Specifications

In order to accurately reconstruct a signal and avoid aliasing, Nyquist theorem says that the signal must

be sampled at least twice as fast as its highest frequency component. This theorem, however, assumes

an infinite record length and a continuous signal. Since no oscilloscope offers infinite record length and

by definition, glitches are not continuous, sampling at only twice the rate of highest frequency

component is usually insufficient.

Many people have seen the effects of aliasing, although they may not be aware of it. If you've watched

a western movie and seen the stagecoach's wheels go backwards while the horses are galloping,

you're seeing an artifact of aliasing: the stagecoach wheels' spokes are turning too fast for the 25 to 30

frame-per-second camera to accurately record their position.

There are two popular types of sampling used with digital oscilloscopes: real-time sampling and

equivalent-time sampling.

Real-time sampling is the sampling method shown above -- the method is best used for signals with

bandwidths less than half the sampling frequency of the oscilloscope. It is the method that must be

used when you have "one shot" at capturing a waveform and its characteristics are illustrated by the

above figures.

Equivalent-time sampling is used when the waveform being measured is periodic. Fortunately, many of

the signals measured in the real world are periodic, so equivalent-time sampling is often used. The

concept is to take many "snapshots" of a waveform over many periods and "piece" them together to

create a picture of the waveform. The benefit is that the method can recreate periodic signals with

frequencies higher than the scope's sampling frequency. Here's how it works.

Figure 21

Suppose the waveform to be measured has a period that is roughly one-third of the shortest sampling

period of the scope. Further suppose the scope's electronics triggers at the indicated positions on the

waveform. The scope's first sampling window samples the waveform at the points labeled 1, the second

window at the points labeled 2, etc. The sampling points are asynchronous with the waveform so the

sampled points are not at the same place in each waveform cycle. Thus, over many sampling periods,

a substantial number of sample points of the waveform's period are constructed in the oscilloscope's

memory.

Copying/reprinting/republishing without express written consent prohibited.