EMC issue with single board computer

Mike Warren · 2006-08-15T03:35:45+00:00

I am using an industrial 5.25" SBC and am having some issues withEMC. All the problem frequencies are harmonics of the 12.288Mhzbit clock on the audio chip.Since high speed digital is not an area I have any experience withI am hoping someone here will be able to advise me if what I amseeing is normal or not.The reference design for the audio chip (ALC655) shows a 22 ohmseries resistor and 22pF capacitor to ground on the bit clock linebut the SBC manufacturer has just used a 33 ohm series resistor.This resistor is about 50mm from the audio chip.There is no room for me to fit a capacitor and even if I could I wouldwant to move the resistor so it was close to the pin of the audio chip.This is also very difficult since the track disappears into the innerlayers of the PCB right at the chip.Here are some pictures of what I see on the bit clock line. It seems tome that the ringing is excessive.WaveformSpectrumThe supplier responded to my request for help by supplying theirCE and FCC reports which don't show any of my problem frequenciesat all. If I were a cynical type, I might think they had switched the audiochip off for the test. ;-)The only options I can see open to me at the moment is the eitherswitch to another SBC or add an extra shield to the SBC insidemy metal case.Any suggestions will be appreciated.-Mike

Aug 15, 2006 61 Replies

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Mike Warren 19 years ago

I tried shorting the ferrite bead but there was no change.

-Mike

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Mike Warren 19 years ago

This was 10 years ago. It probably went the wrong way around the world and since it had to drag a hefty 3MB attachment I guess it had to stop often for rests. :-)

-Mike

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