Dealing with background noise on my USB oscilloscope - Page 1
Forum section: electronics / beginners

author: Hobby73
author_rank: Regular Contributor
timestamp: April 22, 2016, 04:59:27 am
content: I use my USB scope (Picoscope 2205) to measure the voltage ripple of a switched power converter (wall wart that converts from AC to 5VDC). I use the AC coupling setting on the scope to isolate the ripple and noise.I have a few challenging questions about the background noise on my test setup. My USB scope is connected to a laptop which runs on battery and is not connected to any other peripherals that could be a source of noise. The peak-to-peak ripple that appears before I power-on the switch is 8mV, which is typical of what I've read in other forums on this topic for USB scopes. When I power-on the switch, the peak-to-peak ripple displays at 50mV.My questions are:1) Does the 50mV peak-to-peak ripple consist of the 8mV of background ripple plus 42mV of ripple from the switch? Or is the background ripple hidden within the switch ripple rather than additive to it, in which case the switch alone is the source of all 50mV of ripple?2) If my scope can detect all higher frequencies involved, does it matter to the total ripple measurement if the frequency of my background noise is different than the frequency of the switch ripple? In other words, when measuring peak-to-peak ripple, is it customary to include multi-frequency components of the waveform, or do you try to isolate discrete frequency ranges and measure them separately?3) Assuming that the background noise is additive to the switch ripple, what is the common method to account for the background noise so I'm not overstating the ripple from the switch? If they were at completely different frequencies, I could apply a filter to remove the background noise. But let's say they have some overlap in frequency. In this case, do I simply perform a manual subtraction of the background noise when reporting results? My scope supports math channels for transformations of the signal, but I can't figure out a way to subtract a constant noise value from the oscillating waveform that centers on zero.Thanks in advance!

author: Xenon Photon
author_rank: Unknown
timestamp: April 24, 2016, 01:36:13 am
content: Did you watch this video? if not, watch it from minute 20. I think it have an answer to some of your questions.https://youtu.be/Edel3eduRj4?t=20m

author: Hobby73
author_rank: Regular Contributor
timestamp: April 24, 2016, 04:40:16 am
content: Thanks for replying to my questions. Yes, I watched the video (before I posted my questions) and I just watched the segment again at your suggestion. In this video, Dave addresses a lot of topics but not all the ones I have questions about. A couple examples:- He refers to ALL background noise as Common Mode noise. This doesn't apply to my setup as I have multiple sources of noise from my test setup (USB scope connected to a laptop running on battery) which are probably not Common Mode related but originate from my PC's internal voltage regulator, hard drive, fan, etc.- As he investigates the source of his Common Mode noise and finally finds it, the solution he takes is to eliminate the noise. I can fully understand this is the preferred approach. However, I am resigned to keep the noise in my test setup, and I would just like to subtract it out from the gross measurement leaving me with the net signal. I will try the two-channel subtraction method that Dave demonstrates ("poor man's differential probe") and I'll post back with results.Any additional comments are welcome.

author: jitter
author_rank: Frequent Contributor
timestamp: April 24, 2016, 07:43:26 am
content: Quote from: Hobby73 on April 22, 2016, 04:59:27 amSignals add or subtract, they do not "hide" in one another. What you will see, though, is if one signal is huge and the other tiny, the tiny one will become insignificant. That doesn't mean it's not there, but the large signal will push it into insignificance. Of course, in your case with not so huge a signal, you have noticed that the "tiny" signal has become significant.It's likely that the background noise of the scope is more or less wide band white noise. This will be added to the noise from the SMPS which will have a distinct frequency depending on its switching frequency (say 100 kHz or so).QuoteTypically when measuring power supplies, a bandwidth limit of 20 MHz is used. And yes, switching off this limit drastically increases the noise on the measured signal.Noise is a multi frequency phenomenon, when distributed evenly across the (audible) spectrum, it's often called white noise. Ripple is limited to rectification of the 50/60 Hz mains (full wave rectified: 100/120 Hz) and in an smps also the frequencies used in the switching process and their harmonics.Square waves will have much more high frequency content (harmonics) than sinusoidal signals.Rather than just look with a scope at the ripple, also use a spectrum analyser (or the FFT function, if your scope has it). That will show you the noise floor all across the spectrum and peaks rising above this noise floor where the switching noise (with harmonics) is.QuoteAssuming that the background noise is spread across the spectrum, it's no use trying to filter out specific frequencies. You must see how much that random freqency content rides on the frequency content of the smps and account for it. Depending on your scope's options, you could play around with settings like persistence and averaging to give you a cleaner signal.QuoteYou can add or subtract noise to or from a single frequency signal just fine. But there is a catch.I have added a screendump (Screen1) of 1 Vpp noise on CH3 (pink) added to a 5 Vpp 1 kHz square wave on CH1 (yellow) to simulate noise injected into a clean signal. The math trace represents this polluted signal. It is the purple trace and it is magnified to make the result clearer. Unmaginified it would sit right where the yellow trace is but with the added noise.The vertical scale (horizontally placed) cursors are placed 5 V apart at + and - 2.5 V. This is where the clean 1 kHz square wave would be but as you can see, the noise adds to and subtracts from this making the peaks vary randomly with the noise's 1 Vpp amplitude.Quote from: Hobby73 on April 24, 2016, 04:40:16 amIn your setup, you would have to take into account the amount of noise to be able to determine the smps's ripple. But please note that the noise you see riding on this signal may not be just the background noise of the scope, escpecially if the noise's amplitude is significantly higher than that of a terminated scope channel without any input signal. If it is higher, you will have picked up noise from other sources, and measuring low level signals this easily happens, as Dave's video showed. So the techniques shown to prevent this "pollution" of your measurement are really neccessary to prevent false readings and drawing the wrong conclusions.Edit: so you can add and subtract any frequency, but the catch is that noise is random. Subtracting one noise signal from the other will never result in a noisefree signal, unless they're identical, like common mode noise.To show this, the second attachment (Screen2) has CH1 and CH3 connected to two different outputs of the signal generator set to generate the same kind of noise signal. They may look identical, but coming from two different outputs, they're not really (or rather: really not). As a result, subtracting one from the other hardly leads to a flat line. That's what you get with random noise and the background noise from CH1 of your scope is likely not identical to that of CH2.For the last attachment, I tied both CH1 and CH3 to the same generator output. Look at how much flatter the math trace is now that both signals are identical (it's not completely flat due to minute differences in cables and scope inputs, but you get the idea).

author: Hobby73
author_rank: Regular Contributor
timestamp: April 25, 2016, 03:05:11 pm
content: Quote from: jitter on April 24, 2016, 07:43:26 amWell said ... this is fundamental to reading oscilloscope traces and it's helpful to explicitly state how multiple signals are combined.QuoteI've seen lower frequency ripple measured separately from higher frequency transient noise spikes, and other times they are combined for a larger Vpp. I guess either way is alright so long as you're clear about what you are measuring.QuoteI think the key point here is the need to acknowledge and account for the background noise insomemanner. When Vpp is reported without any mention of how background noise was accounted for, I'm suspicious of whether Vpp is overstated because it may contain background noise that wasn't eliminated or subtracted out.QuoteI will try using the differential method with two channels and subtraction.Thanks for your comprehensive answers to my questions!

author: Hobby73
author_rank: Regular Contributor
timestamp: April 25, 2016, 05:31:55 pm
content: Quote from: Hobby73 on April 25, 2016, 03:05:11 pmI just setup the test using both channels with subtraction and it doesnotwork. Even before I turn on the PS, the noise on each channel A and B is different enough from each other that they do not cancel out. This is similar to what @jitter demonstrated in the above post using two channels on a signal generator, but in my case the problem is two channels on my USB scope. My problem likely originates from my noisy laptop (hard drive spinning, fan, voltage regulator) since it is interconnected with the USB scope. Why the noise isn't exactly the same on both scope channels may be due to randomness or something else.My best solution at this point (using my existing test equipment) is a multi-step process to determine and apply a vertical scaling multiplier that will remove the noise from the waveform leaving only the ripple from the power supply or Device Under Test (DUT):Step 1: Measure average Background noise ripple on the scope = VppB.Step 2: Measure Total ripple signal which consists of Background noise (VppB) plus the DUT ripple (VppD) = VppT.Step 3: Compute scaling multiplier as (VppD / VppT) which is the proportion of the Total signal that is attributed to the DUT.Step 4: Create a math channel on the scope which applies the scaling multiplier to the Total signal under measurement on Channel A. The formula is (A = VppT * scaling multiplier). This math channel is now the ripple waveform of the DUT.Step 5: Apply an average Peak to Peak Measurement to the math channel. This result is the ripple value for the DUT.

author: jitter
author_rank: Frequent Contributor
timestamp: April 27, 2016, 08:40:09 am
content: Quote from: Hobby73 on April 25, 2016, 05:31:55 pmThis is as what I expected. The "poor man's differential" proble (nor real differential probe) will not have any effect on the noise from the scope itself. It will only cancel out common mode noise picked up by the leads.QuoteYou wrote that the background noise measures as 8 mVpp. This seems quite a lot as Picoscope specs it as 150 uVrms, which in peak-peak measurement might be a couple of times that, but nowhere near 8 mVpp.It has me wondering if the laptop on batteries (i.e. floating) is actually noisier than with it on the adapter. Have you tried it with the laptop charging?That way there should be a path for higher frequency noise to mains (through the Y-cap between primary and secondary of the adapter).Have you tried the Picoscope on a desktop PC?

author: Hobby73
author_rank: Regular Contributor
timestamp: April 27, 2016, 05:47:05 pm
content: Quote from: jitter on April 27, 2016, 08:40:09 amI have two laptops (no desktop): a 5-year-old Win7 laptop that is my primary PC I rely on for email and business, and a 15-year-old XP laptop that I keep around as a spare. The laptop I'm using with Picoscope is the XP laptop. I want to use this end-of-life XP laptop with the scope in case I have a power-related accident. (I think it's too risky to perform electronics testing with your primary computer).However, I did one quick test on my Win7 laptop and the background noise was a couple mV lower at 6mV when the laptop was plugged into AC. When I pulled the plug so it ran off battery, the noise dropped by one mV to 5mV. It's quite possible that a brand new modern PC may have even less noise. I suspect that the Picoscope spec for noise applies only to their USB hardware device and not the PC which would vary widely by customer and setup.When you see forum posts comparing USB scopes to dedicated scopes you will read about pros and cons for each. The issue of background noise is not often mentioned, but should be.Meanwhile, I'm going to use the math channel approach I outlined above. It's a little tedious and it works with average levels, but it's sufficient.

author: Performa01
author_rank: Super Contributor
timestamp: April 28, 2016, 04:53:28 pm
content: I use USB scopes a lot, both at home and at work. I never noticed any difference when compared to 'regular' DSOs in terms of noise, except maybe that the Picoscopes might even be a bit more quiet.I'm pretty sure the noise you're seeing doesn't come from the scope or the USB connection, but from the probes.Just disconnect the probes from the scope and check the trace - all the noise should be gone.I suspect your lab is highly contaminated with mains hum and all sorts of noise, which would be nothing out of the ordinary. In this kind of environment you see lots of noise, no matter what scope, even with the probe tip shorted to the ground lead.

author: Apollyon25_
author_rank: Regular Contributor
timestamp: April 29, 2016, 02:45:52 am
content: If you want to measure the output noise of a converter you need to remove noise pickup from the probe.i.e. don't use that ground wire and clip.Instead, use the little spring-thing that came in the bag with the scope probe (or should have). Remove the "witch-hat" probe clip plastic thingy-whatsit (30-odd years and I still dont know what they actually call that bit) and push the spring up over the metal ring part. Now push the pointy bit of the spring against the output cap's GND terminal and probe the other. Say goodbye to noise.If this little gem has been thrown out (GASP!) then you can simply buy a bunch of BNC sockets and run a twisted pair up to it from your output cap - better from a hands-free POV.Hopefully youve kept the funny coax bit of metal that also came in your scope probe bag.Don't you dare tell me you threw that out too!These widgets (again - 30 years...) simply push onto the probe tip and then you can plug the scope probe into a BNC socket.If I can find some pictures...

author: jitter
author_rank: Frequent Contributor
timestamp: April 30, 2016, 03:04:06 pm
content: Quote from: Apollyon25_ on April 29, 2016, 02:45:52 amYou mean these?

author: Apollyon25_
author_rank: Regular Contributor
timestamp: May 01, 2016, 08:35:09 pm
content: Yup.(warm fuzzies that someone didn't throw them out)

author: Hobby73
author_rank: Regular Contributor
timestamp: May 02, 2016, 12:21:11 am
content: I don't have a BNC or spring clip for my probes, but I know how to use the tip-barrel method which makes a ground connection without the ground lead.When my probe is removed from the scope, the ripple measures very low at 0.9mV. From here, there are multiple connection steps, for example:- Scope w/o probe attached- Scope with probe attached- Probe connected to PSU (powered off)- Probe connected to PSU (powered on, no load)- Probe connected to PSU (powered on, with load)Which step do you use as your comparison reference to determine the internal noise of your test setup?

author: Apollyon25_
author_rank: Regular Contributor
timestamp: May 02, 2016, 12:46:32 am
content: I have to say, I haven't spent much time debugging my test setups/measurement techniques, they haven't been a big problem. Probably in the early days they were...Glad to hear you know the correct way to measure stuff - its surprising the number of people who dontHowever, in saying that. If my probes are measuring more noise than expected, and I cannot attribute the noise to the DUT, then I'll recheck the probe (short probe tip to GND, both at tip and at board, measure something I know the response of). I'll then correlate what I see with another piece of test gear (another scope, DMM etc)I take the approach of 'if something funny is going on, measure the amount of "funny"'.Once this is done, you can generally find the cause. The time or frequency domain will generally head you in the right direction.Without re-reading the thread, to see if you mentioned the circuit under test, does it share a GND/Earth with your measurement setup? Common-mode currents? Have you floated (isolation transformer) your DUT/scope? Thrown a big ferrite on your scope probe's cable?

author: Hobby73
author_rank: Regular Contributor
timestamp: May 02, 2016, 01:03:46 am
content: Quote from: Apollyon25_ on May 02, 2016, 12:46:32 amMy PC is running on battery (floating) and is not connected to any other peripherals that would create a "backdoor" ground. The DUT (PSU) is isolated from Mains.I suspect my problem may be very specific to my legacy PC and its noise profile.Thanks for your tip on looking at the frequency as a clue to where the noise originates ... good idea.

author: Apollyon25_
author_rank: Regular Contributor
timestamp: May 02, 2016, 02:05:12 am
content: Hmm, sounds like an odd one.FYI you can also use the ground loop connected to the probe tip as a cheap loop antenna to further aid the hunting.The smaller the loop the nearer you'll be able to narrow it down.Post up what you find!