The maximum deviation allowed in an FM broadcast system is 75 kHz. If the modulating signal is a single tone sinusoid of 10 kHz, find the bandwidth of the FM signal. What will be the change in the bandwidth, if the modulating frequency is double? Determine the bandwidth when modulating signal amplitude is also double.
Given that ∆f=75KHz and fm=10KHz
According to the Carson’s thumb rule the bandwidth
BW=2(∆f+fm )=2(75+10)=170KHz
The frequency deviation ∆f=kf Am
Then Bandwidth BW=2(kf Am+fm )
Modulation index β=∆f/fm =75/10=7.5
If the modulating frequency is double then the bandwidth
BW=2(∆f+2×fm )=2(75+2×10)=190KHz
Hence the frequency band (BW) is enhanced by (190-170)=20KHz
If the signal amplitude is doubled with normal modulating frequency then
BW=2(2kf Am+fm )
BW=2(2∆f+fm )=2(2×75+10)=320KHz
The bandwidth is enhanced by (320-170)=150KHz
Explain with suitable block diagram the generation of FM signal using Armstrong method.
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