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Factors in Choosing an Oscilloscope

By Alan Lowne

Digital oscilloscopes have come down in price amazingly over the past few years, but can you get a good scope for around $1,000? Do you really need some of the more advanced scope features like math calculations or deep memory depth? Single-shot, delayed sweep, pre-trigger acquisition, parametric measurements, saving waveforms and data for later analysis, noise reduction, averaging, searching, zooms, and math measurements give you helpful troubleshooting and analysis tools but these can be extras you may not need.

Here are some suggestions to aid your selection process and avoid some common mistakes.

First Thoughts

Ask yourself the following measurement questions about your requirements:

What signal amplitudes will I be measuring (max/min)?
What is their highest frequency?
Do I need frequency domain (spectrum analysis) too?
Do you need automatic measurements?
Will you have a laptop PC available (so you can use a USB PC scope adapter) or must this be a standalone solution?

Answers to these questions and considering the topics below will help in your selection process.

Sampling Rate/Bandwidth


What sort of signals do you want to display? For instance, a microprocessor system clock may be the highest frequency signal the scope you will want to display. So your oscilloscope should have a bandwidth of 3 - 4 times greater than this clock frequency, in order to display the waveform adequately. If, however, you want to accurately see the rise-time of the clock, you'll need around 10 times that frequency as a sample rate.

Bandwidth is arguably the single most important property of an oscilloscope, determining the range of signals that can be displayed. It also dictates price range, since it is much harder and more expensive to make really fast scope circuitry. Bandwidth can be defined as the maximum frequency of signal that can pass through the front-end circuitry (amplifiers, attenuators, ADCs, interconnects, relays), so the analog bandwidth of your scope must be higher than the maximum frequency that you wish to measure.

Most scope manufacturers define bandwidth as the frequency at which a sine wave input signal is attenuated to 71% of its true amplitude (-3 dB point) - the displayed trace amplitude will be 29% in error at this frequency! So try to purchase a scope with a bandwidth five times higher than the maximum frequency signal you wish to measure.

Higher bandwidth scopes are increasingly expensive, so you may have to compromise here. On some scopes, the quoted bandwidth is not available on all voltage ranges, so check the data sheet carefully; and be alert that scopes usually have different sampling rates depending on the number of channels in use. Typically, the sampling rate in single channel mode is twice that in dual channel mode.

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