Maker Pro
Maker Pro

CM Chokes sometimes suck

H

Harry D

Jan 1, 1970
0
I'm working with a group designing avionics equipment and they recently 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).
What is my best arugment to have them replaced with a short?

Harry
 
T

tm

Jan 1, 1970
0
I'm working with a group designing avionics equipment and they recently
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).
What is my best arugment to have them replaced with a short?

Harry
++++++++++++++++++++++++++++++++++++++++++++


Cost?

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

Joerg

Jan 1, 1970
0
Hmm, for some reason I can't see the original post.

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."

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.

[...]
 
N

Nico Coesel

Jan 1, 1970
0
Harry D said:
I'm working with a group designing avionics equipment and they recently hi=
red 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 receiv=
ed by a Pi filter (100K resistor and two 1.0uF caps to ground). The minitur=
e CM chokes are 1k at 100MHz (1.6uH).
What is my best arugment to have them replaced with a short?

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.
 
J

Joerg

Jan 1, 1970
0
John said:
I think it's great. The only way you can lose low-impedance ground continuity is
if the airframe is ripped into pieces.

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 :-(

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

Modern aircraft often have proper ground systems.
 
G

George Herold

Jan 1, 1970
0
 I'm working with a group designing avionics equipment and they recently 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).
 What is my best arugment to have them replaced with a short?

 Harry

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.
 
J

Joerg

Jan 1, 1970
0
George said:
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.)

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.

[...]
 
G

George Herold

Jan 1, 1970
0
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.

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.
 
H

Harry D

Jan 1, 1970
0
I'm working with a group designing avionics equipment and they recently hired an EMI guru. He insists on placing CM chokes on every single ended I/Osignal 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).

What is my best arugment to have them replaced with a short?



Harry
 
H

Harry D

Jan 1, 1970
0
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.

Sorry George when I stated "single ended input line". There is a floating thermistor in a motor winding that could be a double ended input with the CMchoke and Pi filter or single ended into the Pi filter. I'm stating that the added CM choke adds nothing.

Thanks,
Harry
 
J

Joerg

Jan 1, 1970
0
Harry said:
Sorry George when I stated "single ended input line". There is a
floating 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.

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.
 
J

Joerg

Jan 1, 1970
0
John said:
It's like wearing garlic to keep vampires away. ...


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 paralleling three
different values of bypass cap on every power pin. It works, so keep
doing it.

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

Mark

Jan 1, 1970
0
it's a team effort, you really have no choice now but to leave the CM
chokes in because..

if you convince the EMI guru to take them out now ,,,, and in the
future, there is ANY kind of an EMI issue, it will be YOUR fault.

If they don't hurt anything, leave them in and move on.

Mark
 
H

Harry D

Jan 1, 1970
0
A thermistor has two wires. If you don't use the CM choke, where does

the other wire go?





--



John Larkin Highland Technology, Inc



jlarkin at highlandtechnology dot com

http://www.highlandtechnology.com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom laser drivers and controllers

Photonics and fiberoptic TTL data links

VME thermocouple, LVDT, synchro acquisition and simulation

If the CM choke is not used then the other lead goes to ground and the signal is "single ended".
But you knew that already.
Cheers, Harry
 
H

Harry D

Jan 1, 1970
0
it's a team effort, you really have no choice now but to leave the CM

chokes in because..



if you convince the EMI guru to take them out now ,,,, and in the

future, there is ANY kind of an EMI issue, it will be YOUR fault.



If they don't hurt anything, leave them in and move on.



Mark

Mark, you make a good point but this guru will rain on my parade forever and that s$it must stop. He also gets paid a lot more than me and that PMO!!
 
T

Tauno Voipio

Jan 1, 1970
0
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.

There are return lines, at least in my DA42 (you can Google for the type).
 
S

Spehro Pefhany

Jan 1, 1970
0
I'm working with a group designing avionics equipment and they recently 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).
What is my best arugment to have them replaced with a short?

Harry

Maybe I'll be shat upon for this, but IMHO, there's something to be
said for not bringing out antennas connected solidly to various points
on the internal ground plane(s). If there's a Pi filter as you
describe on the "ground" line too, then it doesn't matter.

For something like a thermistor, there's no reason I can think of not
to use pennies-a-piece tiny ferrite beads. You only need a CM choke
when the current is high enough to saturate an inductor.
 
J

John Miles, KE5FX

Jan 1, 1970
0
 I'm working with a group designing avionics equipment and they recently 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).
 What is my best arugment to have them replaced with a short?

 Harry

By "single ended" do you mean separate signal and ground conductors,
or a signal whose ground return is the airframe or something else
besides another wire routed next to the signal line? I understand
that you aren't referring to a differential line, but "single ended"
by itself doesn't say anything about the ground return.

What is the expected offending signal that the EMI guru is trying to
filter out?

How's your 1.0 uF capacitor look at, say, 120 MHz? For that matter,
how does your 100K resistor look?

-- john
 
H

Harry D

Jan 1, 1970
0
If the pcb ground plane is solidly bolted to the box, and especially

if the connector shells are bolted to the box and to the PCB ground

plane, bringing in a ground through a connector pin won't cause EMI

problems.










A pair of beads, one for each end of the thermistor, isn't a bad idea.

EMI rectification errors then to be in the 100+ MHz range, and tend to

follow PCB resonances, both broken up by a bead. A common-mode choke

only attenuates... common mode RF! But grounding one pin, and RC

filtering the other, will usually work fine.



A 100K thermistor in a motor winding does get interesting, especially

if there's a high-frequency motor driver.









--



John Larkin Highland Technology, Inc



jlarkin at highlandtechnology dot com

http://www.highlandtechnology.com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom laser drivers and controllers

Photonics and fiberoptic TTL data links

VME thermocouple, LVDT, synchro acquisition and simulation

I don't get the beads, I am not trying to buy NY city.
The thermistor comes into the PCB in a shielded twisted pair. One lead goes directly to ground, the other thru a Pi filter (220nF, 100K, 220nf) to a Hi Z ADC input. The caps are X7R. The signal BW < 1.0Hz. The noise bandwidth is >10 MHz but the Pi filter has a BW of <7Hz. Now you want to place your stinkin bead where?
Regards, Harry
 
J

Joerg

Jan 1, 1970
0
John said:
In the ungrounded lead. As I said, you can probably get away without it.

This is a thermocouple input, and it's heavily lowpass filtered by RCs, and then
by software filtering. The tc cable is a shielded pair. Unfortunately, the tc is
inside an NMR probe, along with high field levels of RF in the 50-500 MHz range.
The RF did (always will) get into the signal conditioning opamps and get
rectified to small offsets, in a system where 1 microvolt matters. I added this
bead

https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/Bead_L900.JPG

and it helped by something like 30 dB, and especially broke up narrowband
resonances.

I got this business by doing this better than their previous vendor. An RF
generator could shut down his controller from clear across the room. I've sold
over 3000 controllers so far, not bad for a "stinkin bead."

In such unwanted rectification scenarios there is also another option:
Swap the opamp against a CMOS type if one is available that has
otherwise acceptable specs. Those do not have BE junctions behind the
input pins that could rectify.

In one case (across the Bay from you guys) that measure alone killed the
noise dead, as John Wayne would have put it. It was GSM cell phones
getting in there, something where regular beads aren't very effective
anymore unless you use the special Murata thingamagics, and those need
super-direct ground contact plus shield in order to work.
 
Top