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Solved Examples · Example 19

Q.Determine the carrier swing and carrier frequency of a frequency modulated signal which is modulated by a 5 kHz sine wave reaches a maximum frequency of 100.03 MHz and minimum frequency of 99.97 MHz.

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[!TLDR]

The carrier swings over fmax−fmin=60 kHzf_{max}-f_{min}=60\ \text{kHz}, and its rest (centre) frequency is the mean of the extremes, fc=100 MHzf_c = 100\ \text{MHz}.

When an FM carrier is modulated, its frequency moves between a maximum fmaxf_{max} and a minimum fminf_{min}. The carrier swing is the total range of this movement:

CS=fmax−fmin=(100.03−99.97)×106=0.06×106=60 kHzCS = f_{max}-f_{min} = (100.03-99.97)\times10^{6} = 0.06\times10^{6} = 60\ \text{kHz}

Because the carrier deviates symmetrically about its unmodulated (rest) value, the carrier or rest frequency is the average of the two extreme frequencies: …

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