Short note: FM radio receiver

An FM broadcast receiver is a superheterodyne receiver working in the 88–108 MHz band with an IF of 10.7 MHz. It differs from an AM receiver mainly in the limiter, the FM detector and the de-emphasis network.

Ant
 |  +----+  +-----+  +-------+  +---------+  +-------+
 +->| RF |->|Mixer|->|  IF   |->| Limiter |->|  FM   |
    |amp |  |     |  |10.7MHz|  |         |  | det.  |
    +----+  +-----+  +-------+  +---------+  +-------+
              ^                                  |
         +---------+    +-------+  +-------+     v
         | Local   |<---|  AFC  |  | Audio |<-[De-emph]
         |  osc.   |    +-------+  |  amp  |-> Speaker
         +---------+               +-------+

Blocks:

• RF amplifier: selects and amplifies the weak VHF signal; improves image rejection and noise figure.
• Mixer and local oscillator: convert every station to the fixed IF of 10.7 MHz ($f_{LO} = f_s + 10.7$ MHz).
• IF amplifier: provides most of the gain and selectivity, with a bandwidth of about 200 kHz.
• Limiter: clips the IF signal to a constant amplitude, removing amplitude noise and interference.
• FM detector: discriminator, ratio detector or PLL converts frequency variations into audio voltage.
• De-emphasis (75 μs RC network): attenuates high audio frequencies that were boosted by pre-emphasis at the transmitter, which reduces high-frequency noise.
• AFC: a DC voltage from the detector corrects drift of the local oscillator.
• Audio amplifier and speaker: reproduce the sound. In stereo receivers a stereo decoder is placed after the detector.