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Weird EMC circuit

I'm doing some EMC work right now, and created something weird. Has anyone seen it before?

Description:

Suppose you have a shielded cable being terminated to a header. So, you tie the shield to a pin, and ground that on the board. The rest of the wires are signal, power, ground, that sort of thing. For purposes of example, say it's CMOS logic level (impedance controlled for signal quality over the cable, but otherwise pretty boring).

Well, it fails EMC susceptibility, because the header and connection isn't shielded. That one inch of unshielded cable is enough to drop more than V_IH(min) - V_IL(max) and screw up the logic thresholds.

Or if it's differential (say, RS-422/485), the difference might be okay, but the common mode either bangs into the supply rails (transmitter stops transmitting), or hits clamp diodes / TVSs, or exceeds the receiver common mode range (which is usually a few times supply due to internal resistors, but still finite).

So that's the background. Follow?

Accepting that we can't perfectly shield the whole connection, suppose we take half the length of the shield / drain / ground wire, and sense the voltage across it. Ideally, this length has half the voltage drop that's causing our problems. So, run that into a 1:2:2 transformer, where all the signal lines go through the '2' windings. Maybe we just use a ferrite bead, one turn for the ground, two turns each for all the signals.

So...you've got a shorted turn around the ferrite bead, that's got shield/ground current flowing across said turn. And the signal lines make two turns through.

And it works. I just tested a 100%+ improvement doing this -- assuming, of course, that "sense" short is very nearly 50% of the total (or whatever ratio the transformer is).

So, good luck cutting all your cables so they have exactly the right measurements. It seems to be a neat trick, but not something you'd be able to put on cable ends reliably. But all I want to know... has anyone even seen this before?

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams
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Den fredag den 29. august 2014 00.46.28 UTC+2 skrev Tim Williams:

it's getting late but isn't sensing half the voltage and using a 1:2 transformer basically the same as putting all the wires through a 1:1 transformer? so equivalent to the ferrite lump used on many cables

-Lasse

Reply to
Lasse Langwadt Christensen

No -- if the common mode current is just that, a current (or a very high impedance stimulus, which is the case with ESD for example), it has to find its way through, FB or not. So the problem is mutual inductance between the wires.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Drivel:

TI tried to, back in the early 80s. The expansion port on the TI-99/4A was a card edge on the motherboard. There was some grounded fingerstock-ish stuff around it. The mating connector was almost a standard card-edge connector, but it had metal shielding on the outside that contacted the fingerstock. If the mating connector went to ribbon cable, there was an overall shield over the ribbon. This cable was known as the "fire hose".

Computer side:

formatting link
External side:
formatting link

I don't know how this did from an EMC standpoint - the signals involved were probably TTL at a couple of MHz - but from a general reliability standpoint, I learned to save often. Powering down, cycling the connector, and powering back up was also a helpful technique.

Why "shorted"? Is it because the drain wire is bare and it touches itself on both sides of the bead? If it doesn't touch itself, I don't think it qualifies as a shorted turn... it's just another winding with current flowing in it.

Does it work at all (or as well) if *all* the wires have two turns through the bead? (My guess: better than nothing, but maybe not as good as your way.) How about if all the signal/power/etc wires take a turn through the bead but the drain wire doesn't? (My guess: better than nothing, but not as good as the preceding idea.)

Like Lasse said, it sounds sort of like just wrapping the whole cable through a ferrite bead/ring. Figure 7.73 on page 461 of AoE 2nd ed, although that puts all the wires through the ferrite and yields a

1:1:1 transformer. The 1:2:2 thing is perhaps new; I haven't seen enough to say.

Matt Roberds

Reply to
mroberds

On a sunny day (Thu, 28 Aug 2014 22:38:57 -0500) it happened "Tim Williams" wrote in :

mm go optical.

Reply to
Jan Panteltje

Probably not terrifically bad (on EMC or signal quality), assuming the contacts mated up okay of course. But as the tag says, 'drivel' :), not really the thing I did.

Yes, circuit-wise it makes a complete loop around the bead. So it actually doesn't have much current flowing through it, it's just there to impress the voltage on the core.

Didn't try, but should be a marginal case. Performance is very sensitive to the position of the wire, so if it's right next to the bead, the improvement is only ~30%; if it's too far away, same idea. So as odd as it looks, it's doing the balancing act.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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