EEVblog #441 – How To Track Down Common Mode Noise

How to detect and hunt down common mode noise on your oscilloscope.
With the example of testing the output noise on a linear bench power supply.
The aside video for Analog vs Digital Scopes:

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    • Sam Reaves


      It looks like your aside video is set to private.

      Also you might show how different ferrites help common mode noise.

      The other thing you can do is build up a 20MHz filter that you could put in between the DUT and your scope. 20MHz seems to be a very common bandwidth breakpoint for power supply manufacturers. I have found that there are some manufacturers of AC/DC and DC/DC modules (Lambda being one) that outlines the test methods. Who knows what they do in OneHungLow land.

      I think that Jim Williams wrote a note on testing supplys and regulators for noise. It was very good as I remember.


    • Scott


      I noticed that you had something connected to the GND terminal of the PS in some segments. Wouldn’t this be a potential source of common mode intereference?


      • Adam Ward

        That’ll be his noise generator. 😀

    • Goerg

      Hi Dave,

      just wanted to say thanks for your videos!
      Totally like your style of showing and explaining things.
      I find it very interesting so keep up the great work!

      Greetings from Germany

    • Fred

      Hi Dave,

      Thank you for your videos.
      I’ve seen this with a laptop at work.

      NEVER use a laptop on antistatic mats! (you or your friends/colleagues around you)

      Best regards,
      Fred (from France)

    • Hans

      Hi Fred.

      Could you please explain why you tell: “NEVER use a laptop on antistatic mats!”?

      Warnings without explanation is often ignored.

      • elgonzo


        first, watch Dave’s video.

        Only then direct your attention to this sentence in Fred’s post: “I’ve seen this with a laptop at work.”

        Hope, this explanation wasn’t too long… 😉

    • Jimmie

      Hi Dave.
      How would the measurement become if you did the measuring with a differential probe?
      Would you still pickup common mode noise?

    • Kostas

      Hi Dave,
      I always enjoy your videos. Why do not you try something else for noise: connect channel 1 of the oscilloscope to the positive terminal of the power supply and channel 2 to the negative, then display the difference (CH1 – CH2).


    • f4eru

      Take appart the “quili power” :)

      “Turn it off and take it appart”

    • Gord

      Biggest “spiker” on my bench? = Cheapo temp controlled soldering iron.

      Noisiest common-mode source in my house? (actually, from 3 doors down the street!) = “Homeplug” mains networking (also halves the ADSL sync speeds for all the neighbours and even interferes with the analog VHF FM and digital VHF broadcast radios :(

    • Geoffrey Feldman

      Funny story: I had this strange signal on my Rigol, actually while watching EEVBlog. It turned out that a probe was resting near the USB cable from my headphones. I realized this when I paused your video blog and the pulses were less. I was picking up the digital pulses going to my headphone.