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AM vs FM superheterodyne receiver; AGC

Short answer

AM receivers operate MW band, IF 455 kHz, envelope detector, no limiter; FM receivers operate VHF 88-108 MHz, IF 10.7 MHz, use limiter and FM detector (discriminator/ratio detector), with de-emphasis. AGC adjusts RF/IF gain using a DC voltage from the detector to prevent overload from strong signals, compensate fading, and maintain constant output volume.

List the major differences between AM and FM superheterodyne receiver. Why are superheterodyne receivers provided with automatic gain control (AGC) mechanism?

Differences between AM and FM superheterodyne receivers

PointAM receiverFM receiver
RF bandMW/SW, 0.54–1.65 MHz (MW)VHF, 88–108 MHz
Intermediate frequency455 kHz10.7 MHz
IF bandwidthabout 10 kHzabout 200 kHz
LimiterNot usedUsed before detector to remove amplitude noise
DetectorEnvelope (diode) detectorDiscriminator, ratio detector or PLL
De-emphasisNot usedDe-emphasis network (75 μs) after detector
Gain controlAGCAFC common; limiter does most amplitude control
Noise immunityPoorGood (capture effect, limiter)

Why AGC is used

Automatic gain control (AGC) automatically adjusts the gain of the RF and IF amplifiers according to the strength of the received signal, using a DC voltage taken from the detector output.

It is needed because:

  1. Signals from near and far stations differ widely in strength (microvolts to hundreds of millivolts). Without AGC, the volume changes when tuning between stations.
  2. Fading makes the strength of one station vary with time; AGC keeps the output level nearly constant.
  3. Strong signals would overload the RF/IF stages and the detector and cause distortion; AGC reduces gain for strong signals.
  4. Weak signals get full gain, which improves sensitivity.

✓ Answer checked by Claude Opus 5.5 (high effort)

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