• 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: http://www.youtube.com/watch?v=ImyUB3_n9fw

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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.

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