先决条件 – 调制
1. 频率调制：
频率调制是一种调制，其中载波的频率根据调制信号的瞬时幅度而变化，保持相位和幅度不变。修改载波的频率以发送数据或信息。它不能长距离传输，范围更小。它的调制指数总是大于一。在频率调制中，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调制信号的瞬时幅度变化，保持幅度和频率恒定。 Phase Modulation 和Frequency Modulation 类似，但在Phase Modulation 中，载波信号的频率没有增加。相位调制用于移动系统。修改载波的相位以发送数据或信息。调频和调相的区别：
| 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. |