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Modulation Indices for AM, FM, and PM


Modulation index for Amplitude Modulation (AM)

Modulation index in amplitude modulation (AM),

m = Am/Ac

where, Am is the amplitude of the message signal

and     Ac is the amplitude of the carrier signal

 

Modulation index for Frequency Modulation (FM)

Modulation index for FM,
ฮฒ = kf.Am/fm

where, kf is the frequency sensitivity factor or constant

           Am is the amplitude of the message signal

           fm  is the frequency of the message signal

For Eample, when the value of kf = 2 and Am = 1 & fm = 1 Hz, then frequency deviation is 2 radians.

Then the modulation index for FM will be 2

 

Modulation index for Phase Modulation (PM)

Modulation index in phase modulation (PM),
ฮฒ = kp.Am = Phase deviation

where, kp is the phase sensitivity factor or constant

           Am is the amplitude of the message signal

For example, when the value of kp = 2 and Am = 1, then phase deviation is 2 radians. Then the modulation index for PM will be 2

 

We know for AM signal, 
Available bandwidth = 2*fm = (fc + fm) - (fc - fm)
where, fm = frequency of the message signal or modulating signal

On the other hand, modulation index = Em / Ec

where, Em is equal to amplitude of modulating signal and Ec denotes amplitude of carrier signal. 


Modulation index is calculated differently for frequency modulation (FM)

Where modulation index = (frequency deviation) / frequency of modulating signal
 Bandwidth = 2*modulating signal (modulation index + 1)

 and for phase modulation (PM), the maximum change in the phase of the carrier signal in accordance with the highest amplitude of the message signal is termed as modulation index.

Q. In AM modulation, if modulation index increases what happens to the bandwidth, transmission power, and noise
A. So, if modulation index increases then there is no change in Bandwidth while the frequencies of modulating signal and carrier signal remain same.

Now, we're coming to the transmission power part,
Pt = Pc (1 + (modulation index)^2)/2 )
where, Pt = transmitted signal power
Pc = power of the carrier signal
As a result, as the modulation index rises, so does the transmission power.

Bandwidth of ASK, FSK, and PSK
Bandwidth of ASK = 2*Rb
Bandwidth of FSK = 2*Rb + f1-f2
Bandwidth of FSK = 2*Rb
where, Rb = bit rate


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