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Chokes

Discussion in 'General Electronics Discussion' started by Jules Silver, Feb 6, 2015.

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  1. Jules Silver

    Jules Silver

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    Feb 6, 2015
    Could you tell me where I can find some info on what the difference is between common inductor and common mode chokes?


    For example in the attached what is the difference between L1 and L2?


    Sorry to bother you with this but I don’t know where to find the info.


    Regards…j
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello Jules

    They are both inductors it's the application that sets them apart. Common mode chokes are used to primarily to reduce the common mode noise that appears on the power line.

    This comes from the equipment that might be using some kind of switching circuitry. Common mode noise appears on both lines and travels in the same direction.

    The choke is arranged so that the noise current flow around the core in such a way as to cancel each other out. This is not perfect but in most applications is fine.

    A common inductor has many other applications, it can be used as a choke but also for timing circuits, filters, and radiating elements. There are lots of other applications. The core material will also have an impact on what the coil can be used for.

    Adam
     
    Jules Silver likes this.
  3. Jules Silver

    Jules Silver

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    Feb 6, 2015
    Thanks very much Adam for you great reply. I will try and put this into perspective and post another thread. Thanks again...Jules
     
  4. Jules Silver

    Jules Silver

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    Feb 6, 2015
    Hi EP Forum.

    This is sort of a carry on from my first post.

    I am just about to build an ISN circuit (x4) to try and analyse the conducted 'Noise' coming from an ethernet port on a circuit I am currently trying to put through FCC/CE approval. I am told the noise is coming from on board switching power supplies. My quandary is any mods I do to rectify the situation I cannot visualise the difference.

    In the ISN circuit attached there are two types of chokes. I think I have fathomed out one is a coupled inductor style and the other is a common mode.

    I have found both are difficult to source. Any suggestions would be appreciated. Also is it that important?

    I would like to ask the Forum if you have any suggestions, recommendations or alternatives.

    My other question is the 4.7uF caps in the diagram are they electrolytic? Can I use bipolar? Again suggestions would be greatly received.

    Finally where do I put me spectrum analyser probe? is it where the 50ohm resistor is sitting? Is that what the 50ohm resistor represents?

    Thanks for any suggestions you have regarding this circuit.

    Regards...jules
     

    Attached Files:

  5. Jules Silver

    Jules Silver

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    Feb 6, 2015
    Hi EP forum.

    This is my third post regarding a nagging problem I have with a product I am currently putting through FCC/CE approval.

    It is currently failing from conducted emissions through the Ethernet port at between 1 and 2 MHz. See attached.

    I am told that this noise is probably due to on board switching regulators. I am thinking of just attaching a big cap on U1 to try to dampen this out but this is really just a guess.

    Can anyone suggest if I am in the right ballpark or make any other suggestions? I only have one board to experiment with so I am hesitant to try anything without verification.

    My other issue is seeing if any mods I do to the board are effective - see my other threads.

    Again any suggestions are gratefully received.

    Regards...jules

    Attached:
    Photo. my proposed big cap solution - ignore the SM resistor. That was just for scale.
    The schematic for the ethernet board.
    The noise from the testing labs.
     

    Attached Files:

  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Getting the Vcc straight is always a good idea. A big cap alone will not help. To cover a broad range of frequencies you need to use capacitors with different resonant frequencies in parallel. A typical choice could consist of a 1µF tantalum + 100nF ceramic + 100pF ceramic (provide some pads of suitable geometries so you can try different combinations in the layout). Remember to use ultra-short connections between the IC and the capacitors and from the capacitors to GND.
    You may also consider inserting a ferrite in series with Vcc to further dampen high frequency noise.

    Another measure is a GND plane underneath the traces from the IC to the ETH-transformer. You may also want to place the IC as near as possible to the transformer to keep the trace lengths very short.

    Also note that you have GND and EARTH in your design. These are ideally DC-connected only at a single point in the design, typically somewhere in the power supply section. Make sure that you do not create ground loops by having more than one connection between GND and EARTH. Having an AC connection by a small capacitor to divert HF-noise to EARTH is allowed at more than one point, although the number of these connections should be kept as small as possible.

    You should also investigate the quality of your DC/DC converter's output. This type of converter is especially sensitive to layout and can create lots of noise if the layout is unsuitable. Preventing switching noise from being generated in the first place is the best option - much better than trying to suppress it later on.

    Here is some background information on reducing radiated emissions in Ethernet systems.
     
    Jules Silver likes this.
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    You need to look at the use of ferrite beads for the low MHz range. Each signal line may need one. Also as Harald mentioned Layout and incorrect decoupling or insufficient decoupling won't help either.
    Adam
     
    Jules Silver likes this.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    This is a fairly specialised field. I can't help you with specifics on measurement but it sounds like you'll have to do some remedial work on the power supply. I do seem to remember that the eevblog had a video describing some issues with measuring the sort of node you're suffering from.
     
    Jules Silver likes this.
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Jules, I have moved your two newer threads into this one, since they are clearly on the same subject. This keeps all the related discussions together and reduces confusion and duplicated effort.
     
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  10. Jules Silver

    Jules Silver

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    Feb 6, 2015
    Thank you all for your input. It is gratefully appreciated.

    I am still trying to track down suitable chokes for the ISN circuit above. EVERYONE I ask has no answer. Murata are looking into it but I fear they will just come back with the answer - we have nothing suitable. This is what Pace and Panasonic have answered today and Wurth and Toko have yet to respond. If I could just source the correct chokes for the ISN I could visualise the noise on my SA. Any changes I then make to the ethernet circuit to alleviate the noise could then be visualised.

    Thanks again for all your help. I will keep you posted on my progress - if any. lol

    Regards...j
     
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