Maker Pro
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CM Chokes sometimes suck

G

George Herold

Jan 1, 1970
0
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 MHzrange.
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_L9...

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'vesold
over 3000 controllers so far, not bad for a "stinkin bead."

"Good bead" :^)
Say if you passed one of the TC leads through it in the other
direction would that make it a one turn CM choke? (Just trying to
expand my transformer understanding.)

George H.
 
J

John Walliker

Jan 1, 1970
0
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.
Yes, very effective indeed.

John
 
H

Harry D

Jan 1, 1970
0
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 MHzrange.

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'vesold

over 3000 controllers so far, not bad for a "stinkin bead."





--



John Larkin Highland Technology Inc
John, your TC signal came into a diff-amp that rectifies at >1.0MHz. My single ended signal, after filtering, comes into a ADC with a bandwidth <1.0MHz. That stinken bead is worthless at <1.0MHz.
 
W

whit3rd

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.

Yeah, if you're wrong when you say to take them out. It's never
going to be a 'no choice now' situation, though: stand and fight.
The defensive "do EVERYTHING we can" style is bad engineering,
bad tactics. Your job is to use planning time and resources to
good effect, and project hardware bloat is NOT a good effect.
If they don't hurt anything, leave them in and move on.

"No evil, no matter how small, ought to be tolerated." -- Aesop
 
M

Mark

Jan 1, 1970
0
Yeah, if you're wrong when you say to take them out.  It's never
going to be a 'no choice now' situation, though: stand and fight.
The defensive "do EVERYTHING we can" style is bad engineering,
bad tactics.  Your job is to use planning time and resources to
good effect, and project hardware bloat is NOT a good effect.


"No evil, no matter how small, ought to be tolerated."  -- Aesop


============
we are talking about a few pennies worth of parts, i'm not sure I
would use the word evil.

if the EMI guy is responsible for the EMI compatibility issues and he
wants the CM filters in, I would let it go. It's his contribution to
the effort, do not minimize it.

You never know, there may be 5 GHz RF fields and the CM filters help.

You never know, some issue, EMI or otherwise, may come up in the
future and having those CM filter pads on the PWB may be helpful.
It's cheap insurance.

Most importantly, maintaining a good working relationship with other
members of the team is worth ,much more then the few cents for the
parts. If the customer insists on the EMI guy being part of the
team, I would even suggest soliciting the EMI guys input at the start
of the next project.

The real issues here are much larger then engineering.

Mark
 
J

Joerg

Jan 1, 1970
0
Jasen said:
Have you killfiled google?

I used to, until my news provider (www.individual.de) began to do a
really good job of hosing off spam. The ads for the fake Gucci handbags
and all that. But it let posts from David Jones (who unfortunately uses
googlemail) through without a problem, something my own filter wasn't
able to do. Then I took my filter out, there is no killfile in my
newsreader.

The news provider occasionally filter out some other legit posts, but
mostly only Jim's, for whatever reason. Maybe they use a routine that a
certain agency lately ... well, let's not go there :)
 
S

Spehro Pefhany

Jan 1, 1970
0
Clearly Aesop wasn't an engineer.

If the level of evil is guaranteed to be substantially less than the
specified limit, it's probably overdesigned.
 
J

josephkk

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
But they do hurt. They cost money. They do not provide the intended
value. And worst of all they teach erroneous design techniques to the
easily swayed. Costing lots of money permanently.

A far better design would use a tapped inductor 2 turns to the outside
world per turn to inside on the two sides of the tap; then place a cap to
ground at the tap. Toroids or closed pot cores preferred.

Measure the performance difference in front of management.

?-)
 
J

josephkk

Jan 1, 1970
0
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

That is a very different case. The CM choke might actually help. Drawing
the circuit diagram.

___________@@@@___________> to measuring circuit
Therm O=======CMchoke===<_ | |
| _|_ _|_
| ___ ___
| | |
|_____|___________|
_|_
//// Pi filter


Do you see how it is not a transformer with a shorted secondary in this
case?

?-)
 
J

Joerg

Jan 1, 1970
0
josephkk said:
But they do hurt. They cost money. They do not provide the intended
value. And worst of all they teach erroneous design techniques to the
easily swayed. Costing lots of money permanently.

A far better design would use a tapped inductor 2 turns to the outside
world per turn to inside on the two sides of the tap; then place a cap to
ground at the tap. Toroids or closed pot cores preferred.

Measure the performance difference in front of management.

Now what's a deecee-bel? Our MBA professors never mentioned ... oh wait,
it's a cheese, right? Yes, it is a cheese!

http://www.thelaughingcow.com/about-us/
 
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