Before sampling a continuous-time signal, why is an anti-aliasing filter typically used?

• A. To limit the bandwidth so that f_max < f_s/2 and prevent aliasing.
• B. To amplify the signal before sampling.
• C. To convert the signal to digital using a quantizer.
• D. To increase the sampling rate beyond Nyquist.

Answer: (A)

High-frequency content can fold back into the baseband when you sample a continuous signal. This aliasing happens because sampling creates copies of the signal’s spectrum at multiples of the sampling rate, and any energy above half the sampling rate will overlap with the lower frequencies.

An anti-aliasing filter is a low-pass filter placed before the sampler to remove those high-frequency components, effectively limiting the signal’s bandwidth to below the Nyquist limit (f_s/2). By doing this, the spectrum stays distinct and non-overlapping when sampled, so the digital representation can be accurately related back to the original signal.

In practice, the cutoff is set a bit below f_s/2 because real filters aren’t perfect and have transition regions; this provides attenuation of frequencies that could still cause aliasing. Amplifying the signal, quantizing after sampling, or simply increasing the sampling rate do not address the spectral overlap issue in the same pre-sampling way.