Side-by-side comparison
| Parameter | AM | FM Modulation |
|---|---|---|
| Information encoding | Amplitude of carrier varies with message | Instantaneous frequency of carrier varies with message |
| Carrier frequency range | 535 kHz – 1.7 MHz (AM broadcast band) | 88–108 MHz (FM broadcast band) |
| Bandwidth (audio) | 2W_m = 2×5 kHz = 10 kHz per channel | 2(Δf + W_m) = 2(75+15) kHz = 180 kHz (commercial FM) |
| Noise performance | Poor — amplitude noise directly corrupts signal | Good — amplitude limiting removes noise before detection |
| Modulation index | m_a = A_m/A_c; range 0 to 1 (over-mod if > 1) | β = Δf/f_m; can be >> 1 (wideband FM) |
| Power efficiency | Useful signal in sidebands only; carrier wastes 2/3 power at m=1 | All transmitted power carries information |
| Demodulator circuit | Envelope detector (diode + RC); simple | PLL or FM discriminator (ratio detector); complex |
| Capture effect | No capture effect; both stations heard | Stronger signal captures receiver; weak signal suppressed |
| Transmitter complexity | Low — linear amplifier, simple modulator | Higher — needs wideband VCO or phase modulator |
| Typical SNR (broadcast) | ~30–40 dB (AM broadcast) | ~50–60 dB (wideband FM with pre-emphasis) |
Key differences
AM encodes information in amplitude, making it vulnerable to any amplitude disturbance — atmospheric noise directly degrades SNR. FM encodes in frequency; a limiter removes amplitude noise before demodulation, giving FM an SNR advantage of 3β²(β+1) over AM for wideband FM. Commercial FM uses Δf = 75 kHz and 15 kHz audio, giving β = 5 and 180 kHz bandwidth per channel versus 10 kHz for AM. AM demodulation is trivially simple (one diode); FM requires a discriminator or PLL, adding hardware cost. The capture effect in FM means a signal just 3 dB stronger than a co-channel interferer wins completely.
When to use AM
Use AM when receiver simplicity and very long transmission range are the priority — medium-wave AM covers hundreds of kilometres at night and the receiver is as simple as a crystal radio.
When to use FM Modulation
Use FM for high-fidelity audio broadcasting or two-way voice communication where noise immunity and audio quality matter more than bandwidth, such as commercial FM radio at 98.1 MHz or VHF aircraft communication.
Recommendation
For any audio application where SNR and fidelity matter, choose FM — the 3 dB capture effect and limiter action make it dramatically quieter than AM in real-world noise. Use AM only when you need simplest possible receivers or maximum propagation range on HF bands.
Exam tip: GATE problems on AM ask for power in sidebands and carrier: for m=1, P_total = P_c(1 + m²/2) = 1.5P_c; sideband power is only P_c/2 — know this ratio and always verify your modulation efficiency calculation.
Interview tip: Interviewers expect you to explain the FM noise advantage quantitatively: output SNR for wideband FM exceeds AM SNR by the factor 3β²(β+1), and for β=5 that is a 22.5 dB improvement.