When we transmit a signal, whether it is an optical signal or an electrical signal or a wireless signal, if it is transmitted directly, the signal is susceptible to noise interference, and it is difficult to obtain the correct information at the receiving end. In order to improve the anti-interference ability of the system, it can be realized by modulating the signal. Modulation can also improve the channel utilization rate, so modulation has a great impact on the effectiveness and reliability of the communication system.
The Angle modulation described below is for analog signals.
Sinusoidal carrier has three parameters: amplitude, frequency and phase. We can load the information of the modulated signal not only in the amplitude change of the carrier, but also in the frequency or phase change of the carrier. During modulation, if the frequency of the carrier changes with the modulated signal, it is called frequency modulation or frequency modulation (FM); If the phase of the carrier changes with the modulated signal, it is called phase modulation or phase modulation (PM). In these two modulation processes, the amplitude of the carrier remains constant, while the change in frequency and phase is manifested as the change in the instantaneous phase of the carrier, so the frequency modulation and phase modulation are collectively referred to as Angle modulation.
The difference between Angle modulation and amplitude modulation is that the modulated signal spectrum is no longer a linear shift of the original modulated signal spectrum, but a nonlinear transformation of the spectrum, which will produce new frequency components different from the spectrum shift, so it is also called nonlinear modulation.
Both FM and PM are widely used in communication systems. FM is widely used in high-fidelity music broadcasting, TV sound signal transmission, satellite communication and cellular telephone system. In addition to being used directly for transmission, PM is also commonly used as a transition to indirectly generate FM signals. There is a close relationship between frequency modulation and phase modulation.
Compared with amplitude modulation, the most prominent advantage of Angle modulation is its high anti-noise performance. However, there is a trade-off between gains and losses, and the cost of this advantage is that Angle modulation occupies a wider bandwidth than amplitude modulated signals.
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