Comparison of FM and AM
"At the same time of phase modulation, the frequency will definitely change, but the phase will not necessarily change during frequency modulation." The formulation is incorrect. Both frequency modulation and phase modulation are angle modulation, so the phase must change during frequency modulation.
FM can also achieve a very narrow frequency band, the so-called narrowband FM. The bandwidth of narrowband FM is similar to amplitude modulation.
Amplitude modulation: modulating the signal causes the amplitude of the carrier to change; and frequency modulation: changing the frequency or phase accordingly. Transmit-FM, receive-AM: It should be received under certain conditions, but the detection efficiency is not necessarily high. For example: when the receiver (amplitude modulation) loop is in the state of slope detection for the FM signal (see related radio information), the FM signal can be received inefficiently.
Frequency modulation and phase modulation are different. While phase modulation, the frequency will definitely change, but the phase will not necessarily change during frequency modulation.
1. FM has better anti-interference ability than AM
The influence of various external interference, processing industry and sky-electric interference, etc. on the modulated wave is mainly manifested in the generation of parasitic amplitude modulation and the formation of noise. The frequency modulation system can use the method of limiting amplitude to eliminate the parasitic amplitude modulation caused by interference. In the AM system, the amplitude of the amplitude-modulated signal is variable, so amplitude limiting cannot be used, and it is difficult to eliminate external interference.
In addition, the greater the signal-to-noise ratio of the signal, the stronger the anti-interference ability. The signal-to-noise ratio of the signal obtained after demodulation is related to the modulation coefficient. The larger the modulation coefficient, the larger the signal-to-noise ratio. Since the frequency modulation coefficient is much larger than the amplitude modulation coefficient, the signal-to-noise ratio of the FM wave is high, and the interference noise in FM broadcasting is small.
2. FM wave ratio AM wave frequency bandwidth
The frequency bandwidth is related to the modulation coefficient, that is, the modulation coefficient is large and the frequency bandwidth is wide. In FM, the frequency modulation coefficient is often greater than 1, and the amplitude modulation coefficient is less than 1. Therefore, the frequency bandwidth of the frequency modulation wave is much larger than that of the amplitude modulation wave.
3. FM system power utilization is greater than AM system
In the total transmission power, the side frequency power is the effective power of the transmitted modulation signal, and the side frequency power is related to the modulation coefficient, the modulation coefficient is large, and the side frequency power is large. Since the frequency modulation coefficient mf is greater than the amplitude modulation coefficient ma, the power utilization rate of the frequency modulation system is higher than that of the amplitude modulation system.
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