One scope channel noisier - Page 1
Forum section: electronics / testgear

author: Vytautas
author_rank: Regular Contributor
timestamp: September 10, 2021, 07:33:31 pm
content: Hi, guys.Has anyone encountered a similar problem? Recently I noticed that one channel of my GDS-2072E scope (software upgraded to 300Mhz) became noisier. I include two screenshots in different timebases. Both channels are set to exactly same parameters, and nothing is connected to the inputs. Noise is clearly seen. Previously, the noise level of both channels was equal. What could have happened? Any ideas? I would be grateful for anyone's input.

author: Vytautas
author_rank: Regular Contributor
timestamp: September 10, 2021, 07:50:41 pm
content: And here is what happens to the first channel when I connect a 50Ohm terminator. Definitely something wrong. No such thing, when I connect the terminator to the second channel.

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2021, 09:00:57 pm
content: All sorts of weird things show up at max sensitivity.....the frequency counter can give clues to where it's coming from.Go measure something real.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2021, 09:05:23 pm
content: Try changing your trigger to CH2 and LINE and see how it looks. And post a photo of this 'terminator'.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: September 10, 2021, 10:27:04 pm
content: 40khz, my bet is screen noise, does it show with a probe connected?

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 10, 2021, 10:38:17 pm
content: Poor grounding scheme I'd be tempted to say...Is the noisier channel closest to the PSU or the CCFL inverter?The frequency and jittery aspect would definitely point to one of the PSU's.Inceasing the bandwidth also means increasing the chance of hitting on noise somewhere, I'wouldn't expect it to have much noticable effect at 40kHz though.Maybe one of the PSU's snubber frequency? Try going ETS to see if the little spikes are actually MHz range damped oscillations.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2021, 11:09:54 pm
content: A channel can become noisier if the input is damaged by overload. This is usually accompanied by high input leakage which you may be able to test for by measuring the input current.

author: nctnico
author_rank: Super Contributor
timestamp: September 10, 2021, 11:16:51 pm
content: Yup. With the inputs covered with a 50 Ohm terminator nothing should show except random noise. If something does show, then something is wrong.

author: Vytautas
author_rank: Regular Contributor
timestamp: September 11, 2021, 05:29:43 am
content: Thanks all for your ideas. I also guess that I might had done something that might have damaged the input circuitry. Noise has not appeared after upgrading to higher bandwidth, it was not there for a long time after the upgrade. So, this does not seem to be screen noise or poor grounding scheme. Probably, I have damaged the inputs somehow.

author: Vytautas
author_rank: Regular Contributor
timestamp: September 11, 2021, 05:36:18 am
content: By the way, I had ordered a Micsig TO1104 scope a few weeks ago from Banggood (need four channels or portability sometimes). When that comes, it is going to be easier to trace the fault. Hope, it does not go beyond the input amplifier to the ADC.

author: Vytautas
author_rank: Regular Contributor
timestamp: January 04, 2022, 11:53:58 am
content: Hi,this is how things cleared up with my scope, finally. The Micsig scope came and it is a fine device, but it was not needed. I noticed that just by moving around the wire ribbon going from the power supply to the board noise performance of the GW Instek 2000E changes DRAMATICALLY! Try it for yourself. As I was experimenting with moving the ribbon, suddenly the scope died totally. What??!!! I suspected, I created a short somehow. Grabbed my Cat 60 phone, turned on the thermalvision camera and voila - two of the wires heat up. Measured with a mulimeter, - shorted. Then I resoldered the power connector pins on the board and the short is gone. Scope works again. Finally, I found a position for the ribbon to achieve best noise performance - bent it totally to the right side, close against the bunch of power supply capacitors. Now noise performance is good.This is how all turned out.Cheers.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 04, 2022, 03:33:09 pm
content: A photo would have been great.I have trouble believing GW Instek (not the best around but far from worst quality) mass produced a scope that is so sensitive to wire routing unless there is an obvious way they should be placed in the scope. If it is a poor design I would have considered shielding the PS to mainboard wiring.Did you understand what was actually causing the short before resoldering the connector?

author: Vytautas
author_rank: Regular Contributor
timestamp: January 04, 2022, 07:04:01 pm
content: I opened the back and tried moving the ribbon cables again to take some pictures for you, and surprise - now moving the ribbon does not change anything!!! So, maybe when that noise problem had appeared, there was little something already happening and it was enough to cause the noise problem in the first place and then to cause the short? I do not know. But the noise performance was clearly changing when moving the ribbon. And the scope just died. When switched on, a little LED in the main board was regularly flashing and a faint clicking sound was coming from the power supply board at the time of each flash, protection clicking in. And, clearly, the short happened in the board itself as resoldering the connector pins solved the problem. Hmmm. I am happy now the short happened. This helped me to resolve the issue!

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 04, 2022, 08:24:46 pm
content: The only GWI I've been in had a PSU and CPU on upper side, acquisition board on lower side with an aluminium plane separating them.I'd be suspicious that there is (or was) a loose conductor floating between one of the PCB's, it could be a screw, frontend shielding cover, EMC grounding pad...In your case I'd disassemble everything to make sure, if there is a loose part it may move again and cause fatal damage to the scope.

author: Vytautas
author_rank: Regular Contributor
timestamp: January 10, 2022, 06:27:53 am
content: Thanks. I disassembled the unit totally, as I decided to replace the trigger encoder (pushbutton had almost stopped working). At the same time checked for anything lose or anything that may have caused the short. Nothing. So, lets see. The scope now works perfectly. And it has a much better noise performance than my new Micsig scope. But the latter one is four channels.