FM (Frequency Modulation) and PM (Phase Modulation) are two types of modulation techniques used in communication systems to encode information in a carrier signal. Both are forms of angle modulation, which means they vary the phase of the carrier signal in proportion to the information signal.
### Frequency Modulation (FM)
**Definition:** FM varies the frequency of the carrier signal in accordance with the amplitude of the information signal.
**How It Works:**
- The carrier signal's frequency changes in proportion to the amplitude of the input signal.
- If the input signal is an audio signal, for instance, the frequency of the carrier signal will increase and decrease in response to the audio signal’s amplitude variations.
- The greater the amplitude of the input signal, the more the carrier frequency deviates from its nominal frequency.
**Applications:**
- FM is widely used in radio broadcasting (FM radio), television sound, and some two-way radio communications.
- It provides good sound quality and is less susceptible to amplitude noise and interference compared to AM (Amplitude Modulation).
**Advantages:**
- Improved noise immunity: FM signals are less affected by noise and interference because noise tends to affect amplitude more than frequency.
- Better signal-to-noise ratio (SNR) compared to AM.
**Disadvantages:**
- Requires more bandwidth than AM to transmit the same information.
### Phase Modulation (PM)
**Definition:** PM varies the phase of the carrier signal in proportion to the amplitude of the input signal.
**How It Works:**
- The phase of the carrier signal changes according to the amplitude of the input signal.
- For example, if the input signal amplitude increases, the phase of the carrier signal shifts more dramatically.
- Unlike FM, which varies the frequency, PM affects the timing of the signal in a way that’s equivalent to shifting its phase.
**Applications:**
- PM is used in some digital communication systems, including certain types of wireless communications and satellite communications.
- It is also used in conjunction with other modulation techniques in complex systems.
**Advantages:**
- Like FM, PM is also less susceptible to amplitude noise, though the primary difference lies in how the modulation affects the signal.
**Disadvantages:**
- More complex to demodulate compared to FM.
- Phase variations might be more sensitive to frequency changes, which can complicate the receiver design.
### Comparison
- **Modulation Parameter:** FM modulates frequency, while PM modulates phase.
- **Bandwidth Requirements:** FM generally requires more bandwidth compared to PM.
- **Noise Immunity:** Both FM and PM offer better noise immunity than amplitude modulation (AM), but FM is more commonly used due to its superior performance in noisy environments.
In summary, FM and PM are essential techniques in communication systems, each with its specific use cases, advantages, and trade-offs.