Oscilloscope Update Rate

Why the capture frame update rate is important.

Repetitive display of a waveform sweep.

Waveform sweep and spectrum display (click to animate).

Back in the day, all oscilloscopes were analog and the update rate was usually quite high (>50Hz) because the waveform appeared on the display only briefly requiring a high rate to appear "permanently", much like analog TV.

This had implications; it was generally not possible to see waveforms that were not periodic and one-shot capture was also usually impossible.

Most modern oscilloscopes whether they are stand-alone or USB based like BitScope are digital and the refresh rate is irrelevant when it comes to persistent waveform display. It also means they are able to capture single waveform events such as glitches and display them persistently for analysis.

While these oscilloscopes are often designed with one-shot capture in mind there are many situations where repetitive capture is needed and the update rate specification can be quite important.

For most PC based oscilloscopes repetitive capture and display is not a design priority but for BitScope it is; faster refresh rates are usually possible even when capturing logic or mixed signal waveforms and this can provide some significant benefits when compared with devices that focus on one-shot or slow refresh capture only.

So the question is what update rate do you need?

Repetitive display of logic signals.

High speed repeating logic capture (click to animate).

The answer depends on what you're trying to do.

Some modern (and quite expensive) digital scopes can capture at rates as high as millions of frames per second, for a limited time. This is much faster than most scopes and while it can sometimes be useful it's usually well beyond the call of duty.

On the other hand, performing a capture and waiting more than a second or two to the see the result is too slow for practical use with repeating capture.

Most USB oscilloscopes and logic analyzers are designed for one-shot capture and analysis, not for real-time interractive use so they don't support high speed repetitive capture.

While BitScope can be used for one-shot capture too it's been designed for easy interractive use while capture proceeds, even for logic waveforms and spectrum displays. To see what we mean, click any of the images on this page to see what BitScope does; they are five second animated screenshots that should give you the idea.

At the top of the page is a manually swept sinusoidal waveform and its magnitude spectrum captured by BitScope Mini and BitScope DSO. This is displayed in real-time at an update rate of about 50Hz. Next is a logic analyser capture example which shows random captured logic followed by triggered capture (which locks the waveform in place).

Repetitive display of timebase zoom.

Timebase parameter zoom in real-time (click to animate).

In both cases fast display refresh combined with decay phosphor display modes gives a dynamic view as to what is happening in real-time that one-shot or slower rate updates cannot provide.

There are a variety of other reasons higher refresh rates can be important which we'll write about in a another post but one practical one is so you can see the effect of changes made to capture parameters as you make them.

Being able to see these changes "live" can make it much easier to understand what's going on and what parameter settings are optimal for a given test setup.

The last example demonstrates this. It is a fixed frequency sawtooth waveform with noise and it's shown while the timebase zoom is manually adjusted. At the start of the clip the sawtooth waveform can be seen to dominate but as the timebase is increased the (harmonic) noise components in the waveform become evident (in both the waveform and spectrum displays). Fast real-time update makes these dynamic parameter changes easy to manage and their effect easy to see. This is something not easily explained by specifications alone.

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