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Nyquist frequency (folding frequency, fN) A frequency which is half of the sampling frequency. The Nyquist interval is the frequency range from zero to fN, where fN = 1(2Δt) and Δt is the sampling interval. For example, if the sampling interval Δt is 2 ms (i.e. a sampling frequency of 500 Hz), then fN = 1(2 × 0.002) = 250 Hz. It is important to know the value of the Nyquist frequency, as frequencies greater than fN will alias (or ‘fold back’) and appear as lower frequencies from which they cannot be distinguished.