primary voltage is applied to the center tap of the secondary, producing a sum and a difference on the top and the bottom terminals thereof, respectively.
Depending on the changing phase relationship of the input signal â (and therefore ) with , the amplitude of the upper or lower half will be higher, which results in a change of the voltage across the output capacitors. The 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; the phase response of the secondary (and therefore, the voltage response of the circuit) to is an S-curve.
FosterâSeeley 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. The limiter acts as a class-A amplifier at lower amplitudes; at higher amplitudes it becomes a saturated amplifier which clips off the peaks and limits the amplitude.