What is the Nyquist sampling criterion for converting a continuous signal to digital form?

• A. fs > fmax
• B. fs > 2 fmax
• C. fs < fmax
• D. fs = fmax

Answer: (B)

To convert a continuous signal to digital form without losing information, you must sample at a rate that is more than twice the highest frequency present in the signal. This is because sampling creates copies of the signal’s spectrum spaced at the sampling frequency, and if the highest frequency content is not kept below half the sampling rate, these copies overlap. Overlap causes aliasing, where high-frequency components masquerade as lower frequencies and reconstruction becomes impossible. Therefore the condition is that the sampling frequency be greater than twice the maximum frequency component: fs > 2 fmax. In practice, engineers often use a margin (fs ≥ 2 fmax with a guard band) to accommodate nonideal filters and imperfect reconstruction.