Amplitude Modulation0.doc

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Amplitude Modulation

→ In amplitude modulation, the amplitude of the carrier is varied inaccordance with the instantaneous amplitude of the message signal.

→ The carrier signal is a high frequency signal.

→ In the process of modulation, the amplitude of the carrier is varied and the

variations are proportional to the amplitude of the message signal, ie.

whenever the modulating signal reaches maximum amplitude, the amplitude

of the carrier is also made higher and whenever the message signal reaches

minimum amplitude, the amplitude of the carrier is also minimum.

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Modulation Index

→ In amplitude modulation, we define a parameter called as modulation index

which is a measure of depth of modulation.

→ Modulation index may be defined as the ratio of modulating signal amplitude to carrier signal amplitude.

ma=Vm / VC

Where, ma = modulation index,Vm = modulating signal amplitude

Vc = carrier signal amplitude.

→ The value of modulation index normally varies between 0

and 1.

i.e. 0< ma < 1.

AM Wave Equation

→ Mathematically we can represent a AM wave by means of the following

equations.

→ Let modulating signal Vm (t) = Vmsin ωm t .

→ Carrier signal Vc(t) = VC sin ωc t.

V(t)=Vc sin ωc t+ maVC/2[COS (ωc - ωm )t+cos (ωc + ωm )t]

The AM wave basically consists of three terms.

→ The first term is the unmodulated carrier term. The second term is called aslower sideband and the third term is called as upper sideband term.

→ In the above equation ma represents modulation index.

→ The percentage modulation of a modulated carrier is determined by dividing

the change in amplitude by the amplitude of the unmodulated carrier,

multiplied by 100

Ie, % modulation = Vm / VC x 100

→ Modulation index may also be defined in terms of the values referred to the

modulated carrier wave and is given as

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→ Where Vmax and Vmin are the maximum and minimum values of the

amplitude of the modulated carrier wave.

Ma= (V max – Vmin) / (V max + Vmin)

Power Relations in Amplitude Modulated Wave.

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→ It is obvious that the carrier component of the amplitude modulated wave

has the same amplitude as the unmodulated carrier.

→ However, two sideband components are also present in the modulated wave.

→ Thus the modulated wave has more power than that had by the carrier wave

before modulation.→ Now total power in the modulated wave is given as

Ptotal = Pcarrier + PLSB + PUSB

Limitations of Amplitude Modulation.

Amplitude modulation suffers from the following drawbacks:

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→ Low Efficiency. In amplitude modulation, the useful power that lies in the

sidebands is quite small, so the efficiency of AM system is low.

→ Limited Operating Range. Transmitters employing amplitude modulation

have small operating range. This is due to low efficiency.

Hence information cannot be transmitted over long distances.→ Noisy Reception. Incase of AM, the reception is generally noisy. This is

because a radio- receiver cannot distinguish between the amplitude

variations that represent noise and those contain the desired signal.

→ Poor Audio Quality. In order to attain high fidelity reception, all audio-

frequencies up to 15 kHz must be reproduced and this necessitates the

bandwidth of 30 kHz while the AM broadcasting stations are assigned

bandwidth of only 10 kHz to minimize the interference from the adjacent

broadcasting stations. Therefore in AM-broadcasting stations audio quality

is usually poor.

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Amplitude Modulation0.doc