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Testing the quality of twisted pairs

  1. Jan 16, 2015 #1
    Hello all.
    I was wondering if there is a method or a device to check the quality of the twists on twisted pairs cable harness vis-a-vis EMI. i.e. someway (to tell is the twists are good enough to pass EMI testing).
    Cheers
     
  2. jcsd
  3. Jan 16, 2015 #2

    sophiecentaur

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    Science Advisor
    Gold Member

    I think you would need to test it yourself by putting two lengths of the cable side by side and measure the level coming out of one when you feed the other (and terminate it correctly. But this would all depend upon what you would consider to be 'good enough'. There are published figures about this sort of thing (crosstalk) in commercially available cables. How much worse than those figures can you afford to go? If you have 20dB in hand then you're OK but, if they are only just good enough for your application then it may be impossible for your to achieve. That approach would put things into perspective for you and give you something to start with.
    Doing the above test with a similar length of commercial feeder as a comparison would not be too difficult (if you have a reasonable receiver). Without the reference feeder, you would find it very hard to know the actual field strengths of EMI involved. (Much more expensive kit needed)
     
  4. Jan 16, 2015 #3

    berkeman

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    Staff: Mentor

    Can you say more about what you mean? What do you mean by EMI issues? Radiated EMI from twisted pair (TP) cables usually is caused by common-mode RF currents on the TP cable, so the twists have nothing to do with it. If you are talking about cross-talk between adjacent TP, usually cables that have multiple TPs inside them (like 4-pair Cat-5 cable) have different "lay lengths" for each of the TPs. So that on average, the twists do not line up, which minimizes the cross-talk between the pairs.

    For testing TP cables, you can buy or rent a simple cable tester, which will tell you the characteristic impedance Zo of the cable, and depending on the model, may be able to tell you the quality/consistency of the Zo for longer cable lengths:

    http://thumbs1.ebaystatic.com/d/l225/m/miNurJ48AK6d6qkq1SJAx1Q.jpg
    miNurJ48AK6d6qkq1SJAx1Q.jpg
     
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