Class 12th Physics 2017 Set3 Delhi Board Paper Solution
Define the term ‘amplitude modulation’. Explain any two factors which justify the need for modulating a low frequency base-band signal.
Amplitude Modulation is the process in which the amplitude of high frequency carrier wave changes in accordance with the instantaneous value of modulating signal.
Below are the factor justifying the need of modulation:
i. Modulation is also necessary to keep the dimension of receiving antenna within practical limit. Length of antenna bears inverse relation with the frequency of signal it receives. Hence, frequency is kept high to keep anteena length in practical limits.
ii. Size of antenna should be comparable to wavelength for audio frequency signal,
v =15KHz
λ = c/v = 3 x 108 / 15 x 103 = 20000m
⇒ Length of antenna should be = λ/4 = 20000/4 = 5000m, which is practically impossible.
if tranmission frequency is raised to 1 MHz, then,
λ = c/v = 3 x 108 / 106 = 300m
and length of antenna = = λ/4 = 300/4 = 75m,
which is reasonable. Thus there is need of converting information contained in original low frequency base band signal to high frequency setting transmission.
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