Yet another oscilloscope input noise question - Page 1
Forum section: electronics / testgear

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 01:35:21 am
content: Hi! I've just got my beautiful Keysight DSOX1102G oscilloscope, which I won on EEVBlog (thank you, Dave!). I'm setting up a lab at home and one of the first things I did was of course to test the oscilloscope. I know that "high" bandwidth scopes are very unforgiving about the input noise (Dave has a whole video about it). However, I'm able to get a reasonable 5mv "blob" in the 5mv scale if I don't connect the test probe, or even with the test probe touching the GND point without the ground cable clip (first picture). Of course the ground cable clip with the alligator will introduce some noise, but I think the figures I'm getting are kind of ridiculous. I'm connecting the test probe to the GND point as shown in the second picture (both the ground clip and the signal), I've set up the probes to the appropriate values (10:1 in scope and on probe) and I'm getting almost 0.2V of noise even with 20 MHz bandwith limit! Isn't that too much? The third picture shows the captured noise.I know it's not really a scope or probe problem, since I get a reasonable noise figure without the GND clip. But I'm amazed that the short GND clip would pick that much noise.It looks like the noise I pick up is related to my environment (I live in downtown Buenos Aires, which is a really big city). You can even hear a couple of AM radio stations in the land phone line. I've got a somewhat reasonable mains Earth connection, but noise gets a little bit lower if I place my hand over the scope's mains cable, which suggests ambient noise. I've got no LED lights or other switching noises in the vicinity that I'm aware of, at least from my floor, but I live in a 10th floor and I've got neighbors up and downstairs. Looking at the signal (4th picture), it looks like something around 10 MHz modulating a 100 MHz carrier (even with a bandwidth limit in a 100 MHz scope.... mmmm... aliasing?). Perhaps it's someone's internet connection?The questions are: do you think this level of noise is normal and acceptable to work with? And how can I damper the noise as much as possible from outside the scope? I mean, without resorting to BNCs and 50 ohm terminators. I'm trying to get a ferrite for the mains cable and see if it helps, but I'm worried that my home is really too noisy to get a better signal.Please let me know what you think! Thank you!

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2017, 01:57:31 am
content: Congrats on your new scope.Noise is normal, how you interpret it is the key to understanding what you see on the display.Most often when the scope is connected to an active signal source on a DUT noise levels become irrelevant.The reference lead on the probe IS an antenna so if you go looking for noise you will find it for sure.You should have in the probe packets a curly little spring thingy, that's to use instead of the reference lead so rip the lead off and the grabber, fit the reference spring and hold it against the Cal output and go looking for noise now.You should plainly see the difference.Use the scope, become familiar with it and learn to understand its limitations.

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 02:53:10 am
content: Quote from: tautech on May 29, 2017, 01:57:31 amThank you, tautech. I've been using scopes in my office for years (granted, they were always the same low-end Siglent scopes and an old portable 20 MHz Fluke) but they don't look nearly as noisy as this! Isn't 200 mV too much? Connecting signal and GND together is the most stable DUT I can think of. And of course I could use the high-frequency springy, but that's not the point of my question.

author: Joel_l
author_rank: Frequent Contributor
timestamp: May 29, 2017, 03:12:26 am
content: That doesn't seem so bad. With my R&S RTB I see 1.75 mV with a 50 Ohm terminator plugged in. With the supplied probe at 1:1, I see about 55 mV, at 10:1 just over 200mV.

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 03:14:56 am
content: Quote from: Joel_l on May 29, 2017, 03:12:26 amHi, Joel. With the leads open or connected together as in 0V?

author: TK
author_rank: Super Contributor
timestamp: May 29, 2017, 03:27:22 am
content: It doesn't look normal. I am getting 21mV Pk-Pk at 10:1. Downtown Buenos Aires... is the scope plugged to a grounded outlet?

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 03:40:04 am
content: Quote from: TK on May 29, 2017, 03:27:22 amYes, it is a grounded outlet, altough the ground connection is really poor. I live in a 10 th floor and I connected the GND to the building's ground connection, which goes down 50~60 meters and God knows if it's just wrapped around a water pipe. I'm reading ~214V from live to ground, and ~0V neutral to ground (live to neutral: 220V). I don't think I can get a better ground connection than this unless I invest a couple thousands dollars on cabling and contractors. There's not even access to actual dirt because the subway goes just below my building, and the connection would anyway be very very long.

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2017, 03:49:09 am
content: Quote from: guillep2k on May 29, 2017, 02:53:10 amReally it is. Nothing to do with HF, the reference lead is an antenna like I say and will pick up anything it can at high sensitivity settings.Try it.IIRC the KS's have 2x the sampling of the low end scopes you say you've used.Set inputs attenuation and probes the same and just use it.If you've mucked with it heaps, push the Default and get back to known settings and start again.

author: TK
author_rank: Super Contributor
timestamp: May 29, 2017, 03:53:00 am
content: Quote from: guillep2k on May 29, 2017, 03:40:04 amYou can bring it to your office tomorrow and compare with the other scopes in the same environment

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 03:57:14 am
content: Quote from: tautech on May 29, 2017, 03:49:09 amIt's not that I did not try the "springys": I did and those were the first results I mentioned in my original question. But an oscilloscope comes with clip leads that will help you measure everyday things and those -within limitations- should work fine. For example, I can't use the 1 KHz probe compensation output -which is very difficult to measure with the springy- as I get 10~15% of noise over a 2 Vpp signal!

author: TK
author_rank: Super Contributor
timestamp: May 29, 2017, 04:05:42 am
content: Quote from: guillep2k on May 29, 2017, 03:57:14 amYou can compensate without connecting the ground pin to the 1KHz ground pin.

author: TK
author_rank: Super Contributor
timestamp: May 29, 2017, 04:06:51 am
content: Quote from: guillep2k on May 29, 2017, 03:57:14 amHave you tried the signal from the WaveGen? Is it also noisy?

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2017, 04:12:22 am
content: Quote from: guillep2k on May 29, 2017, 03:57:14 amSo you've nuked the spring, it's not good for much now.All screenshots show the Trigger setting outside the waveform, why ?Connect it to the Cal terminals, hit Default then Autoset and show us a screenshot.

author: rdl
author_rank: Super Contributor
timestamp: May 29, 2017, 04:13:38 am
content: I always have some noise on my Rigol (college radio station a couple of miles away probably), but 200 mV seems like an excessive amount to me.

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 04:28:33 am
content: Quote from: tautech on May 29, 2017, 04:12:22 amOh don't worry, that's not the original springy! What kind of animal do you think I am?That's copper wire. The original springy is very difficult to place correctly between those terminals so I first tested with the springy and then, knowing what to expect, I improvised a new one just to take the picture, with same results.Now, look at my setup for compensation: as you've suggested I disregarded the ground pin (just as I do in my office, BTW) as shown in the first picture; no ground clip. You can see in the second picture that the scope wouldn't trigger to that mess properly, even when the signal is 2 Vpp and the trigger level is right at the middle. In the third picture you can see the capture when I hit Run/Stop. So, the signal is there, but very difficult to use.I call that UNUSUAL.

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2017, 04:52:59 am
content: Not having driven a KS I'm almost out of suggestions excepting the probe ?Same result with the other ?

author: JPortici
author_rank: Super Contributor
timestamp: May 29, 2017, 05:53:55 am
content: do an FFT of the input, be sure to see at least 108 MHz. Prepare for a "duh" momentspoiler alert: maybe somebody near you has an FM repeater, as it happens near where i liveI also have FM radio that is picked up by all test equipment and even demodulated by some old audio mixersunfortunately i don't have the equipment necessary to track down who he is and ask him nicely to turn it off

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: May 29, 2017, 06:21:35 am
content: I'd definitely be interested to see if:1. The other probe does this too?2. Other channels do this too?3. Other scopes in the same area do this/the scope in a different environment still does it?

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 12:58:21 pm
content: I'll make more measurements (including my office scope) and get back to you tonight.

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2017, 06:16:17 pm
content: I tried to duplicate your setup and got about 2 minor divisions of peak-to-peak noise (1) at 20mV/div using a x10 probe shorted and connected to the chassis ground so 8mV peak-to-peak. Connecting to ground actually reduced the noise slightly and to about the same level as enabling ground coupling.Maybe there is something about your environment?It is a little difficult to see but your automatic RMS measurement is consistent with the automatic peak-to-peak measurement; it is gratifying that Keysight's automatic RMS measurement works on noise. It is difficult to tell but the graticule seems to show a lot more than 101mV of peak-to-peak noise though.(1) Acquired using peak detection so this is a truly worst case value.

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 08:43:23 pm
content: Quote from: David Hess on May 29, 2017, 06:16:17 pmI'm pretty sure it's the environment. That's why I first asked for an external way of reducing noise, if there's any. I'm not doing peak detection but it doesn't make any difference at 5ns/div (I've tested that, now).Quote from: Keysight_DanielBogdanoff on May 29, 2017, 06:21:35 amChanging probes or channels doesn't make any difference, only moving the probes around; the noise figure is very sensitive to my cable "geometry". I don't have any other scopes around to test with, but I'm pretty convinced it's not the scope. It's got no GND coupling, but shorting the output at the probe lead (without the GND clip) gives me basically the same reasonably good noise figure as if I leave the BNC input open or even short (at the scope), which is ~5mVpp as measured or one division visually at 10 mV/div, 10:1 (first picture). Sorry I've got no better equipment to test with (like BNC cables, terminators, etc.); I will buy all of that eventually, but it's really expensive in my country.Now, to my "new" measurements:If I skip the ground connection (leads as in 2nd picture) and performthe swan lake*with the cables, I can get the much lower noise figure of 20 mVpp (3rd picture). Mind you, this is very carefully placing the cables, looking for good "shapes" and orientations.(*) Or "the eel singing boleros", as we say in Argentina.All things considered, it looks like my mains ground connection is pathetic, and I have no chance of improving it. I'll try to get a ferrite choke for the mains cable of the scope, but that will take a couple of days since I have none available. I don't bet too much money on this since I suppose the scope's got a very good power supply filtering.

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2017, 08:56:45 pm
content: Quote from: guillep2k on May 29, 2017, 08:43:23 pmSo now most things have been checked try other locations at your house, turning off other appliances......walwarts on the same mains supply are notorious for noise.If no better results are seen the last thing is a test somewhere else. Work ?Or bring another scope into the same environment and see if it behaves the same.

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 09:23:14 pm
content: For those who wondered, here's the FFT of the signal. You've guessed it, it starts at 88 and ends at 108 MHz: it's the entire FM band! ("duh" moment, as JPortici said).So, the question remains. Is there any way of reducing/shielding from this kind of noise? ("no" would be a sad but valid answer). Should I cover the entire room with tin foil?

author: Specmaster
author_rank: Super Contributor
timestamp: May 29, 2017, 09:50:40 pm
content: I've always been told to use the probes at 1:1 when looking at such low voltage signals. With the switch set at 10:1 position you are introducing extra noise via the attenuation resistor, give it a go and see if that helps at all?

author: guillep2k
author_rank: Contributor
timestamp: May 29, 2017, 10:26:48 pm
content: Quote from: Specmaster on May 29, 2017, 09:50:40 pmThat would certainly reduce noise (a 10:1 reduction!) but that's not the point because it's not that I'm trying to make any particular measurement right now; I'm just comparing my input noise apples-to-apples to other people's. Also, changing to 1:1 will introduce a lot of capacitance and reduce the bandwidth considerably. In my scope I'm getting a 3dB attenuation from the WaveGen output at 6.1 MHz (which is quite good, actually). Perhaps you already know about all this, but you can watch EEVblog #453 for a nice thorough explanation if not.Nevertheless, 1:1 probing is a good thing to have under your sleeve for times of need.

author: Joel_l
author_rank: Frequent Contributor
timestamp: May 30, 2017, 01:46:12 am
content: So all these responses got me curious. I went back and ran more test with a probe set to 10:1 ( was my worse case ).Probe not connected to anything - about 50mV of noiseProbe and probe ground connected to scope ground - about 50mV of noise ( very faint and fuzzy compared to above)Probe tip connected to probe ground ( no scope ground ) - no visible noiseProbe tip connected to scope ground, probe ground unterminated - about 200 mV. I think my earlier measurement must not have had a good ground.I also ran an FFT with CF at 102MHz and could see some peaks from the local stations. None are very strong, my bench is down in my basement cornered by two concrete walls and dirt. Above me is a bunch of sheet metal duct work.In the end, I'm not worried about anything I see. A good refresher on probing though.Joel

author: macboy
author_rank: Super Contributor
timestamp: May 30, 2017, 01:51:08 pm
content: Quote from: guillep2k on May 29, 2017, 09:23:14 pmNot tin foil, but chicken wire would work at these frequencies. Save the tin foil for microwave frequencies like radar and Wifi. I'm only half joking.You are obviously close to radio transmitters. Every piece of wire is an antenna. Trysnap-on ferrite beadson the power cable, near the scope. You might need more than one. For ~100 MHz I would avoid looping the cable through the ferrite multiple times, this is effective for lower frequencies but hurts high frequency performance. You can also try a ferrite on the probe cable. The signal should be "safe" inside the shield, but it will reduce the RFI. You will want to test for negative effect on the signal at high frequencies.

author: guillep2k
author_rank: Contributor
timestamp: May 30, 2017, 03:09:04 pm
content: Quote from: macboy on May 30, 2017, 01:51:08 pmThanks, I'll do that! (It might take me a while to get those ferrite beads).