After you sample a continuous waveform, you end up with discrete-time values rather than the original signal. The very act of sampling can cause problems when it comes time to analyze the data. You can solve those problems or take advantage of them. Most engineers understand the principle behind the Nyquist frequency, which states; for a sampling frequency of F, you can accurately measure frequency components below F/2, the Nyquist frequency. Some people forget the highest-frequency component isn't always the signal they want to measure. Energy may exist in their signal above the sampling system's Nyquist frequency. As a result, they often find unexpected signals— sometimes noise—in their frequency-domain analysis results.
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