Oscilloscope interference. - Page 1
Forum section: electronics / beginners

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 12:54:03 am
content: Having an issue with my new oscilloscope. It was picking up a LOT of HF interference from *somewhere*, and after some troubleshooting, I was able to track the interference down to my LED driver for my workbench lights. It is pumping a lot of HF into the air and into the mains, and I'm not too sure what to do about it. I bodged in a hand-wound common-mode suppression choke on both the mains and DC sides of the driver, Put a ferrite bead on the power cord at the plug, and ferrite beads on both the +ve and -ve wires right against the LEDs. The interference has lessened somewhat, but it's still buggering up the trigger on the scope. Any thoughts? First picture is with the driver plugged in, second picture is with the driver unplugged. The scope probe is not connected to anything at all.

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 02:03:01 am
content: The problem may be that the probe isn't connected. If you unplug the probe from the 'scope, does the signal disappear? Or try shorting the probe to ground.The question is, where is the signal getting in? Via the probe, the power line, radiation, etc.? One you figure that out you can take steps to eliminate it.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 02:16:58 am
content: Quote from: bob91343 on July 06, 2019, 02:03:01 amProbe is connected, without it connected, it goes away, so the probe is acting like an antenna of sorts.It seems that it also comes in through the mains, though, as it shows up connected to an entirely different outlet.Sent from my ONEPLUS A3000 using Tapatalk

author: JustMeHere
author_rank: Frequent Contributor
timestamp: July 06, 2019, 02:36:29 am
content: Did you watch Dave's video on this?

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 02:47:53 am
content: Quote from: JustMeHere on July 06, 2019, 02:36:29 amWhich one? I've seen a few of his recent ones, but not one that addresses this particular issue. Link?Sent from my ONEPLUS A3000 using Tapatalk

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 05:39:47 am
content: If it's coming in the probe, it's normal. When the probe is connected to a low impedance the noise should fade away. The problem isn't with the oscilloscope but with the environment. EMI is a common problem; we all see it iin varying degrees. If you use a 1:1 probe instead of 10:1 it should also get very small.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 05:46:22 am
content: Quote from: bob91343 on July 06, 2019, 05:39:47 amThe EMI is bad enough to make auto trigger completely useless. It also messes with manual triggering.Sent from my ONEPLUS A3000 using Tapatalk

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 06:04:38 am
content: You need to reduce the EMI but it's not the 'scope's fault. Try my ideas about impedance and attenuation.We all get this. Auto triggering might get confused, so use manual triggering.Some units have bandwidth limiting controls and that might help.If you can find the source, go to work on that.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 06:06:04 am
content: Quote from: bob91343 on July 06, 2019, 06:04:38 amThe source is definitely the LED driver, I'm just not sure how to protect the scope or probes from the EMI.Sent from my ONEPLUS A3000 using Tapatalk

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 06:28:10 am
content: In your first post you indicated that the interference was abated by the ferrites. If so, then use more of them. I have sometimes had to use two or three, and even wind a few turns through them to get enough impedance.If the interference is radiated, shielding may help. Failing all the above, tear out that lighting setup and replace it.

author: radiolistener
author_rank: Super Contributor
timestamp: July 06, 2019, 11:12:35 am
content: Quote from: GadgetBoy on July 06, 2019, 12:54:03 amthen this is a cheap Chinese LED, they don't install filters in order to reduce costs. Just put it into garbage and buy some good quality LED lamp. It should not emit EMI.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 03:34:42 pm
content: Quote from: radiolistener on July 06, 2019, 11:12:35 amI was hoping for a low-budget option, not having a lot of money for these things. Would decoupling capacitors or a low-pass filter help?Sent from my ONEPLUS A3000 using Tapatalk

author: radiolistener
author_rank: Super Contributor
timestamp: July 06, 2019, 04:40:32 pm
content: Quote from: GadgetBoy on July 06, 2019, 03:34:42 pmno, capacitors cannot help. This bullshit is designed to be cheap at expense of increased level of RF interferences.You can try to add ferrite beads inside LED lamp, but it will be too complicated and may not help, because these cheap things also using cheap circuits at expenses of increased level of RF interferences.Also you can try to completely remove LED lamp driver and replace it with ballast capacitor.This cheap bullshit has low cost due to high level of RF interferences. The more cheap way is to put it into garbage and buy good quality LED lamp, because you will spend much more in order to fight with it's noise.If you're need LED lamp for your lab with measurement equipment, you should not use this cheap crap, because it has too high RF noise and will affects your measurements.

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 05:13:32 pm
content: Years ago I refused to use fluorescent lamps due to the high noise they generated. Recent lamps don't have this problem; I use a fluorescent lamp on my workbench with no trouble.The same evolution is happening with LEDs. Eventually the noise will be corrected but you are a victim of the early days. The comments about replacing the offending lamps are spot on. And the cost will be forgotten in the long run.I forget who said it: Quality is remembered long after cost is forgotten.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: July 06, 2019, 06:13:18 pm
content: Quote from: radiolistener on July 06, 2019, 04:40:32 pmSomething like this?https://www.meanwell.com/productPdf.aspx?i=229#1Seems pretty decent at 25$.Sent from my ONEPLUS A3000 using Tapatalk

author: ferdieCX
author_rank: Regular Contributor
timestamp: July 22, 2019, 12:07:20 am
content: I had the same problem 5 years ago, the power brick of the LED bar interfered with the trigger of my scope.My LED bar (Diodor brand) is specified to work between 10 to 14 V so, I made a simple power supply with a 9 V + 9 V transformer, two diodesand a filter capacitor calculated to have less than 1 Vpp ripple. This way, the bar gets a voltage that roughly varies between 11 and 12 V.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: July 24, 2019, 03:20:15 am
content: I had the same problem with my angle poise lamp.It radiates into my audio circuits nicely !I found temporarily lifting the light up a bit until I finished with the scope worked for me.I got caught out similarly many years ago with 50Hz interference on my scope.I had sat my sig gen on top of the scope and the transformer was radiating down into the scope !

author: JustMeHere
author_rank: Frequent Contributor
timestamp: July 25, 2019, 06:49:00 am
content: Quote from: GadgetBoy on July 06, 2019, 02:47:53 amWatch the whole 6 mins.