Duty Cycle Drift  - Page 1
Forum section: electronics / testgear

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 10:36:01 am
content: Morning all,I'm still playing about with cleaning up a gps module signal and for the first time checking the duty cycle.Infinite persistence shows the signal wobbling and veering from 47-53% duty. Not good, I thought.Then I did the same with an (admittedly old) OCXO and got similar results.Threw on the Siglent sig gen and pretty much the same.Is this common, or is my Rigol scope (doing the measuring) a pile of poo?It could of course and more than likely be that I'm a moron.Peter

author: bob91343
author_rank: Super Contributor
timestamp: March 11, 2021, 06:07:39 pm
content: The triggering is important. How fast is the duty cycle changing? You can expect some variation - these instruments are less precise than they seem.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 08:48:54 pm
content: The old trigger deception is why I'm confused.Edge trigger with minimum persistence gives me a solid waveform.Infinite persistence gives it the shakes.How fast is the duty cycle changing?Constantly.You can expect some variation - these instruments are less precise than they seem.That's what I'm hoping it is.Thanks.

author: tautech
author_rank: Super Contributor
timestamp: March 11, 2021, 09:16:41 pm
content: Try another scope.You want zero holdoff and rising edge trigger then set persistence with the trigger point on the display.Any wobbles with the trigger on the display points to the stability of the scopes timebase which will be worse with the trigger point off the display.Screenshots please or we're all guessing.

author: nctnico
author_rank: Super Contributor
timestamp: March 11, 2021, 09:30:43 pm
content: Quote from: purpose on March 11, 2021, 08:48:54 pmThere is always some variation. You do not specify which oscilloscope and frequency. I'm guessing you are measuring a 10MHz signal. You have a period time of 100ns. 6% is 6ns of wander. If you are measuring a sine wave or square with slow edge and/or the trigger level is not set to the steepest part of the signal, this can be normal. Especially if the signal isn't expanded to the full vertical size. All in all there are too many variables you do not provide us with to give an answer.For the best stability you need a steep edge to trigger on. In some case you might want to lower the v/div setting so the top and bottom of the signal are off-screen in order to get the steepest edge to trigger on (but be careful not to overdrive the input amplifiers). However in such cases measurements may no longer work.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 09:35:09 pm
content: Mr. TautechTry another scope.A Siglent, perhaps?If only.You want zero holdoffLooks like I'm limited to 16.0nsrising edge triggeryupthen set persistence with the trigger point on the displayHalf past three.That one is beyond me. Certainly got 8 choices of min to infinite. Infinite is the only one wobbling.Screenshots please or we're all guessing.I'd need video and don't have it.If ultrascope weren't so slow in the update, I could screen video capture it.Ta

author: tautech
author_rank: Super Contributor
timestamp: March 11, 2021, 09:40:17 pm
content: Quote from: purpose on March 11, 2021, 09:35:09 pmAny other scope to discount you haven't got a problem with yours.With your scope:Take a screenshot using Infinite persistence....simple.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 09:43:18 pm
content: Mr. nctnico,You do not specify which oscilloscope and frequencyIts a breathed on 1054Z and you are correct on the freq.You have a period time of 100ns. 6% is 6ns of wander. If you are measuring a sine wave or square with slow edge and/or the trigger level is not set to the steepest part of the signal, this can be normal. Especially if the signal isn't expanded to the full vertical size. All in all there are too many variables you do not provide us with to give an answer.That makes sense.For the best stability you need a steep edge to trigger on. In some case you might want to lower the v/div setting so the top and bottom of the signal are off-screen in order to get the steepest edge to trigger on (but be careful not to overdrive the input amplifiers). However in such cases measurements may no longer work.Marvellous.Many thanks.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 09:48:32 pm
content: Simple when you say it, Mr.T ... I'm just simple.

author: tautech
author_rank: Super Contributor
timestamp: March 11, 2021, 10:10:53 pm
content: Some example.Source 10 MHz square wave from SDG1032XScope SDS1104X-ENO reference link.....both units stand alone.Infinite persistence of ~1min.1. Trigger point center of display.2. Trigger point far off the display to examine jitter.....minimal with these simple settings.Greater vertical magnification would be required to measure any that is there....which will be the sum of both instruments.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 10:11:59 pm
content: Mr. nctnico,For the best stability you need a steep edge to trigger on.That worked a treat.Cheers

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 10:20:03 pm
content: And fine examples they are, Mr. tautech.Very clean.I'm limited to 5ns, but here is a 10MHz square from my SDG5082.

author: tautech
author_rank: Super Contributor
timestamp: March 11, 2021, 10:29:29 pm
content: Quote from: purpose on March 11, 2021, 10:20:03 pmNice and sharp at the zero crossing point too.Now shift the zero crossing point way way off the display, add persistence and then you have source/trigger jitter.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 11:44:29 pm
content: Looks worse to me.Better one