DHO804 Osciloscope not finding my signal - Page 1
Forum section: electronics / beginners

author: MrAlmagro
author_rank: Newbie
timestamp: March 12, 2025, 06:24:48 pm
content: First of all hello, im kinda new to this, i hope i explain myself properllyI bought this christmas the DHO804, im studying robotics, and the osciloscopes at uni are "the good ones" so wanted one at home so that i could do my proyects, as a starter, im meassuring the voltage to see the ripple in a ATX psu i had arround my house. Using the scope on both X1 and X10 i can see properly 3,3V and 5V at all divisions, from 20mV/div to 10 for ex.When i want to inspect the 12V, in a 200mV/div, it doesnt let me, the highest voltage it lets me analyze at 500mV/div would be 10V, and if i go onto 1V/div, it unlocks the rest. Is there any reason for this?This goes as far, as if i click the auto setting while in 12V, it tells me no signal, even tho i can see it in the screen, then if i swap to 5V it properly sees it.Thanks for your time

author: BennoG
author_rank: Frequent Contributor
timestamp: March 12, 2025, 07:34:11 pm
content: you are using the offset function to measure the ripple.To measure ripple you need to set the scope in AC mode and not DC mode.Then the DC part of the signal is ignored and you only keep the ripple.Benno

author: MrAlmagro
author_rank: Newbie
timestamp: March 12, 2025, 08:47:50 pm
content: Thanks, but in both AC and DC coupling mode it doesnt let me look into 12V, so its not about the fact of ripple, its more that it just doesnt tshow me the signal at all

author: SteveThackery
author_rank: Super Contributor
timestamp: March 12, 2025, 09:47:12 pm
content: Sounds like the trigger level needs adjusting.

author: MrAlmagro
author_rank: Newbie
timestamp: March 12, 2025, 09:49:02 pm
content: Doesnt the autoset autoput the trigger?And also if the 5V was being located, then why isnt the 12V, if the 12v is higher than the 5vMaybe im having a concept error?

author: NE666
author_rank: Frequent Contributor
timestamp: March 12, 2025, 10:16:44 pm
content: I think we're all struggling to properly visualise what you are seeing, from your description alone.How about posting some screenshots?

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 10:20:40 pm
content: Assuming you are looking for a small ripple riding on a larger DC level, AC coupling the signal removes the DC and allows you to look at the remaining ripple on some mV/div scale.Similarly, AC coupling the trigger lets you trigger at a given level and slope of the AC ripple waveform.

author: D2236
author_rank: Regular Contributor
timestamp: March 12, 2025, 10:23:29 pm
content: We have seen similar problems on our DHO 4404 in the lab.See post:https://www.eevblog.com/forum/testgear/rigol-dho-4408-hv-ripple-trigger-problem/msg5830737/#msg5830737When this happens, you can either not trigger on the signal, or the screen is blank, which sounds similar what you see. You can see the signal in roll mode but with faster sweeps, even with the scope showing triggered status, there is no trace on the screen. When you adjust the time base, you can momentarily see the trace for a fraction of a second, before it blanks out again. When operating in this state, it won't display in either AC, DC coupling, normal or HD acquisition mode. Auto-setup just reports there is no signal to trigger on. The screen remains blank in auto or normal trigger mode.It appears that rebooting the scope a few times, sometimes temporarily fix the trigger problem, but not the large additional DC offset we get when AC coupled with an HV probe as explained in that posting.

author: MrAlmagro
author_rank: Newbie
timestamp: March 13, 2025, 11:04:47 am
content: Here you got a video so that you can see better whats happening to me

author: Aldo22
author_rank: Super Contributor
timestamp: March 13, 2025, 11:24:01 am
content: I don't have this device, but it looks to me like you are using DC coupling.Try AC coupling.

author: MrAlmagro
author_rank: Newbie
timestamp: March 13, 2025, 12:08:08 pm
content: Exactly the same happened when using AC coupling tho

author: NE666
author_rank: Frequent Contributor
timestamp: March 13, 2025, 12:11:03 pm
content: Thanks for posting the video, a picture is worth a thousand words.It also makes it clear what is happening - as a number of other posters have already told you, you cannot measure such small DC ripple on a large DC offset using DC coupling. You must use AC coupling, to reject the DC bias and leave only the ripple signal, which then won't exceed the dynamic range of the ADC at lower v/div, and will allow the trigger circuit to work correctly.So, set it to AC and show us what you get then.

author: Aldo22
author_rank: Super Contributor
timestamp: March 13, 2025, 01:03:24 pm
content: Quote from: MrAlmagro on March 13, 2025, 12:08:08 pmSo why are you making a video with DC coupling instead of AC coupling? That's wrong anyway.“Auto Set” can also switch back to DC coupling.As I said, I don't have this scope and don't know how it behaves.

author: MrAlmagro
author_rank: Newbie
timestamp: March 13, 2025, 01:05:03 pm
content: So as far as i understand now, In a DC mode i can see signals in a general term right, then i can zoom in for low voltage signals.If i want to see ripple for bigger ones, i use AC mode, which deletes the DC component and lets just see the ripple.In this video i can see the ripple of the signal but on 0V, and as you can see, the auto mode still cant find the signalhttps://youtu.be/6z96-l2uCWoThanks for all

author: SteveThackery
author_rank: Super Contributor
timestamp: March 13, 2025, 01:09:38 pm
content: Quote from: MrAlmagro on March 13, 2025, 11:04:47 amYou are allowed to speak, you know!

author: NE666
author_rank: Frequent Contributor
timestamp: March 13, 2025, 02:47:53 pm
content: Quote from: SteveThackery on March 13, 2025, 01:09:38 pmIn fairness to the OP, if English isn't his first language, he may lack the confidence to do so and/or realise his limitations to fluency for an engineering audience. I know that I wouldn't be attempting Spanish, not that I could do so meaningfully.

author: MrAlmagro
author_rank: Newbie
timestamp: March 13, 2025, 02:58:02 pm
content: Kinda forgotIm not used to forums but thanks for the advice, will do it next time

author: NE666
author_rank: Frequent Contributor
timestamp: March 13, 2025, 03:00:28 pm
content: Quote from: MrAlmagro on March 13, 2025, 01:05:03 pmBefore you go further, I suggest you take a look at this EEVblog video covering scope and trigger coupling:What is clear from your second video is that you are still preoccupied with DC coupling, which cannot be used to do what you are attempting. Furthermore, in the early part of this second video, where your input coupling is set to AC, you don't seem to have any kind of signal level apparent that could be triggered on. That maybe since you seem to be on 500mv/div vertical scaling? Why not set it lower until you actually see something?Things to consider:1. Probe attenuation should be set at 1x, both on the probe and on the scope2. Input and trigger coupling should be AC3. Input Bandwidth limiting should be 'on' (i.e. <20Mhz)3. "AUTO" is not magic, on any scope, and will not lead to a stable trigger in all cases. Especially where the signal level is low, and mixed with noise, as is likely the case here4. Vertical scaling needs to be set so that you (and the scope's trigger) can reliably see something5. You may need to adjust trigger level manually to get a stable trigger on the ripple. If there is a lot of noise (see below), you may be better of with a single-shot and using cursor measurements on that6. You may need a better probing arrangement (especially a shorter ground lead length) if the ripple is masked by a lot of noise