Signals that are sampled with too low of a Sampling Frequency can appear as lower frequencies; using an insufficiently high sampling frequency can cause totally different Continuous-Time Signals to look identical in their Discrete-Time forms. The minimum sampling frequency needed to reconstruct a given CT signal is given by the Nyquist-Shannon Sampling Theorem.
Note: there is also a spatial equivalent of the above that is relevant to phased-arrays
Nyquist-Shannon Sampling Theorem
A Continuous-Time Signal can be reconstructed from its samples if and only if the Sampling Frequency is at least twice its highest frequency component.
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- is called the Nyquist Rate
- Violation of this condition causes distortion called Aliasing