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

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PeteS 20 years ago

Your board vendor probably followed the reference design, which I just downloaded. They do something which has caused holy wars in this group: split grounds.

I would put money on grounds being internally connected in the device, incidentally.

The datasheet is very sparse about power and ground requirements and doesn't clearly state anything at all with reference to grounds.

FB41 is obviously supposed to couple the grounds, but a bead across the split grounds will simply isolate the grounds for highspeed signals, rather than couple them. The reference design shows a short between the grounds (although it should really be the same plane, imo).

I suggest a zero ohm link across the FB41 position, and because it's a highspeed problem, perhaps even 0.01uF across the grounds as well.

Cheers

PeteS

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Eeyore 20 years ago

Hi Mike. I didn't receive your mail btw.

Graham

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Eeyore 20 years ago

Very likely indeed.

Graham

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Eeyore 20 years ago

" perhaps even 0.01uF across the grounds as well. " ????????

Graham

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PeteS 20 years ago

On some unusual layouts, I have found (as a practical matter) that a zero ohm link doesn't always give a solid ac path (internal self inductance perhaps - a lot depends on construction). A small cap seems to help on some occasions.

Cheers

PeteS

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Eeyore 20 years ago

A capacitor across a zerohm link ? You're kidding ?

Graham

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PeteS 20 years ago

Depends on the grounds - in this case we don't know the shapes, separation or anything else.

I had a case with a LM73 where it was sitting on top of a ground plane with high density currents. To make it behave, I decoupled that part of the ground plane to a different part of the same plane (but separated by a small void) and *it worked*.

Strange but true.

Cheers

PeteS

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Mike Warren 20 years ago

Oh. Was I supposed to remove the "REMOVETHIS." bit?

I did remove it and it didn't bounce.

I'll try again.

-Mike

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Eeyore 20 years ago

LOL !

I'll look forward to receiving it. :~)

Graham

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Mike Warren 20 years ago

I've sent another email. If you don't get this one please send something to miwa at csas dot net dot au and I'll reply to it.

-Mike

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jasen 20 years ago

what happens if you replace the cap with a second link?

Bye. Jasen

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Eeyore 20 years ago

Not here yet ! You did delete the dot after REMOVETHIS ?

Graham

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Mike Warren 20 years ago

Yes.

It does begin with rabbitsfriendsandrelations, doesn't it?

-Mike

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Eeyore 20 years ago

It certainly does !! ;~)

Graham

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Mike Warren 20 years ago

And if it finishes with @hotmail.com then both emails were sent to the right address.

I also sent the emails from 2 different accounts in case hotmail didn't like one of them. Maybe hotmail doesn't like Australia. :-)

I once had an email from Norway take 6 months to get to me.

-Mike

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Eeyore 20 years ago

Maybe. If it's not here soon I'll mail you.

You beat me there. Just one month in my case.

Graham

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John Woodgate 20 years ago

In message , dated Sat, 19 Aug 2006, jasen writes

It short-circuits the first one, of course.

-- OOO - Own Opinions Only. Try

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and

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2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

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John Woodgate 20 years ago

In message , dated Sat, 19 Aug 2006, Mike Warren writes

You should have de-selected 'hiking mode'.

-- OOO - Own Opinions Only. Try

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and

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2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

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PeteS 20 years ago

The only time I tried (with the wierd issue on the LM73) a pretty hefty link did not work as well as a cap. As I had a solution and time was short I did not follow up any further at the time.

That said, I would expect it to ease the situation, of course. My suggestion of the cap is only because I have seen it help, even where one might think there should be no effect. Indeed, a zero ohm link may help the situation sufficiently to cure the problem. If not, it's something to try that may or may not help and won't take long to test.

Cheers

PeteS

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