CM Chokes sometimes suck

an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are

Refuse to sign off on it. I did that numerous times when I was contracting at Honeywell... told the management, "I won't sign that. If you think that's right, _you_ sign it."

They never signed off on anything I balked at ;-)

And every time I offered to quit, they declined to accept my offer. ...Jim Thompson

Harry

Cost?

If it's for the government, then that won't work.

an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are

What I do when a customer tells me to do stuff like that is either refuse, or just ignore them. Ignoring is better, because by the time the stuff is shipped, it's too late to change it, too late to argue, and it works, too.

I have one customer who insists that every mounting hole of a PCB be not connected to the ground plane. Then they insist on a parallel RC from each mounting hole to the ground plane. I ignore them.

A lot of EMI and grounding gurus are idiots. Like that HoJo guy.

an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are

Hmm, for some reason I can't see the original post.

One of the peculiarities in aircraft designs is that they often use the fuselage as ground return. Personally I do not like that but t'is the way they are often built. When the box gets bolted to the fuselage and the other side of the GND path of a CM choke is also connected to the fuselage (where else could it go?) then that essentially shorts out the CM choke. Makes it largely inefficient.

It depends where the Pi filter is located. If it is close to the connector then the CM chokes won't add anything. But if there is some length of PCB trace between the Pi filter and the connector then it is wise to use an extra filter. PCB traces tend to pick up all kinds of noise and that noise can be transferred into a connector if there is no filtering at the connector.

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

I think it's great. The only way you can lose low-impedance ground continuity is if the airframe is ripped into pieces.

Well, unless it's made of composites. I wonder how they handle that.

On a signal pin, I usually put a resistor or a ferrite bead first, near the connector pin. That keeps RF current from being pushed into the circuit traces.

CM chokes don't help much to attenuate differential noise, and one shouldn't assume that the outside world is thoughtful enough to provide only differential EMI.

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

is

Our power went in the middle of writing this, Hurumph.

Current takes all paths, not just the one of least resistance. So if anything in the path rattles loose a bit because of old rivets, corrosion or whatever, you can have a bzzzt-phsssst situation. Those can turn into a problem at 45,000ft. It's the same in a car but there you can simply pull over if something smokes. And even then people can die as we have recently seen on the Bay Bridge when the back of a limousine caught fire :-(

Modern aircraft often have proper ground systems.

y hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are re ceived by a Pi filter (100K resistor and two 1.0uF caps to ground). The min iture CM chokes are 1k at 100MHz (1.6uH).

Sorry, I don't understand. If it's a single ended line, what's the other half of the common mode choke attached to? Is the other half just grounded at both ends? So it's just like an inductor in series with the signal line? Is it wound on a toroid? (If not it could add more EMI to the signal.)

I've 'spent' hours collecting data showing that someone's 'assumption' just wasn't true. It seems like a waste of time, but it keeps the project moving forward.

George H.

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

It's worse than that. If you connect both ends of one winding of a CM choke to the same ground then it behaves like a shorted transformer. The only inductance left to fight EMI will be its leakage inductance.

ntly hired an EMI guru. He insists on placing CM chokes on every single end ed I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

dd

Hmm, Scratch scratch... OK I've got no 'intuitive' feel for transformers. Well if that's how Harry's CM choke is hooked up then at least it's not doing any harm.

George H.

ired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are recei ved by a Pi filter (100K resistor and two 1.0uF caps to ground). The minitu re CM chokes are 1k at 100MHz (1.6uH).

tly hired an EMI guru. He insists on placing CM chokes on every single ende d I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The m initure CM chokes are 1k at 100MHz (1.6uH).

Sorry George when I stated "single ended input line". There is a floating t hermistor in a motor winding that could be a double ended input with the CM choke and Pi filter or single ended into the Pi filter. I'm stating that t he added CM choke adds nothing.

Thanks, Harry

Yup. And if you want a ground reference that you can trust you need to pick it off of one point and just use that one point, even if you need long wires.

Or you need to find a Really Good way of sending your sensitive signals in a common-mode resistant sort of way.

Floating isn't single-ended :-)

A CM choke can be beneficial in such situations but only if the thermistor would deliver a signal with higher frequency spectral content. Which thermistors generally don't. So you might as well just RC-lowpass the heck out of it.

If it was a fast magnetic pickup or something I would consider a CM choke.

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

It's like wearing garlic to keep vampires away. Or paralleling three different values of bypass cap on every power pin. It works, so keep doing it.

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

thermistor in a motor winding that could be a double ended input with the CM choke and Pi filter or single ended into the Pi filter. I'm stating that the added CM choke adds nothing.

A thermistor has two wires. If you don't use the CM choke, where does the other wire go?

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

When working in Korea they told me that one day of not ingesting any garlic can easily shorten my life by a month. When I came back my wife almost banned me from the bedroom because I reeked of garlic so bad.

Or growling at approaching clouds. Our Rottweiler did that when he was young. Made the clouds go away, worked every single time, so ...

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

Which is why everybody except Koreans only live to be 2 or 3 years old. ;)

Cheers

Phil Hobbs