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Unusual EMI frequencies

Discussion in 'Electronic Design' started by Matt, Jan 16, 2008.

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  1. Matt

    Matt Guest


    We currently have a system in environmental testing and it is
    having an issue passing a particular EMI test (RE-102) and we have
    spikes above the acceptable limit at and around 22 MHz and 29 MHz. The
    odd thing is is that these spikes occur at completely different
    systems in the same test (different types of power supplies and
    enclosures). Could the issue be with the labs input power (isn't this
    frequency range within the PLC communications frequency range?). I
    should also mention that the spike grows as we turn more components in
    the system on, i.e. it increases with the load.

    Any help or ideas would be great.

  2. Joerg

    Joerg Guest

    What else is in those systems?

    You should be able to narrow down exactly where that comes from, using
    EMCO near field probes or the like. A possibility is that something
    intermodulates. For example one strong TV signal with another, inside
    your unit. But no matter what, you've got to figure out where that
    happens or they won't give you that coveted compliance certificate.
  3. Rich

    Rich Guest


    Thanks for the reply. We have a variety of different peice parts
    in each system. For instance, one uses a linear power supply (AC/DC)
    and the other uses a switching. There aren't and RF communication
    hardware or monitors (except in one system, which was turned off and
    we still saw the spike albeit lower than before. I am heading up to
    the lab tomorrow so I will try using a near field probe on the system.
    I'll report my findings.

  4. Matt

    Matt Guest

    It seems Rich had used my computer while I stepped away. The message
    above was posted by me. Sorry for any confusion.

  5. Joerg

    Joerg Guest

    That's what I thought :)

    Anyhow, what I meant with other stuff: Is there some kind of micro
    controller or other circuit that needs and has a clock source? Or any
    OEM parts that might have something like that inside? If so, then I'd
    first look for anything that is common between the linear and the
    switcher system.

    Switch mode supplies rarely generate noise above a few MHz although I
    have seen a serious blunder by a major power supply manufacturer where
    it emitted 250MHz at full bore. Probably an oscillating opamp or
    comparator that nobody had noticed until I did the EMI, for years abd
    years. In other words one cannot necessarily trust OEM components when
    it comes to EMI no matter what their claims on the datasheets are.
  6. Sounds to me like a room resonance. Are these measurements
    taking place inside a shield room?
  7. Matt

    Matt Guest


    Sorry for such a late delay, I was up at the lab testing quite a
    bit. The room was a shielded room and the input power was running
    threw line filters before it reached the EUT. We believe at this
    moment that it has something to do with an unshielded power cable
    because we were able to run the system using internal battery power
    and we didn't see the same noise level at the 22 MHz frequency range.
    Unfortunately, we got booted from the EMI chamber so the company we
    were using could perform an audit. I will have to try a few more tests
    next week.

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