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Ground loops

Discussion in 'Electronic Design' started by [email protected], Feb 11, 2007.

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  1. Guest

    Hello,

    I am observing this affect on a few components whereby their frequency
    response shows the 60Hz power line frequency. This seems to be
    isolated to just components from a few date codes. However, this
    does indicate that there is interference with the 60Hz frequency with
    some parts. I have tried to tie the chassis of the unit under test,
    the measurement unit and the unit under test to ensure that they are
    at the same ground potential, but the problem still persists. While
    the circuit is powered down, the chassis of the unit under test, the
    unit under test and the measurement unit have no resistance between
    them. But when I power up the circuit, there exists a negligible
    resistance of upto 20 ohm between the chassis of the unit under test
    and the uunit under test and the measurement unit and the unit under
    test. Does this create the possibility of ground loops? Could the
    60Hz power line frequency that I am seeing be related to ground loops?

    I have also tried to run the experiment with/without an isolation
    transformer, but the problem still persists. Any ideas on where the
    60Hz power line frequency could be creeping in? If the failing date
    code is run at a different location, it does not exhibit the 60Hz
    power line frequency. Therefore, it is something in setup that is
    causing parts with some date codes to capture the 60Hz power line
    frequency.

    Please help.
     
  2. Jamie

    Jamie Guest

    i would first check to make sure the bad locations have a real earth
    ground in the receptacle.
    You didn't say exactly what this component is ? it would shed a lot
    more light on it.
    It would appear to me that maybe you have some inductive problems
    generating EMF, but that is just a guess.
    I doubt that your meter is actually reading 20 ohms, it most likely
    is getting energy from it's leads near by.
     
  3. Guest


    The device is an audio sample rate converter
     
  4. On 11 Feb 2007 08:16:16 -0800, in sci.electronics.design
    what sort of components, transformers, inductors, flux capacitors?

    the 60hz, how many dB's worse is it, and what are the specs for 60Hz
    rejection?


    martin
     
  5. john jardine

    john jardine Guest

    Mains frequency injection (and harmonics through MHz) is a real problem
    down at the mV, uV level. It creeps in everywhere, via all paths and all
    mechanisms. Textbook 'solutions' such as isolation transformers, filters,
    star points, baluns, line breakers, shielding etc, provide little help and
    more usually waste time and effort.

    Effective cure is use of battery supplied or isolated power supplies with
    only the signal commons galvanically connected.
    Maybe worth adding that signal sources themselves are prone to injecting
    mains noise along with the wanted reference signals. That 50ohm BNC output
    socket may be delivering say 100uV of signal on it's centre pin and 10uV
    mains noise through it's body.
    (Cleanest signal I've ever seen came from an el cheapo 'Levell TG200' with
    it's internal switched mode battery adapter . Worst, came from a Wavetek
    function gen').
    john
     
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