Signals with very low frequency (for example, 50HZ) will be greatly attenuated in antenna propagation, and it is impossible to reach the required antenna length. Therefore, it should be added to a higher frequency for propagation (waves with cos function). For example, the carrier frequency of our mobile phone is 900MHZ and 1800MHZ. The common modulation methods are amplitude modulation, frequency modulation and phase modulation. The simplest modulation is