Aliasing is when energy outside the bandwidth a sample rate
can represent gets folded back into the captured spectrum, appearing at a wrong
frequency — a phantom that looks like a real signal.1
Aliasing: energy beyond the captured bandwidth folds back to a false position inside it.
How it works
It is the Nyquist theorem violated. SDR front-ends include
anti-alias filtering, and choosing an adequate sample rate keeps real signals safely
inside the usable window. It also constrains the order of
filtering and decimation.
Relevance to SDR
Recognising an alias prevents chasing signals that are not really where they appear.
Sources
Aliasing — Wikipedia, on out-of-band energy folding to false frequencies when undersampled. ↩
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