Scope with fast waveform averaging (can your scope go faster?)

[LaurentR]

A quick 3000T datapoint (include disclaimer on methodology here):

The 3000T 1Mwfms/s starts dropping around 20ns/div. While at 1Mwfms/s, if enabling Averaging, the wfms/s drops to 26kHz (at least according to the trigger output), regardless of the # of averages (which is 2-64k on the 3000T).

Below that, there is a very variable ratio between Normal wfms/s and Averaging wfms/s (in the 1-50 range). The ratio gets smaller as longer time bases, with an asymptote at 1.

At and above 20us/div, the sample rate is locked at 5GSa/s in both mode. Below 20us/div, the Normal sample rate starts dropping and the Averaging sample rate is locked at 1/4 the Normal sample rate (as reported by the scope).

In all cases, the number of averages (2-64k) has no impact on wfms/s.

Rough wfms/s (as measured by Trig Out - disclaimers apply):
500ps/, 1ns/, 2ns/, 5ns/, 10ns/: 26.4 kHz
20ns/: 34.2 kHz
50ns/: 13.8 kHz
100ns/: 6.9 kHz
200ns/: 1.7 kHz
500ns/: 1.1 kHz
1us/: 1.1 kHz
2us/: 1.0 kHz
5us/: 950 Hz
10us/: 820 Hz
20us/: 650 Hz
50us/: 96 Hz
100us/: 96Hz
200us/: 62 Hz
500us/: 78 Hz
1ms/: 62 Hz
2ms/: 44 Hz
5ms/: 18 Hz
10ms/: 9 Hz
20ms/: 4 Hz
50ms/: 2 Hz
100ms/: 1Hz

[maxwell3e10]

Thanks for detailed measurements. It looks like there is also a sweet spot for 20 nsec/div. What was the waveform length for these measurements? That could have an effect on the speed.

In principle, running average requires only one multiplication/addition per waveform, so it's no more complicated calculation than scaling/offset that needs to be applied to every waveform to display it. Display calculations are often done on decimated data. But for low number of waveform points there should not be a big difference in number of points.

To compare it to Rigol measurements (which does not have a useful Trig out signal in this case), one needs to use:

On a Keysight 2000X series scope I can get up to 13.3k wfms/s in averaging mode (dependent on the timebase setting but not the number of averages), down from 220k wfms/s in normal mode. This is measured with a frequency counter connected to the trigger output.
The -3dB frequency of a moving average filter is approx. f_s * 0.443 / N (for large N). So that works out at around 5800 averages in your methodology. Depending on the actual implementation, things may be a bit more complicated (don't believe it is actually purely a FIR filter, so the frequency response may be somewhat different).

So roughly speaking it looks comparable between Rigol 8000, 6000 and Keysight 3000T.

An (unintentionally) correlated trigger could make that average to 0 very quickly and provide extremely misleading results.

The whole idea is that if the triggering is fast and without gaps the whole averaging completes before the slowly changing sine wave has a chance to change sign. Now, one does have to be careful if using auto trigger that it is set on a different channel or outside of waveform range, so the scope does not introduce display delay when getting a trigger on auto setting.

[tchiwam]

Record in a long buffer then download the wave and make the averaging yourself ?
 

[maxwell3e10]

Record in a long buffer then download the wave and make the averaging yourself ?

No, that defeats the whole purpose of a scope to look at signals in real time and diagnose things based on small features in averaged signal.

[LaurentR]

Thanks for detailed measurements. It looks like there is also a sweet spot for 20 nsec/div. What was the waveform length for these measurements? That could have an effect on the speed.

I used a 30MHz sine wave to get enough triggers. I also tried in auto trigger with noise and saw no difference at any of the time bases.

[maxwell3e10]

What was the waveform length for these measurements? That could have an effect on the speed.

[Someone]

Record in a long buffer then download the wave and make the averaging yourself ?

No, that defeats the whole purpose of a scope to look at signals in real time and diagnose things based on small features in averaged signal.

Averaging without a trigger defeats the whole purpose of averaging....

[maxwell3e10]

Yes, in most cases that's true, when measuring a fast signal. But in this case it doesn't matter (assuming auto trigger is fast enough), because the signal is very slow. Think of it as a kind of box-car high-resolution averaging.

Still curious what is the waveform length LaurentR used. I haven't used a 3000T. Does it just default to auto length equal to number of points displayed on the screen?

[maxwell3e10]

According to the manual, 3000T uses waveform length to fill the screen or half the working memory, it's not so clear. On my EDUX1002 it uses 50k points all the time in run mode independent of time scale and 100k points in single shot mode. In single shot mode with averaging on it is supposed to do just one set of N averages and stop.

I am leaning toward Rigol 7000 series scope, it's likely to have one of the fastest averaging for a scope under $10k, new or used. Having long memory would help if one has to resort to downloading the data.

[JehTeh]

On the rigol front, I was playing around more with my 8k this morning and trying to see if I could hit the 600k trigger rate, but I have been having no luck. The baby rigol watching trig out is seeing ~110k max. Interestingly whether I have averaging on or not seems to make no difference (200ps/div, 20pt 'auto' memory - all settings from 1k averages to 64k show same trigger rate).

Scope is supposed to do 600k/s but I am not sure under exactly what circumstances, would be interesting to actually get it there and see if turning on averaging makes a difference.

[maxwell3e10]

On Rigol 6104 I was able to hit the advertised trigger rate of 180k/sec, but interestingly not at the shortest time scale. The averaging also does not seem to affect the trigger out rate, the scope must just pick some fraction of the waveforms to average.

You should be able to get from Rigol tech support the conditions to get 600k/sec since they advertise it so prominently.

The datasheet has this note for waveforms update. I wonder how narrow are the conditions to get highest rate.
Note[3]: Maximum value. single-channel, 10 ns horizontal time base, input amplitude 4 div, sine wave signal with 10 MHz frequency. Others are default settings.

[JehTeh]

Quickly messing around, it does indeed hit >600k wfms/s (~700 actually) at 10ns/div. Funnily enough, only there, but I have not bothered to play with it more to see where else it might be running at that rate.

In this case turning on averaging brings the trigger rate back to roughly 110k/s.