Excessive Noise with Siglent SDS1102CML+ - Page 1
Forum section: electronics / repair

author: fatalelieberi
author_rank: Contributor
timestamp: September 15, 2020, 01:49:17 am
content: Hi,I think I'm having a problem with my SDS1102CML+. It has excessive noise even when the probes are disconnected from the inputs. But, I don't know if this is normal because I haven't used the scope for a year and I forgot how it was before then. You can see the waveform below. Can someone confirm that this is not normal with this model?

author: Circlotron
author_rank: Super Contributor
timestamp: September 15, 2020, 02:38:21 am
content: I had a similar problem recently. Turned out it was the SMPS for the LED bench light.

author: wraper
author_rank: Unknown
timestamp: September 15, 2020, 02:41:09 am
content: Yeah, quite likely this is not a problem with oscilloscope itself. Try disconnecting everything else from mains. Or take this oscilloscope elsewhere and check there.

author: fatalelieberi
author_rank: Contributor
timestamp: September 15, 2020, 04:29:57 am
content: So, it is not faulty oscilloscope? Okay, I'll look at other appliances that are connected to the mains. Anyway, if I find the causing device and it is still needed, is there a way to isolate it so it doesn't affect my scope?

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2020, 06:37:06 am
content: Quote from: fatalelieberi on September 15, 2020, 04:29:57 amIt won't affect the scope badly in any way and certainly it's from SMPS nearby.Even just a phone charger or any SMPS wallwart can do this along with local LED lighting and the distances can be quite surprising like from another room.Averages can reduce the impact on the waveform however if the signal you're working with is not repetitive averaging can effect it too.Sometimes it's something you need just live with and know at increased sensitivity it will be there.

author: fatalelieberi
author_rank: Contributor
timestamp: September 15, 2020, 06:46:59 am
content: I thank you guys who gave a pointer! You were right! It wasn't the oscilloscope, but it was the LED strips.So, there are two shelves above my work bench, and my scope is sitting on the first shelf. Under each shelves, there is an LED strip powered by a single AC adapter. There's a Y splitter to power those strips and each strip has its own switch. If I turn on one of these strips, I get the noise. However, if I directly connect the strip under the second shelf to the power adapter and power it on, I don't get the noise. If I directly connect the strip under the first shelf and power it on, I get the noise. But, if I move my scope away from the shelf, the noise is gone. In conclusion, I think the cause is an electromagnetic interference from the first shelf strip since the scope is sitting on it.Also, while testing, I do notice that when the scope is in AC coupling mode, there's still a DC offset about 4mV in each channel when the input is disconnected, and with strips turned off. It's not a sign of damage from the EMI, is it?

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2020, 08:22:28 am
content: Quote from: fatalelieberi on September 15, 2020, 06:46:59 amIt is and sometimes you can use this to your advantage by just clipping the probes reference lead to the grabber and this makes a poor mans RF pickup where if you hold it close to a switching circuit you have often measure the frequency it's operating at.QuoteNo, you won't have hurt the scope in any way unless you have applied 400+V into its inputs.Some 0V offset is common in AC mode and this changes as the instrument warms however when warm you can activate the scopes internal Self Cal from within the Utility menu for it to correct the offset.It takes a few minutes to complete and needs be done with probes disconnected.

author: fatalelieberi
author_rank: Contributor
timestamp: September 15, 2020, 08:43:13 am
content: Quote from: tautech on September 15, 2020, 08:22:28 amThat's a nice trick!Quote from: tautech on September 15, 2020, 08:22:28 amSo, that means the offset will drift with temperature changes of the scope? Would it be a good practice to do Self-Cal every time I notice non-zero offset in AC mode?

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2020, 09:27:18 am
content: Quote from: fatalelieberi on September 15, 2020, 08:43:13 amEverything in an instrument undergoes some slight changes while it warms up. Some scopes have Auto Cal to compensate for both the ambient and instrument temp however the entry level CML+ does not.QuoteYes, no, maybe.Get to know your how your scope behaves over time is best advice and keep it at similar temps when Self Cal was last done for best accuracy. Remember scopes are only rated to+3% vertical accuracy and that is normally after a 30 minute warm up.

author: fatalelieberi
author_rank: Contributor
timestamp: September 15, 2020, 11:31:41 am
content: I've done testing my scope now. I timed when the DC offset stabilized after turning on the scope. It took about 30 minutes just like what you said. Also, I am amazed by how much it drifted. It started with -40mV and after 30 minutes, it got to +10mV, so about 50mV drift! No wonder I had some difficulties measuring voltage across current shunt resistor.As a side note, when testing the drift, I found out that my second channel waveform sometimes disappeared and came back again, like a blink. It was random though, I couldn't find any pattern of blinking second channel. Is it a known bug? I have flashed with the latest firmware from Siglent EU website.Another side note, I think the buzzer speaker inside is affected by the temperature. When it was cool, the speaker just worked well. But, after it warmed up, it stopped sounding anything.

author: aqibi2000
author_rank: Regular Contributor
timestamp: September 15, 2020, 07:32:41 pm
content: Sell that scope

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2020, 07:46:36 pm
content: Quote from: fatalelieberi on September 15, 2020, 11:31:41 am10x probe ? If so that's a 5mV drift.QuoteSome prefer a beeper, I don't. However it shouldn't do that. Report it to your supplier.

author: fatalelieberi
author_rank: Contributor
timestamp: September 16, 2020, 01:56:05 am
content: Quote from: tautech on September 15, 2020, 07:46:36 pmSilly me forgetting the 10x attenuationQuote from: tautech on September 15, 2020, 07:46:36 pmI think I can live with it. It has just been repaired because of its broken knobs and keypad and it took 3-4 months to get the replacement part. That's why I am very cautious from any damage to my scope.Anyway, I thank you so much for your help. It's a relief that my scope is not broken, except for the beeper.