Maybe I didn't get the point but I can't imagine that any scope on the market isn't interpolating the displayed waveform from "true" sample points. At least in single shot mode there simply isn't any choice, or is it?
Yes, it's hard to believe, yet rumours have it that such scopes do exist. You can always calculate something completely new from a number of original samples, which then just get replaced by the result of the computation (like most modern digital cameras do).
And in the "digital phospor" / intensity grading modes. I would assume the samples of previous acquisition are not really existing anymore, just the previous waveform(s) must be stored somewhere to process the grading.
Yes, there are some scopes that use only the screen (or some small secondary) buffer for intensity grading.
For Siglent X-series scopes there is one irrevocable standard: In normal mode, it will always show all original sample data from within the last 40ms (if available) on the screen at once. In Sequence recording mode it will even show the complete history buffer, i.e. up to 100000 records and/or a total of 440 Megasamples at once. This way you can detect any anomaly at a glance.
This is true for normal acquisition mode; Peak Detect
might, Average and Eres
will replace the original sample data by the corresponding processed data, i.e. min/max data pairs in case of Peak Detect, the sliding average of corresponding samples over a number of subsequent records in case of Average and the result of a FIR lowpass filter operation within one record for Eres. Nevertheless, the number of samples doesn't change though and we still get all the data displayed.
This is by the way the major difference to LeCroy scopes, which never ever manipulate the original sample data, thus cannot have peak detect and implement Average as well as Eres as Math functions.
Now for the mystery of intensity grading, which some confuse with persistence (of course we can have persistence as well, but that has absolutely nothing to do with intensity grading):
It is simply a matter of the number of samples falling into the exact same position on the screen – similar to an analog scope, where intensity is determined by the speed of the electron beam moving over the screen. Since each sample represents a certain time step (1/samplerate) the accumulation of these results in a total time which in turn determines the trace intensity.
For this process, a lot of data is involved in the display mapping. It is the record length multiplied by the number of acquisitions within a span of 40ms (which is the display refresh period).
Anyway, I still find it puzzling that any kind of interpolation divides the acquisition rate by a factor of four (or more).
With the explanation given above, we could measure/calculate the data rate for different timebase settings:
200ps/div => 120kpts/s => 4.8kpts/frame;
20ns/div => 110Mpts/s => 4.4Mpts/frame;
1ms/div => 950Mpts/s => 38Mpts/frame;
So while at even slower timebases we can exceed 1Gpts/s, there seems to be no dedicated graphics processor to handle such an amount of graphic data in a fast and efficient way.
The amount of additional dots required for interpolation depends on the signal amplitude, waveform and frequency. This can easily lead to a multiple of the original data to be drawn on the screen. This seems to be the bottleneck and both position mapping and interpolation require additional processing time.
Looking at their incomplete manual, at least Siglent included Stdev to compute AC RMS and RMS to compute AC + DC RMS. But the multimeter has separate AC RMS and DC RMS selections. Is Siglent always so consistent?
Well, these are different worlds, I suppose. Experienced DSO users should be familiar with the Stdev measurement (even though I’ve come across “experts” who confuse it with the standard deviation in the measurement statistics), whereas on a DMM the terms AC RMS (often just RMS) and AC+DC RMS are common. But that’s irrelevant, because other than a true scope meter, a pseudo DMM is just a marketing gimmick in my book, that doesn’t belong into a DSO – not in the midrange class anyway. Yet since it was introduced by one of the big boys, every manufacturer feels the need to offer one as well. I think Siglent is the last one to follow that trend.