EMC issue with single board computer

What's the source of the 12.288MHz clock ?

If you have a metal case, your emissions must be down to some kind of poor grounding IME. Where exactly is it getting out ?

Graham

The audio chip.

I think that as well but don't know what I can do differently. The board is mounted straight to the chassis, as is everything else.

The SBC only has 4 mount points, one on each corner. It might be worth looking at what changes if I don't attach it to the chassis.

Everywhere it can. The worst area is through the LAN cable. I come out of the RJ45 on the SBC with a short cable which goes to a socket on my I/O PCB. This is the same technique used by the board manufacturer in their sample kit.

Here is a picture of the prototype.

I've tried shielding various cables but nothing makes more than about

1dB of difference. Switching the audio chip off in the CMOS makes all my problems go away.

-Mike

Firstly, know ye that EMC is a very black art !

What exactly are you measuring ? It is important to note the difference between near and far field. Near field means putting your SA probe directly on a cable or board. Unfortunately, what you see doesn't tell you much about far field measurements, where the test receiver is placed a considerable distance from the 'apparatus'. Far field is what you have to pass for EMC tests. The bad news is that far field tests are difficult to do without a quiet site or anechoic chamber, plus a suitable amplifier for your SA.

If far field measurements show that you have a problem, then the fun starts ...

Dave

I'm using a near field probe kit to localise the source of problem frequencies discovered during a pre-compliance test in an anechoic room.

One of the probes is an amplified antenna which I place 1M from the machine.

Although this isn't calibrated, I see some similarities to the proper test.

The thing that bothers me is that *all* my problem emissions come from the same source and yet the SBC has many other potential sources of problems.

If I didn't need the audio, I would just switch it off in the CMOS config and be happy.

-Mike

I'd guess that the xtal waveform itself is not a major part of the problem.

First thing to do is find how it is getting off the SBC. What happens if you remove the Audio input and output cables from the SBC ?

Dave

In message , dated Tue, 15 Aug 2006, Mike Warren writes

Shielding is not appropriate; you need ferrite beads on the cables. But this is a 'fix', not a proper cure.

I wonder if the audio chip maker 'improved' it and it now has much faster edges than before. It's happened.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

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

Sorry, I forgot to mention we tried ferrite beads at the pre-compliance test and they also made very little difference. Even completely removing all the internal cables except the power and network only dropped the problem signals by a few dB.

Maybe our case maker has used fake steel. :-)

It's possible. I would love to be able to see if fitting the recommended R/C network helps.

I have just been trying some weird things and am amazed by how little difference anything makes.

First, I made a metal shield to mount over the board without interfering with the heatsinks. It was grounded at the board mount points. This would be commercially viable if it worked.

Then I tried putting the board in a bubble wrap bag without grounding it and finally covered the bag with aluminium foil.

There has to be a clue in how little the signals were affected.

The numbers here are dB relative to the centre of my spectrum analyser display.

Freq Start Metal Bag Bag+foil Shield

110.60 +22 +18 +22 +18 122.89 +12 +14 +12 +14 135.17 +8 +12 +10 +10 159.75 +18 +16 +16 +18 172.04 +10 +8 +12 +14 184.33 +16 +14 +16 +16 258.06 +6 +6 +8 +6 282.64 +6 +2 +2 +8 515.99 -12 -12 -12 -12 573.98* -4 -4 -6 -8 749.60 -6 -4 -4 -6 774.18 0 0 +4 0

• This is the only emission not a harmonic of the 12.288MHz bit clock. It didn't show up on the pre-comp test.

I designed a similar device a couple of years ago using a different SBC and it passed EMC testing first time.

Looks like I'm going to have to bite the bullet and try another board. That will however, mean major redesign.

-Mike

Very little change. It seems to me that it is being radiated from the PCB, not conducted down cables.

There is more info in my reply to John.

At over AUS$325 per hour there wasn't a lot of time to play at the test lab.

-Mike

Sorry Graham. Just realised you probably wanted more info.

The ALC655 has a 24.576MHz crystal which I assume is divided by 2 internally to generate the bit clock. The chip is capable of using an external bit clock but in this case it is being generated by the chip.

-Mike

That's an AC97 clock, driven from the ALC device. Assuming you aren't driving a huge load (it *should* only be the AC97 master), you could get away with up to 100 ohm series resistance, although it's hard to know.

I would be tempted to put a 49.9 ohm device in series rather than the

1. If you *did* move the resistor, it would probably do more good near the destination rather than the source (that varies board to board though).

If you want a link to the AC97 spec (which has the physical layer) post again and I'll dig it out (You could simply google, but it's hidden in the bowels of Intel's site).

Keep in mind that there is data travelling on the SDataout (to sound device) and SDatain (from sound device) lines and these will

**definitely** give you 12.288MHz harmonics as they will have (typically) stochastic activity, splattering all over the spectrum.

I would suggest you look closely at those lines too. I had some issues with my data lines on the AC97 bus I have in my handheld device in the early stages.

Cheers

PeteS

Thanks for the information. I'll give that a go

Something else for me to check. Those lines also have 33 ohm resistors in series. All go to an Intel 6300ESB

-Mike

You put a ferrite lump over that specific cable for sure.

Are all your connector shells firmly bonded to chassis ? That's a classic issue.

Is that a Class D audio amp inside btw ? Whose bits are you using ?

Graham

It's sounds to me as if you're trying to use audio screening style techniques.

Provided that your enclosure is effective, the emissions will be comoing out on the leads. I know very well that it's popular in audio not to directly ground the connector shell ( signal low/cold) to chassis but you really have to do this here.

Does that make sense ?

Graham

In message , dated Tue, 15 Aug 2006, Mike Warren writes

Yes. But without sight of the board, it's difficult to say what it's a clue to! The 33 ohm being that far from the audio chip suggests a less than skilled board layout. Maybe a look at the supply line decoupling of the audio chip, especially at the ground end, might be helpful. Even check that the ground end really finds ground! No open-circuit via!

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

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

What material is that ? I'm guessing it's electrogalvanised steel.

If it has a phosphated 'anti-fingerprint' finish, it can be a pig to ground stuff to it adequately. Just try it with a meter !

Graham

Putting a ferrite on the external cable at the test lab lowered the emission slightly but fitting one to the internal cable actually made the one frequency we were looking at worse. We tried both ends of the cable. There was no change whether the cable was connected to the SBC or not. Disconnecting it at the I/O PCB end did make it better.

The sockets are insulated but I have 10nF capacitors from the socket ground to chassis ground on the PCB next to the sockets. I did try shorting across the capacitors but there wasn't any difference. I'll try putting foil between the sockets and the case today.

Yes. They are made by Hypex in the Netherlands.

-Mike

Zincanneal. Its definitely not 'anti-fingerprint' :-) It's supposed to have a good conductive surface but I haven't actually checked it. I'll do that today.

-Mike

I have tried 2 boards so it's unlikely. One other puzzling thing is the schematic shows a SM ferrite between the audio chips A_Gnd and power ground. When I remove that there is still 0 ohms between the

2 grounds. It may be in the chip or there is another ground path.

I'm awaiting a response from the manufacturer on that.

-Mike

Is that a tradename ? Over here in the UK we used 'Zintec' from British Steel but the Asians don't seem to have any direct equivalent. Either that or the purchasing guys don't know about it. The phosphating can be an utter PITA.

Graham