In a digital communication system employing Frequency Shift Keying (FSK), the 0 and 1 bit are represented by sine waves of 10 kHz and 25 kHz respectively. These waveforms will be orthogonal for a bit interval of

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This question was previously asked in

ESE Electronics 2014 Paper 2: Official Paper

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Answer 1

Derivation:

Given, In FSK modulation, let f1, f2 be frequencies for bit ‘0’

s(t) = A sin (2πf1t)

For bit ‘1’

s(t) = A sin (2πf2t)

Let Tb be bit duration

For both waveforms to be orthogonal,

It is possible if both 2π(f1 - f2)Tb and 2π(f1 + f2)Tb are integral multiplies of π(i.e. = nπ), i.e.

Application:

Given, f1 = 10 kHz and f2 = 25 kHz

It is possible for , with minimum m and n value, i.e.

i.e. Tb = 200 μs. (minimum value)