Foster-Seeley Discriminator

The Foster-Seeley discriminator extracts audio from FM signals using phase shifts instead of simple amplitude slopes. High-quality analog receivers rely on this circuit for low distortion.

Core Concept

The circuit uses a special tuned transformer. The primary and secondary windings are coupled to produce specific phase shifts. The Foster-Seeley discriminator relies entirely on these varying phase shifts.

At the unmodulated 10.7 MHz center frequency, both diodes receive exactly equal voltages. Their rectified outputs cancel out perfectly to zero volts. This establishes the center point of the S-curve.

When the frequency shifts, the phase relationship changes. One diode conducts more than the other, producing a net positive or negative output voltage. This perfectly reconstructs the original audio waveform.

Key Equations

Output voltage formula: Vout = VD1 - VD2

Primary-secondary phase shift: θ = 90 ± Δθ

Here, VD1 and VD2 are the rectified DC voltages from the top and bottom 1N4148 diodes. The phase angle θ rests at exactly 90 degrees during the unmodulated state.

Given:
Diode 1 output voltage VD1 = 12 V
Diode 2 output voltage VD2 = 9 V

Why this formula:
The discriminator subtracts the rectified voltages from the two secondary branches.

Formula:
Vout = VD1 - VD2

Substitution:
Vout = 12 - 9

Calculation:
Vout = 3

Final Answer:
The final audio output voltage is 3 V.

Exam Tip: Foster-Seeley circuits respond heavily to amplitude variations too. They absolutely require a separate limiter circuit before them to strip off AM noise.

Key Properties

• Provides excellent linearity over a very wide frequency range.
• Relies on a doubly-tuned transformer to create a 90-degree phase shift.
• Both primary and secondary tanks are tuned to the exact same 10.7 MHz frequency.
• Suffers from zero inherent immunity to amplitude modulation noise.
• Uses two matched diodes working in opposite polarity.

Quick Revision

• Foster-Seeley depends on phase shift changes.
• Center frequency yields zero output volts.
• Diodes subtract their rectified outputs.
• Requires a dedicated limiter stage.
• Provides excellent audio fidelity.
• Exam trap: Assuming the primary and secondary are tuned to different frequencies.