FM superheterodyne radio receiver

An FM superheterodyne receiver converts the received FM station (88–108 MHz) to a fixed intermediate frequency of 10.7 MHz, amplifies and limits it, and then recovers the audio with a frequency discriminator.

Block diagram

Ant
 |
[RF amp]->[Mixer]->[IF amp]->[Limiter]->[Discri-]
88-108M      ^     10.7 MHz             [ minator]
 |           |     BW 200k                  |
 |      [Local osc]<------AFC---------------+
 |       fLO=fs+10.7M                       |
 +--ganged--+               [De-emph]<------+
                               |
                            [AF amp]-->Spk

Working of each block

1. RF amplifier: tuned to the wanted station $f_s$. It gives gain at a low noise figure (important at VHF), improves sensitivity and rejects the image frequency $f_s + 2(10.7)$ MHz.
2. Local oscillator and mixer: the oscillator is ganged with the RF tuning so that $f_{LO} = f_s + 10.7$ MHz. The mixer produces sum and difference frequencies; the difference, 10.7 MHz, is selected. The frequency deviation (up to ±75 kHz) is unchanged by mixing.
3. IF amplifier: several fixed-tuned stages at 10.7 MHz with a bandwidth of about 200 kHz (Carson bandwidth for $\Delta f = 75$ kHz, $W = 15$ kHz is 180 kHz). Provides most of the gain and the selectivity.
4. Amplitude limiter: clips the IF signal to a constant amplitude, removing amplitude variations caused by noise, interference and fading. The information is in the frequency, so nothing useful is lost. This is the main reason FM is less noisy than AM.
5. Discriminator (FM detector): converts frequency variations into voltage variations, e.g. a Foster–Seeley discriminator, ratio detector, slope detector or PLL. Output is proportional to $k_fm(t)$.
6. De-emphasis network: an RC low-pass filter with 75 µs (or 50 µs) time constant. It undoes the transmitter's pre-emphasis and reduces high-frequency noise.
7. AF amplifier and speaker: amplifies the audio to drive the loudspeaker. In stereo sets a stereo decoder sits between discriminator and AF stage.
8. AFC (automatic frequency control): the DC part of the discriminator output shows any mistuning; it is fed back to a varactor in the local oscillator to keep the IF centred at 10.7 MHz.

Differences from AM superheterodyne

Example: for a station at 100 MHz, $f_{LO} = 110.7$ MHz and the image is at 121.4 MHz, outside the FM band.