Anti-Aliasing

Aliasing is the phenomenon that frequencies greater than the Nyquist frequency are shifted erroneously to lower frequencies.

The Nyquist frequency is calculated with the following equation:

f _Nyquist = sampling rate / 2

When you acquire data with an NI dynamic signal acquisition (DSA) device, aliasing protection is automatic in any acquisition. The sharp anti-aliasing filters on DSA devices track the sampling rate and filter out or attenuate all frequencies above the Nyquist frequency.

When performing frequency measurements with an NI E Series, M Series, or X Series DAQ device, you must manually eliminate aliasing using the following methods:

• Increasing the sampling rate
• Applying an external lowpass filter
• Using an inherently bandlimited DUT

Simulated data also can contain aliasing. Simulated signals usually are generated according to a time-domain expression. Therefore, simulated signals usually have high-frequency components that are aliased in the discretely sampled data. The following figure shows an example of the aliasing for a simulated square wave.

The only way to protect data from aliasing is to apply appropriate aliasing protection before the data is generated or acquired. Aliasing occurs when the data is generated or sampled. You cannot remove aliased components from the data without detailed knowledge of the original signal. In general, you cannot distinguish between true frequency components and aliased frequency components. Therefore, accurate frequency measurements require adequate aliasing protection.