EMI, fail at SDRAM

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.

--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
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Larry Brasfield
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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.



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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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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, Boki.

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