先决条件–调制
1.调频：
调频是一种调制，其中载波的频率根据保持相位和幅度恒定的调制信号的瞬时幅度而变化。修改载波频率以发送数据或信息。它不能长距离传输，范围较小。其调制指数始终大于1。在调频中，幅度和相位保持不变。
2.相位调制：
相位调制是一种调制，其中载波的相位根据保持振幅和频率恒定的调制信号的瞬时振幅而变化。相位调制和频率调制相似，但是在相位调制中，载波信号的频率没有增加。相位调制用于移动系统中。修改载波的相位，以便发送数据或信息。调频和调相之间的区别：
| S.NO. | Frequency Modulation | Phase Modulation |
|---|---|---|
| 1. | In Frequency Modulation amplitude and phase remain the same. | In Phase Modulation, the frequency and amplitude remain the same. |
| 2. | Frequency Modulation is proportional to modulating voltage. | Phase Modulation is proportional to modulating voltage. |
| 3. | Associated with the change in frequency, there is some phase change. | Associated with the change in phase, there is some frequency change. |
| 4. | It is possible to receive FM on a PM receiver. | It is possible to receive PM on a FM receiver. |
| 5. | Noise immunity is poor than AM and PM. | Noise immunity is better than AM but worst than PM. |
| 6. | Signal to noise ratio is better than in phase modulation. | Signal to noise ratio is poor than in frequency modulation. |
| 7. | Frequency Modulation is widely used. | Phase Modulation is used in mobile system. |
| 8. | In FM, the frequency derivation is proportional to the modulating voltage only. | In PM, the frequency derivation is proportional to the modulating voltage as well as modulating frequency. |
| 9. | Amplitude of FM wave is constant. | Amplitude of PM wave is also constant. |
| 10. | In FM, received signal is of high quality. | In PM, received signal is of low quality. |