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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  1. 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.


  2. Dave,

    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?


  3. 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

  4. 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)

  5. Hi Fred.

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

    Warnings without explanation is often ignored.

    • @Hans,

      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… 😉

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

  7. 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).


  8. Take appart the “quili power” 🙂

    “Turn it off and take it appart”

  9. 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 🙁

  10. 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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