Answer :
The spectrum of the sampled signal will have replications of the original 20 MHz wide rectangular spectrum centered at multiples of the sampling frequency, 60 MHz, resulting in centers at ..., -180 MHz, -120 MHz, -60 MHz, 0 MHz, 60 MHz, 120 MHz, 180 MHz,... The correct answer is option b.
The question is about the spectrum of a sampled signal. Given that the highest frequency component of the original signal is B = 10 MHz and the sampling frequency is f = 5B + 10 MHz = 60 MHz, according to the sampling theorem, the sampled signal will have copies of the original spectrum centered around multiples of the sampling frequency. Each copy of the spectrum will be 2B = 20 MHz wide because the original signal has a rectangular spectrum with a bandwidth of B, and the spectral copies will be double-sided.
Our sampled signal's spectrum will therefore present replications of the original 20 MHz rectangular spectrum centered at 0 MHz and at every multiple of the sampling frequency (60 MHz). The centers of these replicas will be at ..., -180 MHz, -120 MHz, -60 MHz, 0 MHz, 60 MHz, 120 MHz, 180 MHz, ... since these are the multiples of 60 MHz. Therefore, the correct answer is the Repetitious rectangular spectrum, 10 MHz width, centered around......-180,-120,-60, 0, 60, 120, 180...MHz.
