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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    • Dave,

      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?


      • 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.

    The EEVblog Store generally ships twice a week, on Tuesdays & Fridays, Sydney time. Dismiss