Amplitude Modulation
Amplitude Modulation
In Process of amplitude modulation the amplitude of the carrier is varied in proportional to the instantaneous value of the baseband signal with the frequency and phase of the carrier remaining constant.
In this Process or case the information is being transmitted by the amplitude variation of the carrier.
Equation for Amplitude Modulated Carrier signal
Let us consider the base band signal consisting of a signal frequency component fm given by the equation.
em(t)=Emsinwm(t)
ec(t)=Ecsinwc(t)
We know that in amplitude modulation ,the amplitude of the carrier is varried in proportion to the instantaneous value of the baseband signal.
During amplitude modulation
Ec α Emsinwm(t)
Ec = kEmsinwm(t),
k=proportional const called the amplitude sensitivity of the amplitude modulation
assuming k=1
Ec =Emsinwm(t)
eam(t)=[Ec+Emsinwmt]sinwct
eam(t)=Ecsinwct+Em/2 cos(wc-wm)t-Em/2cos(wc+wm)t
The above equation indicates that the amplitude modulated carrier signal consisting of three frequency components.This type of signal is called AM signal or double side band(DSB) signal.From the above equation we can say that the LSB and USB frequency components are the results of modulation.
em(t)=Emsinwm(t)
Amplitude Modulation
eam(t)=(Ec+Emsinwmt)sinwct
The maximum amplitude of AM signal
Emax=Ec+Em
Emin=Ec-Em
Modulation index or depth of modulation:
Modulation index in AM is defined as the ratio of maximum change in carrier amplitude to original carrier amplitude. It is denoted by ‘m’.
m=Emax-Ec/ Ec
m= Em/ Ec
Bandwidth of AM signal:
Range of frequency: fc-fm to fc+fm
=2fm
=2XBandwidth frequency.
Power in the AM signal Pt=Pc(1+m2/2)
where Pt=Total power of the Amplitude modulated carrier signal.
Pc=unmodulated carrier power
m=modulation index.
Efficiency of transmission in AM:
Since all the useful information is carried by the side band component efficiency of transmission in AM is defined as the ratio of power carried by the side band components to the total power in the AM signal.
η=side band power/total power in the AM signal= m2/(2+m2)
%of efficiency=m2/(2+m2)x100
%of efficiency=33%
That means only 33% of the total power is carried by the sideband components and the remaining 67% is carried by the unmodulated carrier which is a wastage. Because all the information are carried by the sideband components.
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