Rigol DS1054Z Trigger weirdness? - Page 1
Forum section: electronics / testgear

author: Ondre
author_rank: Unknown
timestamp: January 24, 2017, 09:00:43 pm
content: Hi,I've just encountered some strange (at least to me) behavior from my relatively new Rigol DS1054Z. I was probing four signals, triggering on falling edge on channel 1 (normal, not auto). With auto-memory depth setting enabled, it looks like the scope is triggering a second time and puts two waveforms on top of each other, while when I manually set the memory depth to 300K, the waveforms look like expected. Turning anti-aliasing settings off or on doesn't make any difference. Any clue whats going on? Maybe just a pebkac?CheersAndré

author: ebastler
author_rank: Super Contributor
timestamp: January 24, 2017, 09:27:31 pm
content: Well, your second screenshot shows that the signal on channel 1 has four different falling edges to choose from. Why shouldn't the scope "randomly" select any one of them to trigger, depending on which one comes along next when the scope is ready to trigger again?I assume that, by setting the memory depth to 300k, you have influenced the scope's processing speed for each scan, and (by coincidence) have brought the scope into sync with the signal on channel 1. So the scope happens to always trigger on the same phase of channel 1 due to this synchronization.If that's what is going on, the clean way for obtaining a stable trigger would be to use one of the more complex trigger conditions the scope offers. E.g. trigger on a falling edge preceded by at least 1µs "high" state or such, to always trigger on the first falling edge in the group of four.

author: Ondre
author_rank: Unknown
timestamp: January 24, 2017, 09:42:54 pm
content: Ok, I'm fine with the scope triggering on any falling edge that comes along, but shouldn't it display only one of them at a time? The two trigger events are probably really short one after the other. So is this maybe some persistence that is displayed on the screen? It seems that you can't turn off persistence completely, or at least I haven't found this setting yet

author: ebastler
author_rank: Super Contributor
timestamp: January 24, 2017, 09:53:16 pm
content: Quote from: Ondre on January 24, 2017, 09:42:54 pmDid you use "single trigger" mode then? If so, I had not realized that from your initial post, sorry.[EDIT: Looking more closely at the screenshots, I think you arenotin single trigger mode. If you were, the trigger status should read "STOPPED" instead of "WAIT" in the upper left, if I recall correctly.]If, on the other hand, you use "normal" triggering, the scope will keep triggering and writing traces whenever another valid trigger event comes along. They all get overlaid on the screen. So what you are seeing on the screen are probably not two traces, but the overlay of many more. (With different brightness in case they are written with different likelihood.) -- Apologies if this is trivially clear to you already; don't know your level of scope experience.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 24, 2017, 10:11:42 pm
content: This can happen on analog scopes too. If you want to continue with Edge triggering, you can try adjusting the Trigger Holdoff setting. The default value is 16.0 ns; try entering a value greater than the length of the pulse train. Like 2.6 µs or so.

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: January 24, 2017, 10:32:01 pm
content: Quote from: alsetalokin4017 on January 24, 2017, 10:11:42 pmThis. The scope is re-arming the trigger in the middle of the "pulse train" state and triggering on the falling edge of one of the middle pulses instead of one the first pulse.The "Holdoff" modifies how fast the trigger re-arms. By increasing the length of time the scope takes to arm the trigger, you ensure that the scope isn't arming in the middle of a pulse train like it's doing now.

author: Ondre
author_rank: Unknown
timestamp: January 25, 2017, 06:24:38 am
content: Ok thanks folks! I think I made a bit of a fool out of myself. Didn't realize that the two signal sequences were spaced apart that short. Looking at the same signals with my old scope, I haven't noticed this. Next time, no forum posts prior to dinner.