Nyquist limit

(Harry Nyquist, 18891976, Swedish - American physicist), the maximum frequency of a signal that can be measured with a method that employs sampling of the signal with a specific frequency, the sampling frequency. According to Shannons sampling theorem, a signal must be sampled with a frequency at least twice the frequency of the signal itself. The maximum measurable frequency the Nyquist limit or frequency is thus half the sampling frequency. If the signal frequency is higher than the Nyquist limit, aliasing occurs. Electrical signals are sampled with a specific sampling frequency, e.g. in the analogue to digital converter ADC , and in the sample and hold unit of pulsed Doppler instruments. In pulsed Doppler ultrasound, the sampling frequency is equal to the pulse repetition frequency PRF , and the Nyquist limit is thus PRF.

HJS