Frequency Aliasing in Digital Storage Oscilloscopes (DSO)

Digital Storage Oscilloscopes (DSOs) have become extremely popular and affordable due to improvements in Analog to Digital conversion circuitry. DSOs have all but replaced traditional analog scopes on most workbenches and in schools. These new DSOs are extremely powerful but the user must be aware of potential measurement errors inherent in any digital sampling circuitry.

Since these scopes are based on digital technology, the analog signal that is applied to the oscilloscopes input terminal is sampled at discrete time intervals and converted to a digital value so that it may be processed and displayed.

Sampling theory requires that a signal be sampled at a rate of at least twice the frequency of the signal that is being measured. But to obtain an accurate representation of the signal this number should be at least 5 and preferably more than 10 times the frequency of the signal. At low sampling frequencies it is easy to be misled by the waveform displayed on the oscilloscope screen. Therefore DSOs, unlike their analog counterpart, could potentially display an inaccurate display of the signal. For instance if the sweep time is reduced to an extremely slow rate, the displayed signal appears to be at a significantly lower frequency that the actual value.

To check for frequency aliasing, the sweep rate should be momentarily changed to see if the apparent frequency of the waveform changes. The measurement feature of most modern Digital Storage Oscilloscopes can also serve as a validation of the actual signal’s frequency. This frequency (or period) value should be compared with the time/div setting of the Horizontal axis.

It is important for the user to understand this anomaly of digitally sampled test equipment, so that the displayed waveform may be accurately interpreted.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

Leave a Reply