Limiter
In electronics, a limiter is a circuit that allows signals below a set value to pass unaffected, as in a Class A amplifier, and clips off the peaks of stronger signals that exceed this set value, as in a Class C amplifier.
An FM radio receiver usually has at least one stage of amplification for this purpose. It provides a constant level of signal to the FM demodulator stage, reducing the effect of signal level changes in the output. If two or more signals are received at the same time, a high performance limiter stage can greatly reduce the effect of the weaker signals on the output. This is commonly referred to as the FM capture effect.
Generally, FM demodulators are not affected by amplitude variations, since the baseband is contained in the frequency deviations. Some detectors, like the Ratio Detector, automatically limit gain as a nature of the circuits design. In AM radio, the intelligence is located in the amplitude variations, and distortion can occur due to spurious signals that could cause the baseband to be misrepresented.