FM Demodulation
FM transmission encodes audio in frequency variations. Demodulation does the reverse: it extracts the audio from those frequency changes. Several elegant circuits accomplish this — each with different tradeoffs.
What We Need to Recover
An FM signal's amplitude is constant — the information is in its instantaneous frequency. To demodulate, we must convert frequency variations into amplitude variations.
Slope Detector
A tuned circuit has a slope in its frequency response. Detune it slightly: now different frequencies produce different amplitudes. Apply an envelope detector — you have FM demodulation.
Foster-Seeley Discriminator
Two detuned circuits, one above and one below center frequency. Their outputs are subtracted: at center frequency, they cancel (zero output). As frequency shifts, one dominates — the difference is the audio.
Ratio Detector
A variant of the Foster-Seeley with an added capacitor that rejects amplitude variations. Since FM noise often appears as amplitude variations, this gives natural noise rejection — no limiter needed.
Phase-Locked Loop
A Voltage-Controlled Oscillator (VCO) tracks the FM signal's frequency. The control voltage needed to keep the VCO locked is exactly the original audio signal.
Why the PLL Works
The VCO frequency is proportional to its control voltage. When locked, the VCO output matches the FM signal's instantaneous frequency — so the control voltage directly tracks the message signal.
Digital FM Demodulation
Software-defined radios digitize the FM signal first. Demodulation becomes simple arithmetic: compute the phase difference between successive samples.
FM Receiver Block Diagram
- BPF — selects desired station
- LNA — low-noise amplifier
- Limiter — removes AM noise before demod
- De-emphasis — undoes pre-emphasis from transmitter
The Capture Effect
When two FM signals arrive at the same frequency, the stronger one wins completely. The limiter suppresses the weaker signal. A 3 dB power advantage is enough for full capture. AM has no such protection.
Key Takeaways
- FM demodulation converts frequency → amplitude
- Slope detector: simple but nonlinear
- Foster-Seeley: better linearity, needs limiter
- Ratio detector: natural AM rejection
- PLL demodulator: elegant, widely used in ICs
- Digital: just compute phase difference between samples
- Limiter + FM demod gives the capture effect
FM Mastered
You now understand FM — concept, math, bandwidth, and demodulation, and you have seen how a PLL elegantly recovers the original audio. One lesson left in Module 4: putting AM and FM side by side, with every number on the table.