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EMI, fail at SDRAM

Discussion in 'Electronic Basics' started by Boki, Apr 21, 2005.

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  1. Boki

    Boki Guest

    Hi All,
    Not familiar with EMI.

    My baord encountered EMI problem in SDRAM clock.

    1. I am using 4 level board.
    2. The problem comes from SDRAM clock of core chip.
    3. It not all fail, it fails on some high ( multiplier ) frequencies of
    SDRAM clock.
    4. I had added iron...
    5. The layout of other circuits is very compact.

    I had add some 0 ohm resistor / RC filter...

    I think my major problem is the SDRAM clock.

    Thank you very much for your comment.

    Best regards,
  2. Is it surface mount? How many plane layers?
    Is the ground plane pretty contiguous or is it
    broken up a lot for routing use?
    On what evidence do you conclude that?
    What frequencies are giving you trouble?
    What is the clock frequency?
    Where? Is the circuit in an enclosure? What is
    that made of? How are the signals entering or
    leaving the enclosure treated? Is your board
    connected to the enclosure near the openings
    that signals enter or leave?
    I would expect the address and data lines to be about
    as spread out as the RAM clock.
    That might help with high order harmonics. Have you
    tried increasing the resistor value? What kind of
    termination do you have on the clock and other
    high speed signals?
    With a spectrum analyzer you should be able to
    tell whether it is the constant frequency clock or
    other, less constant transitions whose spectra
    are around the clock but more spread out. The
    address and data lines could easily produce the
    kind of problem you have described.

    Do you have a means of evaluating incremental
    changes in your circuit's radiation at the frequencies
    causing the problem in your own lab? Or must you
    rely on test results from an EMC testing house?
    You are welcome.
  3. PeteS

    PeteS Guest

    In addition to what Larry said (and I completely agree with all the
    comments), other considerations are :

    Was the layout of the signals set for impedance controlled tracks?
    (this implies a ground plane somewhere, and if you get the impedance
    right - which can be forced with the correct terminations - helps
    enormusly with EMI issues)

    If you know the source of the emissions is the core device, can you use
    a spread spectrum oscillator to run it? That helps with EMI issues (by
    spreading the energy across a wider band, thus reducing the emissions
    at any one frequncy, at least for long period type issues).

    What is the clock running at? If it's high, you may be able to slow it
    down and that may very well help too.


  4. Joerg

    Joerg Guest

    Hi Pete,
    In some countries it is called "clock dithering". There are or at least
    used to be chips that could even dith a clock after the fact. If I am
    not mistaken Cypress is one of the companies.

    Regards, Joerg
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