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| Scope with fast waveform averaging (can your scope go faster?) |
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| 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: --- Quote from: switchabl on August 05, 2022, 10:43:20 pm ---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. fs * 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). --- End quote --- So roughly speaking it looks comparable between Rigol 8000, 6000 and Keysight 3000T. --- Quote from: Someone on August 10, 2022, 04:27:10 am ---An (unintentionally) correlated trigger could make that average to 0 very quickly and provide extremely misleading results. --- End quote --- 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:
--- Quote from: tchiwam on August 10, 2022, 01:03:48 pm ---Record in a long buffer then download the wave and make the averaging yourself ? --- End quote --- 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:
--- Quote from: maxwell3e10 on August 10, 2022, 12:43:25 pm ---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. --- End quote --- 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. |
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