EMC issue with single board computer

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

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

Graham

Very likely indeed.

Graham

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

Graham

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

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

Graham

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

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

I did remove it and it didn't bounce.

I'll try again.

-Mike

LOL !

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

Graham

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

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

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

Graham

Yes.

It does begin with rabbitsfriendsandrelations, doesn't it?

-Mike

It certainly does !! ;~)

Graham

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

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

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

Graham

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

It short-circuits the first one, of course.

-- OOO - Own Opinions Only. Try

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

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

You should have de-selected 'hiking mode'.

-- OOO - Own Opinions Only. Try

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

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