EMC issue with single board computer

That's the good stuff then !

Graham

Something I didn't ask earlier; do the AC97 signals come *off* the board? If so, you'll have stubs and they will be an absolute b*tch to deal with.

If they *do* come off the board, put a terminator on the line to ground (try 47 in series to a 1k paralleled with 100pF to ground to start with. You might get away with something simpler). I had precisely this issue when doing some early prototyping with a premade board.

Cheers

PeteS

Thanks Pete. Unfortunately the signals do not go to any headers. I've pretty much resigned myself to stopping the emissions at each wire. Very slow work and there will be unpleasant side effects to deal with. :-(

-Mike

What does the OEM say about your problems? You may not be the only one having trouble with their board. They may have to redesign them to make them pass the required tests if they expect to sell a lot of boards.

Good advice. They may even know of a fix.

Graham

why dont you look closely at the offending board, figure out how its emitting the mess, and fix it? slugging oscillator outputs with caps to reduce slew rate is brutal, but effective. Of course if its a crap layout there wont be much you can do, but another rough trick I have used is to make up a "ground plane" pcb - 0.8mm FR4, single sided, solid copper. put holes in it for all the component legs to poke thru, place it on the bottom of the PCB (Cu facing outwards), and solder all the 0V connections to it.....

a good way to tell a crap layout from a good one is to hold the pcb up to the light. If you can see through it, its probably crap. Although just because you cant see through it, doesnt mean it isnt crap (I've seen numerous atrocious 4,6,8,10 and 12-layer PCB layouts).

Cheers Terry

Their response was to send me their CE and FCC test reports showing it had passed.

-Mike

There are very few places to attach a shield since the PCB is so dense. Anyway, I need a way that is practical for production. We won't be buying a high enough quantity to make the manufacturer redesign.

I'm currently waiting on a board from another supplier to see if it will do the job.

I can get the emissions down a few dB by doing multiple mods to the board and it would probably pass but these are not practical for production.

-Mike

In what configuration ?

Graham

It's not about a 'shield' !

I suspect your background is primarily audio ??? Like myself. I can see where you possibly haven't grasped the subtleties of EMC. I was lucky to have an tutor early on back in ~ 1987.

There is truly a very different approach required to EMC compared to classic audio. I've heard comments like some of your own only too many times.

The thing is, if you get the concept right, it's actually not *that* difficult to tame. I'm concerened you may do another iteration and find it no better.

Fancy offering me a paid vacation ?

Graham

Yes.

Are you talking about the shield or ground plane needing to be bonded to ground everywhere, much like a ground plane on a PCB? If so, I can't do that here as there is very little ground available to solder to. I have actually done what Terry suggested a couple of days ago but could only get 3 connections to ground. Of course, it made very little difference.

I'm concerned about that too. :-(

I would. Just have to convince the powers that be. ;-)

How about some private tuition over phone/email? If you think that's viable, email me with your rate.

-Mike

At least in part but not exclusively.

A common situation !

Terry's good !

I'll bet !

I'll certainly follow up on this. I'm intruiged by what you're doing as it looks like it may overlap with something I have in mind too.

The email address in my headers works btw !

How many hours ahead of London are you ? What format are your schematics / pcb files in too ?

Graham

I have just sent an email to the hotmail address.

-Mike

whats the emoticon for blushing?

looks

Cheers Terry

looks

pcb

One thing for you to check before going too much further - I have had some very noisy devices that have relatively highspeed clocks and D-A and A-D converters onboard.

Look carefully around the AC97 device and check the decoupling for it's analog side. I've had excessive noise from system clocks 'leak' through the converters into the analog system, and then it has a really nice big antenna, exacerbating the problem.

If you have a schematic for the board, check what decoupling they used for the analog side - a common problem is not to provide a separate power rail into the audio section (I derive mine from the primary rail with a ferrite bead and fairly hefty decoupling). Failure to provide a separately decoupled supply for the analog section _could_ lead to this issue. If the noise is getting in by way of the analog power, you might have a chance to suppress it near the device. Even if there's separate decoupling, providing a cap _across_ from one power domain to the other can suppress a lot of radiated hash.

Cheers

PeteS

The analog supply has 2 10uF ceramics and 2 100nF ceramics on it. The digital supplies have one 10uF and 2 100nFs. All are fed through ferrite beads.

I'll give that a try.

-Mike

That attitude would make me look for another supplier.

They may not have fully understood the question / reason for it and might actually have thought they were being helpful !

Especially likely if both parties don't share a common first language.

Maybe Boki can act as interpreter ? It's time he contributed.

Graham

That's quite possible. I asked where the connection was between AGnd and Gnd when ferrite bead FB41 was removed and they answered that it would be OK for me to sort out the ferrite bead if I wanted.

-Mike