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I came across this question which goes

An FM signal with modulation index mf is passed through a frequency tripler. What will be the modulation index at the output of the tripler?

and according to the reference book the answer is 3mf.

Modulation index as we know is the ratio between the frequency deviation and the maximum modulating frequency, both of which are multiplied by the same factor when the signal is fed to say a class C amplifier. So I suppose the modulation index will not be affected by any frequency multiplication since the factors would just cancel each other out. Am I correct or is there something I'm missing out on?

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  • \$\begingroup\$ Why do you think, that modulating frequency is affected by whatever you do with the FM signal? \$\endgroup\$ – Curd May 25 '17 at 11:00
  • \$\begingroup\$ My bad. I confused modulating frequency with the carrier. \$\endgroup\$ – DorkOrc May 25 '17 at 11:07
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    \$\begingroup\$ FM index multiples but FM deviation ratio of carrier of centre f does NOT change since the later does not include input frequency . Modulation index, β, is used to describe the ratio of maximum frequency deviation of the carrier to the maximum frequency deviation of the modulating signal \$\endgroup\$ – Sunnyskyguy EE75 May 25 '17 at 13:09
  • \$\begingroup\$ FM deviation ratio is simply delta f/f \$\endgroup\$ – Sunnyskyguy EE75 May 25 '17 at 13:15
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The baseband modulating frequency WILL NOT be affected when the modulated FM signal is passed through a frequency tripler. The deviation will of course. This means that the modulation index WILL BE multiplied by 3.

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