Frequency Modulation (FM) and Phase Modulation (PM) are two popular modulation techniques used in analog and digital communication systems. While both FM and PM involve modulating the characteristics of a carrier signal to encode information, there are differences between the two techniques. In this blog post, we will discuss the similarities and differences between FM and PM and make comparison between them.
Similarities between FM and PM
- Both FM and PM are types of angle modulation, where the angle of the carrier signal is modulated to encode information.
- Both FM and PM provide a higher signal-to-noise ratio than amplitude modulation(AM) techniques, making them more immune to noise. See Frequency Modulation vs Amplitude Modulation.
- Both FM and PM can be used to transmit analog and digital information.
Differences between FM and PM
- Encoding Information: In FM, the frequency of the carrier signal is modulated in proportion to the amplitude of the modulating signal, whereas in PM, the phase of the carrier signal is modulated in proportion to the amplitude of the modulating signal. There are many methods of generating FM signal such as FM generation using VCO.
- Bandwidth: FM signals have a wider bandwidth than PM signals. This is because frequency variations in FM signals are proportional to the amplitude of the modulating signal, while in PM, phase variations are proportional to the amplitude of the modulating signal. Therefore, the frequency variations in FM signals are greater than the phase variations in PM signals.
- Distortion: FM signals are more prone to distortion due to amplitude variations in the signal, while PM signals are more prone to distortion due to frequency variations in the signal.
- Applications: FM produced by FM transmitter is commonly used in applications such as radio broadcasting, music recording, and navigation systems, while PM is commonly used in applications such as radar, phase-shift keying (PSK) in digital communication systems, and optical communication.
- Modulation Index: In FM, the modulation index is defined as the ratio of the frequency deviation to the maximum frequency of the modulating signal. In PM, the modulation index is defined as the ratio of the phase deviation to the maximum phase deviation of the modulating signal.
Comparison between FM and PM
FM PM Modulation index is proportional to both modulating voltage and frequency:\(\beta_f = \frac{A_m f_m}{f_m}\) Modulation index is proportional to only modulating voltage:\(\beta_p = \frac{A_m k_f}{f_m}\) Frequency deviation is proportional to modulating voltage Phase deviation is proportional to modulating voltage Associated with carrier frequency variation, there is also change in carrier phase Associated with carrier phase variation, there is also change in carrier frequency FM signal can be received by PM receiver PM signal can be received by FM receiver FM SNR is better than PM SNR PM SNR is poorer than FM SNR FM has better noise immunity than PM and AM M has bit poorer noise immunity than FM but better noise immunity than AM FM wave equation:
\( s_{fm}(t) = A_c cos[2 \pi f_c t + 2 \pi k_f \int_{0}^{t}m(t) dt] \)PM wave equation:
\( s_{pm}(t) = A_c cos[2 \pi f_c t + k_p m(t)] \)
In conclusion, FM and PM are two popular modulation techniques used in analog and digital communication systems. While they share some similarities, they have distinct differences in terms of encoding information, bandwidth, distortion, applications, and modulation index. Understanding the differences and similarities between FM and PM is essential in choosing the appropriate modulation technique for a given application.
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