Measuring O-scope - Flatness? - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: November 09, 2016, 07:31:07 pm
content: I have access to a leveled sweep signal generator so I hooked it up to my Rigol DS1054Z sweeping 0 to 200 MHz.RF to Channel 1 via BNC0-10V sweep ramp to channel 2Set scope to X-Y mode and adjusted the channel sensitivities to maximize the display. I get a sideways funnel type image, but is not smooth. For example, the amplitude rises around 10 MHz then drops around 40 MHz, then rises around 60 MHz, then falls around 100 MHz and is a little flat to about 150 MHz, then gradually falls to 200 MHz. I was expecting something more like the shape of a bullet.Where do I take my initial reading to determine the 3db rolloff point? Max is around 10 MHz, but 25 to 75 MHz is fairly flat.What would be a good setup for this and are there any other interesting measurements I can make using the sig gen?

author: metrologist
author_rank: Super Contributor
timestamp: November 09, 2016, 09:13:24 pm
content: I changed my setup just to trigger and long sweep. That does not look very flatAnd then I found some modulation...

author: barry14
author_rank: Regular Contributor
timestamp: November 09, 2016, 10:32:14 pm
content: Did you terminate the cable going to the scope with a 50 ohm feedthru? At these frequencies, proper impedance levels must be observed if you expect flat responses.

author: tggzzz
author_rank: Super Contributor
timestamp: November 09, 2016, 11:12:55 pm
content: Don't forget to plot it on log-log scales, i.e. dB and decades. The same data plotted on lin-lin axes looks very different.In other words, give numbers, not adjectives

author: newbrain
author_rank: Super Contributor
timestamp: November 09, 2016, 11:28:59 pm
content: Read the top line on your first screenshot:5 Ms/sSo, that's good till 2.5MHz, that is to say meaningless (a minor miracle something vaguely resembling the real signal is shown at all)Is the 2nd screenshot the same signal? That's more difficult to explain.As for XY display, this being a DSO, you'll have sampling artifacts as in in the 1st picture, depending on the settings.

author: tggzzz
author_rank: Super Contributor
timestamp: November 10, 2016, 08:50:09 am
content: Quote from: newbrain on November 09, 2016, 11:28:59 pmIf there's no modulation, then not necessarily - it is ultimately limited by the analogue front end.To put it simply, if I have a 1MHz bandwidth signal on a 1GHz carrier, then I only need to sample at >2MS/s. However, the sampling head has to have useful behaviour at 1GHz.

author: newbrain
author_rank: Super Contributor
timestamp: November 10, 2016, 09:09:28 am
content: Quote from: tggzzz on November 10, 2016, 08:50:09 amOn paper, yes, no doubts about it, and that's, in fact the principle ETS DSO are based upon, if I'm not mistaken:if the signal is repetitive, and band limited you can under sample it and obtain a correct reconstruction.But here we have a signal that varies in time and I can expect the result to depend on interference between the signal period, the waveform update rate and the scan blind time.So I would not trust this kind of results.Am I completely off-track? I admit my digital signal theory is a bit rusty, to say the least, so thanks for any clarification.

author: tggzzz
author_rank: Super Contributor
timestamp: November 10, 2016, 09:24:34 am
content: Quote from: newbrain on November 10, 2016, 09:09:28 amThat's correct; I first saw it done without a scope, where the output device was an XY pen plotterIn addition "subsampling" is often used in "digital" radios with non-repetitive signals, which is a more direct example of the principle.QuoteQuite possibly, but not necessarily. I would want to play with the scope and signal before deciding what's happening.Ifthere analogue front end isn't the limiting factor, then the key issues will be the sweep speed and any digital processing of the sampled signal.

author: newbrain
author_rank: Super Contributor
timestamp: November 10, 2016, 10:07:03 am
content: Quote from: tggzzz on November 10, 2016, 09:24:34 amOk, then I think we agree: we can not conclude much form that screenshot.(Admittedly, I was a bit too dry in the first post. I should not answer when I'm >50% asleep...)As for the analog bandwidth:https://www.eevblog.com/forum/testgear/new-rigol-ds1054z-oscilloscope/msg743080/#msg743080might be an indication that it should not be so bumpy (though I don't know how it was measured).I'm still puzzled by the second picture, though: is that an artifact of the persistence + changing frequency + wfm/s ?

author: tggzzz
author_rank: Super Contributor
timestamp: November 10, 2016, 01:25:07 pm
content: Quote from: newbrain on November 10, 2016, 10:07:03 amI've never done that. Oh no. Never everBut is it always difficult to know how many "escape clauses" should be put in a quick statement!

author: Kalvin
author_rank: Super Contributor
timestamp: November 10, 2016, 03:03:37 pm
content: Did you calibrate the probe using the oscilloscope's internal square wave generator?

author: metrologist
author_rank: Super Contributor
timestamp: November 10, 2016, 05:33:24 pm
content: I did not use a 50 ohm term. BNC to BNC. Yeah, I realized I missed my 50 ohm feedthrough termination though.In the first image, the right side is 200 MHz. I see there could be a problem with a 5MS/s there.I also tried to take a bunch of screenshots by adjusting the time base for each frequency to give a couple cycles on the display, and then compare p-p voltage at say 10 MHz intervals, but I had not noticed that the scope was telling me that screen save failed! (Aside the point, during this testing, I noticed that my menu flyout's were not happening.)The second image is after I found some modulation, AM saw wave...Anyway, I have access to a sweep signal generator and just wanted to test out my scope.

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: November 10, 2016, 11:14:18 pm
content: Instead of sweeping, you could do it more manually and measure the average Vpk-pk at increasingly higher frequencies and plot it. This allows you to change the time/div scaling to optimize sample rate. It's not uncommon for oscilloscopes to have a non-flat frequency response, as long as it doesn't dip below -3dB it's technically ok. That being said, we design scopes to have as flat of a frequency response as possible up to the bandwidth point.

author: metrologist
author_rank: Super Contributor
timestamp: November 11, 2016, 12:34:30 am
content: Thanks, Daniel. That's what I tried to do later, but had not noticed the scope was not saving screen grabs. I don't recall if Rigol even specs flatness or if it must be determined from absolute accuracy.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 11, 2016, 02:36:38 am
content: Quote from: newbrain on November 10, 2016, 09:09:28 amETS works well for simple signals but for complex ones.it falls flat on its face.The classic case,which I quotead nauseumis that of trying to look at one field of a PAL video signal using an early '90s DSO.To see one field,it is necessary to display 20ms worth of signal on the screen.At the required time/div settings,the sample rate on these devices reduces to one not quite sufficient to reproduce a 15kHz line rate signal,& totally insufficient to reproduce 4.434MHz colour information,or 5MHz HF luma.The resulting display is totally unuseable.In the OP's case,using a far more capable instrument,the DSO sees the signal as a very low frequency one,with a huge range of frequencies superimposed upon it.It is stretching the bounds of friendship to ask the poor DS1054Z to sample & store at its native maximum sampling rate over such a large time.It will run out of memory!It does the obvious thing & reduces the sample rate to one which is still quite reasonable for most signals that would be viewed at this time/div setting.The anti-aliasing filters don't do anything until the top end of the range,so "funnies" happen after the sweep approaches & then passes 5MHz.People have checked the frequency response of their DS1054Z's "retro-style" using a series of "spot " frequencies,without the result looking anything like the OP's screenshot.

author: rstofer
author_rank: Super Contributor
timestamp: November 11, 2016, 03:14:00 am
content: Quote from: vk6zgo on November 11, 2016, 02:36:38 amI think it's important to get this into the thread lest newcomers see the traces and just assume the problem is with the scope.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2016, 10:31:55 am
content: Quote from: rstofer on November 11, 2016, 03:14:00 amWell, the problemiswith the scope, but it doesn't mean the scope is at fault! Such issues can occur withanymeasuring instrument, if its operation and limitations aren't understood.(In generalI don't think it is fair to punt the issue and say the "problem is with the operator" - although sometimes that is the inescapable deduction!)

author: JPortici
author_rank: Super Contributor
timestamp: November 11, 2016, 10:41:01 am
content: Quote from: metrologist on November 11, 2016, 12:34:30 ampendrive? check the size. not specified if not in the troubleshooting guidebut size > 2GB will not work.. or not work reliably (i have funny stories about that)

author: rstofer
author_rank: Super Contributor
timestamp: November 11, 2016, 04:43:09 pm
content: Quote from: tggzzz on November 11, 2016, 10:31:55 amI thought the issue reduced to something bizarre with the signal generator. Is that not the case?We know full well that the scope doesn't roll off to -3dB until somewhere around 150 MHz. This has been documented and there's a video out there somewhere. If I had a signal generator, I would simply duplicate the test using a simple sine wave and varying the frequency.I was pretty careful to avoid the term 'operator error'. I have no idea why the display is showing what it is except that I am not quite ready to throw the scope under the bus given that the signal wasn't even terminated. And where did the modulation term come from?I must be missing something. There is nothing in the discussion that makes me believe it is a scope problem. But then there is the issue of 'confirmation bias'. Maybe I really did miss something!

author: metrologist
author_rank: Super Contributor
timestamp: November 11, 2016, 05:29:02 pm
content: Quote from: metrologist on November 09, 2016, 07:31:07 pmI made no claim about my screen grabs other than they weren't excatly what I was expecting, so that's why I switched my setup. I just forgot my 50 ohm load, but what was I gonna do, not even hook it up?The pen drive is 1GB. I think something else happened to stop it from saving. There was a partially written file name with ~ in it. That's probably also when my menu flyout's stopped working. Anyway, I got rid of that file and the scope saved fine after that.

author: metrologist
author_rank: Super Contributor
timestamp: November 11, 2016, 05:37:38 pm
content: Quote from: rstofer on November 11, 2016, 04:43:09 pmNothing here, really. I just brought my scope to work where we have a very nice sig gen. I had an idea to measure something. Perhaps the method is flawed. Just take it at face value, I hooked the stuff up, set it up this way, and here is what it looked like.I learned anyway that there was not enough samples the way I had it set up to see enough of the higher frequency signal.After that, I just thought a saw tooth AM modulation looked cool. I have one of sine wave AM too, but that's been done before.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2016, 07:28:09 pm
content: Quote from: rstofer on November 11, 2016, 04:43:09 pmMy understanding, and I don't think it has been confirmed, is that there is a "missing" 50ohm cable termination and possibly something to do with the way the scope captures/processes the samples. Any such capture/processing problem may well be understandable and predictable; if so there is no fault in the scope, but there is a problem with it. You can get such problems when using any piece of equipment; that is why it is so valuable to "play around" with the settings to see if you can understand what's happening.An example of another such problem with a scope. With Tek 24x5s and the cal out, when you change the timebase the displayed trace remains unaltered. That's because the scope adjusts the cal ut frequency so there are 5 cycles on the display. The scope isn't faulty, its behaving in an unexpected way is a problem.

author: _Wim_
author_rank: Super Contributor
timestamp: November 12, 2016, 11:28:09 am
content: You can make a nice frequency response graph by using the graph function on the measurements. Here is a graph I made from my "upgraded" DS1054Z trending Vrms and Frequency using the frequency generator of my CMU200 and the FreRes software. Scope has a 50ohm adaptor on its inputs. For reference, same cable directly to CMU200 input. As can be seen by the plots, -3db point is well over200 MHz! Not bad for a 50Mhz scope...

author: rstofer
author_rank: Super Contributor
timestamp: November 12, 2016, 03:42:53 pm
content: Nice test!I can see the scope has yet another capability I had never seen. I really should RTFM.