It follows from the statement of the Nyquist criterion above that it is only possible to reproduce signals that are band-limited by using a digitally based stimulus generator. If an attempt is made to generate a non-band-limited stimulus however, all the energy in the signal that is supposed to be above the Nyquist frequency (f_N) will be aliased into the pass-band of the system and a distorted waveform will be produced. The group of signals that most commonly fall into this category is square-waves. Fourier analysis of a square wave (f_s) with a fundamental frequency f₀ shows that it is composed of the sum of the odd harmonics (f_n) with decreasing amplitude to infinity.

f_s=f₀/1-f₃/3+f₅/5-f₇/7 … etc.

Even if the sampling frequency is arranged such that the fundamental or even the lower harmonics pass the Nyquist criterion, the relatively slow convergence of the series will mean that significant energy is available for aliasing.

To avoid this problem and ensure that the stimulus is as faithfully generated as possible without artefacts think in terms of sine waves only and shun square waves or anything with sharp edges. This applies equally to spatial and temporal waveforms.

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