What is the primary purpose of an anti-aliasing filter before sampling a signal?

• A. It converts analog to digital
• B. The primary purpose is to limit input bandwidth to below half the sampling rate to prevent aliasing.
• C. It increases the sampling rate.
• D. It removes all noise before sampling.

Answer: (B)

The main idea is to prevent aliasing by trimming the signal’s high-frequency content before sampling. When you sample, frequencies above half the sampling rate would fold back into lower frequencies and distort the signal. An anti-aliasing filter acts as a low-pass filter, limiting the input bandwidth to below the Nyquist frequency (half the sampling rate) so those problematic frequencies are suppressed. This helps ensure the sampled data can be accurately reconstructed later.

It doesn’t convert analog to digital by itself—that’s the job of the ADC. It doesn’t raise the sampling rate—the filter doesn’t change how often you sample, just what you sample. And it doesn’t remove all noise; it mainly attenuates high-frequency components, reducing potential aliasing while leaving the desired in-band content intact.