Question

A continuous-time sinusoid of frequency 33 Hz is multiplied with a periodic Dirac impulse train of frequency 46 Hz. The resulting signal is passed through an ideal analog low-pass filter with a cutoff frequency of 23 Hz. The fundamental frequency (in Hz) of the output is ____

• A. 13

Answer 1

The correct option is A 13
If x(t) is a message signal and y(t) is related to x(t) as

$y\left(t\right)=x\left(t\right){\sum }_{n=-\infty }^{\infty }\delta (t-n{T}_s)$

$y\left(t\right)=f\left(s\right){\sum }_{n=-\infty }^{\infty }X(f-n{f}_s)$

Spectrum of X(f) and Y(f) are as shown


Cut off frequency of LPF = 23 Hz

​​​​​​​Hence, frequency at the output is 13 Hz.
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