Frequency Modulation (FM)
In Frequency Modulation, the frequency of the carrier signal varies in accordance with the message signal's amplitude.
sFM(t) = Ac cos(ฯct + kf∫m(t)dt)
where ฯ = 2ฯf & kf = Frequency Sensitivity
Modulation index, ฮฒ = (kf * Am) / fm
Change the parameter values to see the effect.
Kf (sensitivity):
๐งช Experiment for Students:
"Set the Message Amplitude to 1 and Frequency Sensitivity ($k_f$) to 5. Observe how the wave 'stretches' and 'squeezes'. Next, increase the Message Amplitude to 2.5. Notice that the frequency swings become much wider because the peak frequency deviation ($\Delta f = k_f \cdot A_m$) has increased."
Frequency Modulation Knowledge Check
In FM, what happens to the signal when the amplitude of the message signal increases?
Understanding FM Performance
Modulation Index ($\beta$)
The FM modulation index ($\beta$) is the ratio of frequency deviation to message frequency. Unlike AM, $\beta$ can be much greater than 1. When $\beta$ is small (< 1), it is Narrowband FM (NBFM); larger values result in Wideband FM (WBFM).
Bandwidth (Carson's Rule)
The bandwidth of an FM signal is theoretically infinite, but practically determined by Carson’s Rule: $BW \approx 2(\Delta f + f_m)$. It depends on both the frequency deviation ($\Delta f$) and the highest message frequency ($f_m$).
Noise Immunity
FM provides superior noise performance compared to AM. Since the information is encoded in frequency shifts, the receiver can use a "limiter" to clip off amplitude-based noise and interference without losing the original signal data.
MATLAB Codes for Modulation
View MATLAB Codes for Frequency Modulation (FM) =>Interactive FM Spectrum Analyzer
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