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.
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.
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.