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From the Q&A

With TJ Byers


Nyquist Said It First

Question:

I have heard that there is a way to be able to read higher frequencies on an oscilloscope than those for which the oscilloscope was built. I was wondering what circuit or devices are needed to accomplish this task.

Dustin Enns, Lehigh, KS


Answer:

What you are talking about is sampling. That is, you sample small parts of a series of waves to reconstruct them. Confused? Look at the figure below. Most scopes have two different sampling rates (modes) depending on the signal being measured: real time and equivalent time sampling — often called repetitive sampling. However, sampling only works if the signal you’re measuring is stable and repetitive because this method works by building up the waveform from successive measurements. When sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal in order to be able to perfectly reconstruct the original from the sampled version.

Otherwise, the frequencies will overlap — a condition called aliasing — where the reconstructed waveform has nothing at all in common with the original signal. The aliasing failure was first formulated by Harry Nyquist in 1928 and is (duh) known as the Nyquist theorem. For an in-depth discussion on sampling and other oscilloscope-related facts, read Tektronix’s XYZs of Oscilloscopes that you can find at XYZs_OSCILLOSCOPE.PDF.


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