Is my Oscilloscope broken ? Update June 23rd. NO, it is not broken ... - Page 1
Forum section: electronics / testgear

author: NHSA
author_rank: Regular Contributor
timestamp: June 22, 2021, 09:45:46 pm
content: Hi,No probes connected straight line.Probes connected sine wave with 60 Hz.Is this just terrible nosie from something or is my Oscilloscope broken ?Two days ago eveything was fine. What sort of electric device can make such Noise if this is Noise.I just hope it is noise from something ...Thanks for any tips and comments ...Rainer

author: tom66
author_rank: Super Contributor
timestamp: June 22, 2021, 09:58:40 pm
content: That's a bit noisy but it may be by design (or lack of design, maybe it needs a shield can or two.) You haven't specified a model number so nobody can do a comparative test.The most common source of this noise in my experience is SMPS power supplies without adequate filtering, have you plugged in or bought anything new into your office/lab with a power supply like that? Could also be your neighbours or power line communication.

author: NHSA
author_rank: Regular Contributor
timestamp: June 22, 2021, 10:24:09 pm
content: Quote from: tom66 on June 22, 2021, 09:58:40 pmHi Tom,Sorry. It is a Siglent SDS 1102CML+. No, I have not plugged in anything new in the room.And no I have not installed any powerline communication device at all. What about Baby radios or something like that, WiFI extender ?, in my neighbours house. Could that be a reason and it transmits noise into my house cabling too ?I have Solar panels on the roof but when I got the Oscilloscope new I did not see anything like that.regards Rainer

author: NHSA
author_rank: Regular Contributor
timestamp: June 22, 2021, 10:51:57 pm
content: Hi,I just tested it again. Strange curve.Rainer

author: AVGresponding
author_rank: Super Contributor
timestamp: June 22, 2021, 10:53:31 pm
content: You're measuring quite low magnitude signals there, some noise is to be expected.What is the 60Hz sine source? And are you certain the ground is continuous between source and scope?It does look quite a bit like the kind of random noise you get from not having your probe properly grounded.EDIT: The second set of pics look like the output from a really crap inverter or generator. Also similar to what you get from a ferro-resonant transformer fed with a square wave (an old kind of crap inverter).

author: StillTrying
author_rank: Super Contributor
timestamp: June 22, 2021, 11:01:33 pm
content: "Strange curve."Looks like normal mains noise.Have you got the probe's point shorted to 0V with the GND clips, and both probes on the same X1 or X10 so both channels show the same.Look at the amount of noise I got from a faulty street lamp outside.https://www.eevblog.com/forum/beginners/scope-trace-interference/msg1251655/#msg1251655

author: NHSA
author_rank: Regular Contributor
timestamp: June 22, 2021, 11:27:16 pm
content: Quote from: AVGresponding on June 22, 2021, 10:53:31 pmHi,I am not measuring anything. It is just the probe hanging in the air or laying on the desk.I was working on an amplifier circuit and suddenly, yesterday, that noise appearead and is transmitted into the input of my circuit and is amplified too and the real input device, a flexible Piezo element dos not work anymore. Day before yesterday eveything was ok.QuoteI have no idea who could have connected an inverter or something. What about a faulty inverter of a Solar panel installation ?regards Rainer

author: radiolistener
author_rank: Super Contributor
timestamp: June 23, 2021, 12:10:18 am
content: Quote from: NHSA on June 22, 2021, 09:45:46 pmSince you're using 10 mV/div, its pretty normal.High impedance wide bandwidth amplifier input always have pretty high noise power. This is just a property of nature.Higher impedance = higher noise floor.Wider bandwidth = higher noise floor.Quote from: NHSA on June 22, 2021, 09:45:46 pmSuch noise is a noise of amplifier with a high impedance input on the oscilloscope frontend.Any oscilloscope with 1 MΩ input have such noise.The reason for this noise is high impedance and wide bandwidth of the oscilloscope input.Quote from: NHSA on June 22, 2021, 11:27:16 pmThis is pretty normal.First, the probe has very high impedance (1-10 MΩ), so it is very sensitive to interferences.Second, your probe is not connected to anything, so it means that the probe is connected to an air space between probe GND and probe tip. Air impedance is even higher than 1 MΩ, it leads to a higher noise level.Try to connect the probe to circuit with low impedance, it should reduce the noise.Also note that short circuit with a long wire (for example 5-10 cm) doesn't means low impedance for RF, because such wire has inductance and it leads to a inductive impedance. At high frequency (like 100 MHz) even a small inductance produce noticeable effect.For example, if you put your probe ground wire on a probe tip (short circuit) it doesn't means that noise will be reduced. Just because your ground wire forms a loop. Such ground loop works as antenna and can increase noise interference because it will suck electromagnetic noises from environment.The best way to test your oscilloscope self noise is to disconnect your probe from oscilloscope connector and keep any metal things and your body away from the connector for 0.5-1 meters. In such configuration you can measure noise floor of your oscilloscope. If noise with your probe or other connected circuit is higher than oscilloscope noise floor, then it means that this noise is come from your probe or circuit.Note: such test with open connector is relevant for 1 MΩ input. If your oscilloscope has 50 Ω input, then you're needs to connect a small size 50 Ω terminator on the input connector for testing.

author: NHSA
author_rank: Regular Contributor
timestamp: June 23, 2021, 01:18:19 am
content: Hi radiolistener,Thanks for the explanation.Two days ago everything was OK. It started yesterday,Here the noise floor of the oscilloscope with no probes connected at all. I think the noise is very low.Here the two probes connected and amplifier no running. As you can see that is OK but probe2 (violet) has that noise.Here the wave generator is not connected and amplifier is running and the signal from the probe 2 (violet) is amplified. This did not happen two days ago.And here the wave generator is feeding a signal into the amplifier input. This is OK as it also was two days ago.Now if attach the sensor which is a flexible piezo sensor which gives voltage when bend nothing happens. Two days ago I had a nice signal and now nothing happens as the probe noise kills any input from the pieze sensor.Will follow you advices but I have no 50 Ohm terminator. Will need to check what the input of my oscilloscope is.The question here really is that everything was OK two days ago and yesterday it started with that horrible noise.Thanks everybody for the advices and I will test all your tips.regards Rainer

author: mindcrime
author_rank: Unknown
timestamp: June 23, 2021, 01:54:01 am
content: Huh. Any time I see spurious signals that just happen to be right around 60hz (or whatever the local mains frequency is), my first hunch is always "environmental noise". But the magnitude on CH2, while small in absolute terms, seems a bit high for just noise leaking in from something mains powered, unless it's *really* doing some weird shit. Do you have a fluorescent light with a ballast nearby or anything like that?I tried with my scope just now, just letting a probe hang freely in the air, and turned the volts/div all the way down, and fiddled with the trigger level and could see some environmental noise, but the magnitude pretty much never made it out of the uV range, or maybe peaked at like 1mV briefly here and there. And the signal was far from a pure sine wave.... in fact, it was so noisy the scope wouldn't even measure and report the frequency.Getting a pretty much pure 60Hz sine wave at 36mV definitely seems odd. I wouldn't necessarily jump to concluding the scope is broken, but it is a little weird.Here's a thought: could you try switching the probes between CH1 and CH2 and see if anything changes? Just curious if the probe itself is involved somehow.

author: radiolistener
author_rank: Super Contributor
timestamp: June 23, 2021, 09:32:59 am
content: Quote from: NHSA on June 23, 2021, 01:18:19 amIt looks that there is some noise source in your circuit. may be noisy power supply or a long wires.If such noise was missing several days ago, then something changed. Just look at your circuit for any changes.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 23, 2021, 09:50:01 am
content: As just about everyone else has said it's normal to see some noise at high input sensitivity with probe left floating you get a little capacitive coupling (antenna) between probe tip and surrounding power lines.Considering that you have noticed a sudden difference however it may be worth checking that the scope's earth connection isn't broken somewhere. First check with an ohmmeter between BNC shield and earth connection on a mains socket. Of course if there's no problem there you could still have a bad earth connection in the building itself.The spikey look of your noise could fit the no-earth scenario as it would show some of the scopes own SMPS noise.

author: Brumby
author_rank: Unknown
timestamp: June 23, 2021, 03:14:25 pm
content: I, too, am suspicious of the earthing - but as you are saying the probes aren't connected to anything, then I don't think that will be particularly relevant.What I will ask, though, is this: Are the probes "laying around" in the same position as they were a couple of days ago? Is the scope? Are any and all other cables and equipment in the same position as they were before - AND - are they all powered/unpowered as they were before?What I see there is what others have said: the picking up of signals that are floating all around us, especially in our electrified society - and when you have a high impedance input, you are going to see a good portion of that crap.

author: NHSA
author_rank: Regular Contributor
timestamp: June 23, 2021, 07:59:49 pm
content: Hi,OK, my Oscilloscope is not brokenand thanks for all your answers. I found a partly culprit and that was myself. Many, many years ago I added an outlet to my normal installation and did not include Earth on this. Today I got some 3 wire cable and exchanged the old cable.The noise went down to 1/3 rd of what I had before measuring in the same way I posted in a pic in a previous message.Interestingwise measurung with a scale of 50mV I get now ~ 20mV peak to peak. Measuring with a 2 mV scale I get ~ 8 mV peak to peak. Tested another pair of probes (not originalSiglent but 100MHz rated too) and same result.So I need to put more attention to my electrical installation. Shame on me.regards Rainer