Learning Library · Oscilloscope specs, explained · 3
Memory depth
An oscilloscope stores every sample of a capture in acquisition memory. Memory depth is how many samples fit. It decides how long a window the scope can capture at its full sample rate, and what happens to the sample rate when you ask for more time than that.
Top: the whole record, ten divisions wide. A 2 ns glitch sits at the marked point. Bottom: the 40 ns around the glitch, showing the actual samples the scope stored.
The equation that governs it
The time base fixes the record time: ten divisions times the seconds per division on most scopes. If memory can hold that many samples at the maximum sample rate, the scope samples at full speed. If not, it has only one way to fit the record into memory: it lowers the sample rate.
Why a lower sample rate hurts
Every halving of the sample rate doubles the gap between samples. A narrow glitch can fall between two samples and never appear. Fast edges lose detail, and frequencies above the new Nyquist limit alias, as the sample rate and aliasing explainer shows.
| Memory depth | Full rate at 2.5 GSa/s holds for | Time base |
|---|---|---|
| 64 kpts | 25.6 µs | 2.56 µs/div |
| 100 Mpts | 40 ms | 4 ms/div |
| 500 Mpts | 200 ms | 20 ms/div |
Try this
- Pick the 64 kpts scope and set 10 µs/div. To fit 100 µs into 64 k samples, the scope drops to 640 MSa/s, still fast enough to catch the glitch.
- Slow the time base to 1 ms/div. The sample rate falls to 6.4 MSa/s, samples are 156 ns apart, and the 2 ns glitch is gone.
- Switch to 500 Mpts at the same setting. The scope still samples at 2.5 GSa/s and the glitch is there.
Deep memory has a cost
Long records take longer to process and draw, so a scope can slow its update rate when every capture is hundreds of millions of points. Most deep-memory scopes let you choose a shorter record when you need fast updates, and use the full depth when you need to capture a long event in one shot. Some scopes also split memory between channels, so check the per-channel figure on the datasheet.
For example, UNI-T's UPO2000HD series offers 100 Mpts with a 2.5 GSa/s sample rate, and the MSO3000HD series offers 500 Mpts at 2.5 GSa/s.
Segmented memory is a different tool for the same problem. Instead of one long record, it stores many short ones, one per trigger, and skips the dead time in between. The capture rate and Sequential Mode explainer covers it.
Keep the sample rate at long time bases. Deep-memory oscilloscopes.
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