The function of modulation is to move the frequency spectrum of the modulation information from low frequency to high frequency, so as to be suitable for channel transmission. The principle of modulation has already been discussed in the previous section. In this section, the demodulation principle of FM is mainly explained. Corresponding to modulation, the demodulation process is required at the receiving end to restore the modulated information, so demodulation is an important technology that affects the performance of the communication system.
The demodulation method is divided into non-coherent demodulation and coherent demodulation according to the standard of whether carrier recovery is required. Coherent demodulation needs to extract and restore the carrier information through the phase-locked loop during demodulation, and demodulate the modulation information through the recovered carrier signal and the input signal. Non-coherent demodulation, as the name suggests, means that the carrier information does not need to be extracted for demodulation during demodulation. In terms of demodulation performance, coherent demodulation performance is better than non-coherent demodulation, so coherent demodulation is widely used, and the phase-locked loop is a key component in the coherent demodulation system. In terms of hardware implementation complexity, coherent demodulation requires the extraction of coherent carriers, so the circuit is complex and difficult to implement; correspondingly, non-coherent demodulation circuits are easier to implement, so they have practical applications in many occasions.
1 PCM/FM demodulation principle