Which statement about sampling and aliasing is correct?

Which statement about sampling and aliasing is correct?

• A. Aliasing occurs when sampling frequency is well above the highest frequency component.
• B. Aliasing occurs when sampling frequency is equal to the loop bandwidth.
• C. Aliasing occurs when the sampling frequency is less than twice the highest frequency component.
• D. Aliasing is not a concern in digital control.

Answer: (D)

Understanding how fast you sample relative to the signal’s highest frequency is crucial. If you sample too slowly, higher frequency components in the signal fold back (alias) into lower frequencies in the sampled data, which distorts the signal you observe. This is captured by the Nyquist criterion: to accurately capture and reconstruct a signal, the sampling frequency must be at least twice the highest frequency present in the signal. When the sampling rate is less than twice that maximum frequency, aliasing occurs. In digital control, this matters because measurements and the controller’s actions are based on discrete samples. If aliasing happens, sensor readings can misrepresent rapid changes, leading to incorrect control decisions or instability. That’s why anti-aliasing filtering and choosing a sufficiently high sampling rate relative to the loop’s bandwidth are standard practices. The statements that say aliasing occurs only when sampling is well above the highest frequency, or that aliasing isn’t a concern in digital control, aren’t accurate. Aliasing is directly tied to sampling speed being too low relative to the signal’s highest frequency, and it can impact digital control performance if not addressed.

Understanding how fast you sample relative to the signal’s highest frequency is crucial. If you sample too slowly, higher frequency components in the signal fold back (alias) into lower frequencies in the sampled data, which distorts the signal you observe. This is captured by the Nyquist criterion: to accurately capture and reconstruct a signal, the sampling frequency must be at least twice the highest frequency present in the signal. When the sampling rate is less than twice that maximum frequency, aliasing occurs.

In digital control, this matters because measurements and the controller’s actions are based on discrete samples. If aliasing happens, sensor readings can misrepresent rapid changes, leading to incorrect control decisions or instability. That’s why anti-aliasing filtering and choosing a sufficiently high sampling rate relative to the loop’s bandwidth are standard practices.

The statements that say aliasing occurs only when sampling is well above the highest frequency, or that aliasing isn’t a concern in digital control, aren’t accurate. Aliasing is directly tied to sampling speed being too low relative to the signal’s highest frequency, and it can impact digital control performance if not addressed.