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Power content of frequency modulation and phase modulation

Nasser M. Abbasi

September 6 2009 compiled on — Wednesday July 06, 2016 at 08:26 AM
This demonstration illustrates frequency modulation (FM) and phase modulation (PM) using one tone sinusoidal as the modulating signal.

For FM, the modulating signal m (t) is defined as

m (t) = Am cos(2πfmt )

And for PM

m (t) = Am sin(2πfmt )

where f
 m  is the signal frequency in Hz and A
 m  is its amplitude. This definition for m(t)  is used to simplify determining the spectra of the modulated carrier s(t)  by the use of BesselJ functions.

With m (t)  defined as above, the modulated carrier s(t)  can now be defined as

s(t) = Accos(2πfct+ βm (t))

where β  is the modulation index, fc  is the carrier frequency in Hz and Ac  is the carrier amplitude.

For FM modulation,

β =  fm

where K is the deviation constant in Hz per volt, and for PM modulation,

β = kAm

where K  is in radians per volt. The above units for β  assumes that the unit of m (t)  is volts.

The bandwidth of the modulated carrier s(t)  is defined as approximately 2(1 + β)f
         m  . This bandwidth contains 98  small β  the modulated carrier spectra become a narrow band and for a large β  the spectra becomes wideband.

The parameters Am, Ac, fm,fc,k  can be adjusted and the effect on the spectra of the modulated carrier can be observed.

The demonstration also calculates and plots the power content (normalized) of the modulated carrier as a function of the bandwidth. This is also called the power ratio, and defined as

            2    ∑              2
BesselJ(0,β) + 2    BesselJ(n,β)

where BesselJ is Bessel function of the first kind, and M is the number of sidebands on each side of the carrier frequency fc  .

This plot is useful in the design of FM and PM modulators as it allows one to determine the size of the bandwidth needed for a given power ratio.