What is Limiter Circuit in FM circuits

A limiter is a circuit that maintains a constant amplitude for all input signals exceeding a critical threshold. In the context of an FM receiver, its primary role is to eliminate any residual unwanted amplitude modulation and amplitude variations caused by noise. These variations could negatively affect the audio signal if passed through to the speaker. Additionally, the limiting action offers Automatic Gain Control (AGC), as signals between the critical minimum and maximum values provide a steady input level to the detector.

By design, the FM discriminator (or detector) ideally doesn't respond to amplitude variations since the information is encoded in frequency deviation and the rate of that deviation around the center frequency.

The circuit diagram below illustrates a transistor limiter circuit. 

 Notice the dropping resistor, which limits the DC collector supply voltage, resulting in a low DC collector voltage. This makes the stage easily overdriven, which is the desired behavior. When the input signal becomes large enough to cause clipping at both extremes of the collector current, the critical limiting voltage is reached, and the limiting action begins.

The limiter's input/output characteristic is shown in the graph below.

The graph demonstrates the desired clipping behavior. When the clipped signal is fed into an LC tank circuit tuned to the signal’s center frequency, the natural flywheel effect of the tank filters out all frequencies except those near the center frequency, resulting in a clean sinusoidal output. For certain FM demodulator circuits, it may be preferable to omit the LC circuit at the limiter's output. For example, the quadrature detector benefits from using the square-wave-like waveform produced by the limiter.