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CM Chokes sometimes suck

Discussion in 'Electronic Design' started by Harry D, May 21, 2013.

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  1. Harry D

    Harry D Guest

    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
     
  2. tm

    tm Guest

    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.
     
  3. Joerg

    Joerg Guest

    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.

    [...]
     
  4. Nico Coesel

    Nico Coesel Guest

    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.
     
  5. Joerg

    Joerg Guest

    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.
     
  6. 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.
     
  7. Joerg

    Joerg Guest

    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.

    [...]
     
  8. 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.
     
  9. Harry D

    Harry D Guest

     
  10. Harry D

    Harry D Guest

    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
     
  11. Joerg

    Joerg Guest

    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.
     
  12. Joerg

    Joerg Guest


    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 ...
     
  13. Mark

    Mark Guest

    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
     
  14. Harry D

    Harry D Guest

    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
     
  15. Harry D

    Harry D Guest

    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!!
     
  16. Tauno Voipio

    Tauno Voipio Guest

    There are return lines, at least in my DA42 (you can Google for the type).
     
  17. 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.
     
  18. 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
     
  19. Harry D

    Harry D Guest

    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
     
  20. Joerg

    Joerg Guest

    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.
     
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