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 Foster–Seeley 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.
The circuit makes use of the
90\circ
Through the coupling capacitor
Ck
Vcenter\plusmn
| Vsecondary | |
| 2 |
Depending on the changing phase relationship of the input signal —
Vprimary
Vcenter
Vsecondary
L3
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
\Delta{f}
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. 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.