The calculation method depends on the waveform type measured and the signal reconstruction method adopted by oscilloscope, such as example sinusoidal insertion method, vector insertion method, etc. To reproduce the signal accurately and avoid confusion, the Nyquist theorem states that the sampling rate of the signal must be no less than twice its highest frequency component. However, the premise of this theorem is based on signals of infinite length of time and continuous period.Since oscilloscopes cannot provide a record length of infinite time, and low-frequency disturbances are by definition discontinuous and not periodic, sampling rates of twice the highest frequency components are usually insufficient.
In fact, the accurate reproduction of the signal depends on its sampling rate and the interpolation method of signal sampling point gap, namely waveform reconstruction.Some oscilloscopes provide the operator with the following options: sinusoidal interpolation for measuring sinusoidal signals, and linear interpolation for measuring rectangular waves, pulses, and other signal types.
A useful rule of thumb for comparing sample rates and signal bandwidths: if the oscilloscope you are looking at has interpolation (filtered to regenerate between sample points), the ratio (sample rate/signal bandwidths) should be at least 4∶1.Without sine interpolation, the ratio of 10∶1 should be adopted.