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How is frequency deviation calculated for a FM wave? Suppose we have a same carrier and a sinusoidal message signal of frequency 5Hz and maximum amplitude of 5V and another sinusoidal message signal of frequency 5Hz but amplitude of 10V will frequency deviation be different or same in both case?

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2 Answers 2

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Frequency deviation is proportional to the amplitude of the modulating signal. So the deviation produced by your 10 V signal will be twice that produced by the 5 V signal.

does that mean maximum frequency deviation of a carrier is proportional to the maximum value of amplitude of message signals the carrier can carry whatever maybe the frequency of message signals?

A practical system will have a maximum permitted frequency deviation and a maximum permitted frequency for the modulating (message) signal. Within those limits then yes.

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  • \$\begingroup\$ does that mean maximum frequency deviation of a carrier is proportional to the maximum value of amplitude of message signals the carrier can carry whatever maybe the frequency of message signals? \$\endgroup\$ Nov 20, 2020 at 12:07
  • \$\begingroup\$ @user55092 Reply added to answer. \$\endgroup\$
    – Graham Nye
    Nov 20, 2020 at 12:45
  • \$\begingroup\$ that means of if I don't change my frequency of my message signal but I change its amplitude it will increase its modulating index and hence will decrease the amplitude of the central tone but will increase the amplitude of the side bands of the same message signal? \$\endgroup\$ Nov 20, 2020 at 13:34
  • \$\begingroup\$ Yes, for specific values of mi as tabulated e.g. here in the carrier column. \$\endgroup\$
    – Graham Nye
    Nov 20, 2020 at 15:10
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One useful equation is

  • Phase Deviation == FrequencyDeviation / Frequency_of_Modulation

Thus if your frequency deviation is 10,000 Hertz, and the Modulation rate is 100Hz,

then the phase_change is 100 radians.

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