Learning Library · Oscilloscope specs, explained · 4
Capture rate and Sequential Mode
A digital oscilloscope is blind most of the time. After each capture it stops to process and draw the waveform, then re-arms for the next trigger. Anything that happens during that dead time is never seen. Waveform capture rate, in waveforms per second, measures how quickly it gets back.
Real time runs left to right, slowed down enormously. Bright blocks are capture windows. Dark gaps are dead time. Red marks are glitches: solid if a window caught them, faint if they fell in dead time. Glitch timing here is illustrative; the readouts are calculated.
Live time is what catches glitches
The capture window is the time on screen: ten divisions times the time base. At 20 ns/div the window is 200 ns. At 500,000 waveforms per second the scope is live for 500,000 × 200 ns = 0.1 s of every second, so it misses 90 % of everything that happens. For an event that occurs at random, the average wait to see it is one divided by its rate times the live fraction.
What Sequential Mode changes
In normal acquisition, the scope processes and displays each waveform before it re-arms. In Sequential Mode, listed as sequence mode on some datasheets, it fills a block of memory segments with back-to-back triggered captures first and draws them afterwards. Skipping the per-capture display work shortens the dead time, which raises the capture rate. The trade-off is that the screen updates in bursts rather than continuously.
| UNI-T series | Standard acquisition | Sequential Mode |
|---|---|---|
| UPO2000HD | 250,000 wfms/s | 1,000,000 wfms/s |
| MSO2000X | 500,000 wfms/s | 2,000,000 wfms/s |
| MSO3000HD | 500,000 wfms/s | 1,500,000 wfms/s |
| MSO5000HD | 800,000 wfms/s | 1,500,000 wfms/s |
Maximum rates from the UNI-T datasheets. Capture rate depends on the time base and settings, so the maximum applies at the scope's fastest-updating configuration.
Try this
- Start at 500,000 wfms/s, 20 ns/div, and 100 glitches per second. The average wait is 0.1 s.
- Switch to 5,000 wfms/s, the class of rate older or entry-level scopes reach. The same glitch now takes about 10 s on average to appear.
- Switch to Sequential Mode. The live fraction triples and the wait drops to a third.
- Slow the time base. Each window covers more time, so the scope is live a larger fraction of the time, until the windows touch and the scope is effectively always watching.
A higher capture rate makes a rare event likely to appear. A trigger makes it certain. If you know what the glitch looks like, a pulse-width, runt, or window trigger captures it on the first occurrence, and the capture rate only decides how quickly the scope is ready for the next one.
Catch the glitch the first time. Oscilloscopes with high capture rates and Sequential Mode.
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