Q. Illustrate Frequency-shift keying?

Digital transmission by FSK is a nonlinear modulation method that is appropriate for channels that lack the phase stability needed to perform carrier-phase estimation. Figure shows the functions of a coherent FSK system in which the transmitted signal s_FSK(t) is generated by frequency modulation of a voltage-controlled oscillator (VCO). The digital signal d(t) has a polar format with amplitudes ±1, corresponding to binary digits 1 and 0, respectively.Modulation keys the VCO's angular frequency between two values, such that

where ω is the frequency deviation from a nominal or carrier angular frequency ωc. In order to conserve the bandwidth in the signal sFSK(t), ω is usually selected not much larger than the minimum allowable value given by

When Equation is satisfied, the bandwidth of the channel needed to support sFSK(t)is about 2ωb rad/s. The transmitted signal in Figure can be seen to be two unipolar ASK signals in parallel, one at carrier frequency ω2 and the other at ω1. The receiver then becomes two coherent ASK receivers in parallel.

The noncoherent FSK version results when each product device and the following integrator in the receiver are replaced by a corresponding matched filter followed by an envelope detector.