1) If an FM signal is input to a phase-locked loop (PLL), which of the following

Question

1) If an FM signal is input to a phase-locked loop (PLL), which of the following is true regarding the PLL’s output y(t)?

(a) The signal y(t) needs to be integrated to get the message signal m(t).

(b) The signal y(t) needs to be differentiated to get the message signal m(t).

(c) The signal y(t) needs no special attention to get the message signal m(t)

2) A signal m(t) has bandwidth W=10kHz and is to be modulated via indirect FM. The narrowband FM signal has f nb c =200kHz and in order to avoid distortion it must have k nb f |m(t)| 20Hz. The desired wideband FM signal must have fc=4.5MHz and frequency deviation ratio D = 2.5. Design a configuration for this system that consists of frequency doublers and mixers such that no signal in the system exceeds 100MHz.

Explanation / Answer

1.

The blocks in Phase locked loop are phase detector, filter and voltage controlled oscillator. the output of the phase detector is V1 = Kp*Delta(phi). and the output of filter is V2 = kf*V1 and output of VCO is Y(w)= (k/jw)V2

by observing the transfer function of the VCO we can notice that it is of integrator. the input and output of an integrated are related as Y(s)= (1/s)X(s)

So, the signal y(t) needs to be integrated to get the message signal m(t)

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