Shielding for common mode currents

  1. Let's say a shielded cable is in the vicinity of noisy environment with plenty of EMI.
    How to go about cancelling the common mode currents, so that the magnetic field gets cancelled.
    It's pretty simple in case of differential mode currents. The mag field from the forward current and return current cancel each other, provided both the conductors are close to each other.
  2. jcsd
  3. berkeman

    Staff: Mentor

    Can you use common-mode chokes?
  4. Nope.
    How about grounding the shield at both sides. When there is EMI, a current will flow on the shield due to mag field interference and cancel some of the mag field. Some (small)current will flow in the center conductor, since the EMI is common mode.
    This should help a bit, right?
  5. berkeman

    Staff: Mentor

    How come no chokes? It can be as simple as coiling your transmission line cable a few times through a ferrite toroid...
  6. This is for RF immunity test. All I get to use is a shielded cable (not even twisted pair).
  7. berkeman

    Staff: Mentor

  8. I did a lot of CE tests include RF immunity tests, we use common mode chokes all the time. Look at your laptop supply, connecting cables, a lot have choke on it. This is standard way of fixing this kind of problem. Double check.
    Last edited: Mar 1, 2012
  9. I know ferrite chokes help a lot with CM noise. But if I use them for the test, the product has to be shipped with chokes.
    Last time when I did the test, grounding on both sides helped pass the test.
    Both cable ends were well grounded because the cable was short (5m).
    The test was failing with just one side grounded.
    As berkeman had mentioned, the noise was capacitively coupling from shield to the inner conductor.
    I agree, if cables are long grounding both sides is a bad idea (ground loop etc).
  10. Yes, I wasn't thinking straight on the second part about grounding both side.

    Is it the EMI emitted from the signal inside the cable? Is it a coax cable? Is it connect from one module to another one. If so, make sure shield on both sides grounded at the outer chassis.

    If that don't help, are you sure the EMI is not from the board. I don't recall have issue with coax cable emitting noise except in case that the manufacturing mistaken to put an insulated BNC panel connector. As soon as we grounded the connector to the chassis with copper tape, that fixed it.

    I have seen the board driving the signal onto the coax gave out emission, but that was because of layout mistake in the board.

    This is one of the case that picture can speak a thousand words. If you can take picture of the connection from the pcb output of the transmitting side to the pcb input of the receiving side.
  11. It would be interesting to see you draw a sketch of your setup and how you think the currents flow and where your noise generators are. There's no one-size-fits-all answer here and you're not providing enough information for a meaningful discussion imo.
  12. berkeman

    Staff: Mentor

    Are you doing the EN 61000-4-3 Radiated RF Immunity test, or the EN 61000-4-6 Conducted RF Immunity test? What frequency range are you testing, and what are your problem frequencies? What test levels are you trying to achieve?

    Can you use ferrites and caps at the input of the cable to your product? Those can be more effective than using the cable clamps in some cases. If you are having trouble passing the RF Immunity tests, then you will most likely need to use some standard circuit-type solution.
  13. Shielding is definitely a good way, but the shield termination needs to be well implemented. Running the termination in on a pin and then through a trace to a ground is about the worst way to do it (but mandated by some standards, like DeviceNet). Almost as bad is soldering the shield to a skinny lead and then soldering that to the shell.

    To get performance from a shield, it should be grounded to the shell of the connector promptly. This is because the current passing through the shield can become fairly high when you hit a resonance of the cable. Then, any inductance on the shield allows voltage to build, which is then coupled through to the inner leads.

    Depending on the type signal you're conveying, a great deal con be done at the circuit board.
    If your signal is low frequency, and going into a high impedance, a simple series resistor will typically knock out the noise. A 4.7K resistor is like a brick wall to a 10V/m application. If you need a lower impedance, a 100 ohm, followed by 470pF to ground will likely knock it out.

    If your signal is low frequency and going out, a Zobel network is usually a good way to ensure that your driver is stable and interference doesn't come back. Values vary, depending on the frequency of your interference but 220nH inductor with 47 ohms followed by a 220pF capacitor with a 47 ohm series resistor to ground is a good place to start for 30-150MHz. Above this, a series ferrite bead is useful to about 350-400MHz. After that, a small value inductor, such as 20nH.

    If you're working with higher bandwidth signals, the common mode choke is your best friend.

    One thing that's also good to remember when performing band-aids, is that the smaller the id of a ferrite, the more effective it is. Sometimes, you can slip small beads over individual leads. With flex cables, a slip-on ferrite can help, but the effectiveness on a flex goes down dramatically with width.

    For future reference, I suggest the writings of Dan White Consultants and Henry Ott.

    Hope something in this helps,

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