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An FM modulator operates at carrier frequency of 250 kHz with frequency deviation sensitivity of 1.5 kHz/V. A PM modulator operates at carrier frequency of 500 kHz with phase deviation sensitivity of 1.5 rad/V. If both FM and PM modulators are modulated by the same modulating signal having peak amplitude of 5 V and modulating frequency of 5 kHz, then what is the relationship between frequency modulation index and phase modulation index?

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  1. PM = FM
  2. PM = 2FM
  3. PM = 4FM
  4. PM = 5FM

Answer (Detailed Solution Below)

Option 4 : PM = 5FM
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Detailed Solution

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Given that

FM modulator

fc 

KF =

PM modulator 

fc

KP  =

Peak amplitude = 5V.

fm = 5KHz

Frequency Modulation index (hf):

Phase modulation index (hp):

pm =   = = 1.5

pm = kp Am = 

pm =5 fm

Hence, PM = 5FM.

Here, option 4 is correct.

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