The chapter begins with the discussion of an FM receiving system that is developed from basic principles. It involves an amplitude limiter and a frequency discriminator. The use of preemphasis and deemphasis to improve system performance is discussed next. Comparison with AM and PM is presented. It is seen that for higher input SNR, FM scores over AM. PM on the other hand is useful in frequency division multiplexing as FM tend to perform worse at higher frequency bands due to its rising noise power in output. Like AM, FM too exhibits threshold which is of higher concern in communication context. Threshold phenomena can be related to occurrence of spikes at low input SNR. A detailed discussion on this is presented. Reduction of threshold remains an important concern in FM communication. A technique that uses basic limiter-discriminator circuit but also a feedback through voltage controlled oscillator is discussed. It is shown how occurrence of spikes is reduced by introduction of this feedback which in turn reduces threshold. MATLAB simulation in Experiment 37 gives a clearer idea of this. The other popular threshold reduction technique using phase locked loop is discussed in next chapter.
