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| Help me understand oscilloscope peak acquisition mode |
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| AJ528:
Hi everyone, Thanks for the responses! This is the sort of discussion I was hoping to prompt; I've already learned so much from this! --- Quote from: Performa01 on December 05, 2023, 11:39:19 am ---You might want to read the section "Peak Detect" in my review document part 2 (SDS1104X-E Review 26-50): https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1371771/ This is for the little SDS1104X-E, but the working principle is thesame on all higher class models too. So you should easily be able to reproduce the examples given in that article. EDIT: Maybe you shoud read the section about the Normal Acquisition mode first, to see the problem that can be solved by peak detect. --- End quote --- Thanks for pointing me to that in-depth review (frankly it's more of an instruction manual/characterization at that length, which is not a bad thing!) The example problem you highlight is actually what prompted me to investigate how peak detect worked on my scope. --- Quote from: Performa01 on December 05, 2023, 02:11:46 pm ---It should be obvious, that it cannot be the raw sample interval, which might be as short as 500 ps at 2 GSa/s. It is the decimated sample interval instead, which is now twice as wide as for decimation in norml mode, because it contains a sample pair (min/max) instead of a single sample. --- End quote --- --- Quote from: RoGeorge on December 05, 2023, 03:47:09 pm ---For each "sample" in peak mode, the oscilloscope always displays 2 points, the max and the min value. Not added, not subtracted, but both the min and the max one after another. Always pairs of dots, even when max == min. For your example with +1 and -1 in the same interval, it will display two consecutive dots, one display dot at -1V, followed by another dot at +1V. --- End quote --- This is very interesting. Until now, I was assuming that, when in peak detect mode, the oscilloscope would measure at its max sample rate, then throw out all samples but one for each sample period (I couldn't figure out how it was deciding to keep the min vs max value each time, which is why I originally posted this question). The idea of collecting samples for two sample periods, then retaining a pair of min/max samples hadn't occurred to me. However, that certainly appears to be what is happening here. I generated a single 100MHz sine wave, and even though the entire signal was complete within 20ns, I still see a minimum and maximum point plotted 100us apart from each other. I have attached the actual sine wave being generated, as well as what peak detect mode shows me when I am zoomed out to 10kSa/s |
| Performa01:
--- Quote from: AJ528 on December 05, 2023, 09:35:22 pm ---I generated a single 100MHz sine wave, and even though the entire signal was complete within 20ns, I still see a minimum and maximum point plotted 100us apart from each other. I have attached the actual sine wave being generated, as well as what peak detect mode shows me when I am zoomed out to 10kSa/s --- End quote --- Yes, this is a very nice demonstration of Peak Detect mode – let’s examine that for the ones who can’t imagine exactly what’s going on: That single signal period is 10 ns wide and the original sample rate is 2 GSa/s, i.e. 500 ps sample interval. No problem capturing a 10 ns period. Now you lower the effective sample rate to just 10 kSa/s by increasing the time base to 200 ms/div and limiting the record length to 20 kpts at the same time. Now Peak Detect has to come into play: Of course, the ADC still samples at 2 GSa/s, so we have an accurate representation of the input signal in the original data. Now these data at 500 ps sample interval have to be decimated to a 100 µs sample interval. That means keeping only one out of 200k samples. Now we work on two decimated sample intervals at the same time, by finding the min. and max. value within a 200 µs interval. It is most likely that we will get one such min/max pair that is [-0.5V,+0.5V]. This is now split again to get the final effective sample rate with 100 µs sample interval by generating two samples, -0.5V followed by the next one +0.5V 100 µs later. For this to work, we need also consider the order of the two extrema, i.e. know which one comes first, the minimum or the maximum. If the transition between two 200 µs sample intervals happens to be exactly in the middle of the signal period, then we would get [0V,+0.5V] for one 200 µs interval and [-0.5V,0V] for the following one. In this case, we also get the correct decimated samples with 100 µs interval following the same scheme as above: 0V, -0.5V, +0.5V, 0V. |
| Performa01:
--- Quote from: PeDre on December 06, 2023, 12:14:54 pm ---I find the Siglent display unusual, I had never seen it before. --- End quote --- Thank you for showing us how Peak Detect is implemented on a scope from a so called A-brand. It's so much nicer to have just a fat blob instead of a hint on the true waveform (which we can't get with the peak detect crouch anyway)... |
| colorado.rob:
--- Quote from: PeDre on December 06, 2023, 12:14:54 pm ---For comparison, here are three pictures of the R&S RTB2000, if permitted. With the RTB you get two samples per time interval for min and max. Even if you export the data. I find the Siglent display unusual, I had never seen it before. --- End quote --- Given the description in the Siglent manual, this is exactly how I would expect it to be displayed. I find this much more intuitive. In vector mode, is it joining all minimum points together and all maximum points together to form two distinct waveform displays per channel? |
| 2N3055:
--- Quote from: PeDre on December 06, 2023, 05:15:46 pm --- --- Quote from: Performa01 on December 06, 2023, 04:19:06 pm ---[It's so much nicer to have just a fat blob instead of a hint on the true waveform (which we can't get with the peak detect crouch anyway)... --- End quote --- I cannot understand your statement that the displayed zoom signal gives an indication of the real signal. But I'm glad I could contribute to the amusement. Peter --- End quote --- No need for argument on any side... R&S has one display representation. Siglent has other. On every "vertical double point" R&S plots in time, Siglent will plot two samples, one Max other Min. When viewed at some normal timebase they will show exactly the same. This super high zoom ratio displayed was very useful to see what scopes actually do. But in practice it is not supposed to be used like that. Purpose of Peek detect mode is to show very short pulses on longer timebases. If you look at main (not zoomed window) both scopes show a spike, positive and negative (that would not be there in normal mode and slow sampling) and that is extent of it's usability. On both RTB2000 and even more on Siglent SDS2000X+ (that has order of magnitude more memory), Peak mode is pretty much unnecessary. I know I didn't use one in ages... |
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