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Impedance matching between two 50 ohm cables through a joint

  1. Nov 18, 2015 #1
    Hi,

    I'm in a bind trying to determine how to handle impedance matching through a joint of copper wire sandwiched between two 50-ohm coaxial cables (the joint is an interface between atmosphere and a UHV system). There is a signal pulse with a rise time of 2 ns coming through one of the coaxial cables, through the joint, and into the other cable, where it then continues through to a series of electronics. I am trying to avoid reflection as much as possible. Any ideas? Thank you very much.
     
  2. jcsd
  3. Nov 18, 2015 #2

    davenn

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    hi

    draw a pic Am trying to envision where the 3rd wire comes into the picture ??1

    and can you clarify this please .....


    cheers
    Dave
     
    Last edited by a moderator: Nov 19, 2015
  4. Nov 19, 2015 #3

    CWatters

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    The impedance of the "joint" needs to match that of the coax as closely as possible and be suitable for the frequency range. What connectors are being used?

    Perhaps scroll down this page to the "COAXIAL CONNECTOR CHART"

    http://ecee.colorado.edu/~kuester/Coax/connchart.htm
     
  5. Nov 19, 2015 #4

    CWatters

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    Are you sure any reflection is coming from the joint?
     
  6. Nov 19, 2015 #5
    Hi everyone,

    Thank you for your responses. The joint is a copper wire feed-through within a flange on a UHV chamber. I've attached a poor drawing, but hopefully it portrays what I'm trying to describe. I think we've found another solution, but I'd still like to understand how this particular issue could be solved by impedence matching.

    Thank you very much,

    ~Randall

    https://www.dropbox.com/s/k1aer8mmaszst9p/Note111915_1.jpg?dl=0
     
  7. Nov 19, 2015 #6

    f95toli

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    That is going to cause reflections, and with a rise time of 2ns you might run into some problems. I doubt there is an easy way to solve this without a complete re-design.
    Is the copper just a feed-through? Or, does it have some other purpose?
    If your goal is just to get an RF signal into a vacuum chamber the best solution is just to buy a proper RF feedthrough.
     
  8. Nov 19, 2015 #7

    berkeman

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    How is ground fed through that connection? If you don't feed the ground through as well, you will have serious problems.

    (Assuming that ground is fed through as well and you just didn't draw it...) What are the source impedance and load impedance? If they are both 50 Ohms, then you will not get ringing at the load due to the impedance mismatch of the feedthrough. You will get a reduced amplitude pulse at the load due to energy lost in the reflection at the feedthrough, but the "back termination" effect of the 50 Ohm source will keep you from getting ringing at the load. If either the source impedance or load impedance are something other than your transmission line's 50 Ohms, you will get ringing at the load due to the feedthrough impedance mismatch.
     
  9. Nov 19, 2015 #8

    marcusl

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    Manufacturers of vacuum equipment sell vacuum-tight feedthroughs with coaxial connectors on each side. I suggest that you replace your feedthrough with one of those.
     
  10. Nov 19, 2015 #9
    Hi again everyone,

    Thanks to all of you for replying. Yes, we have a flange with a coaxial feedthrough on another chamber. The initial issue was because of a deadline that prevented us from taking the time to order a new flange. Fortunately, things have changed and this was solved. However, I continued the conversation because I was curious if this could be solved in any other way for future reference.

    If this problem is simply not possible to solve easily (and since the problem was essentially solved using a different flange), then feel free to discontinue the thread. As for the other questions, the signal is actually coming from the vacuum chamber from a detector through the flange and to other electronics and the ground connection is actually the chamber itself.

    Thank you very much.
     
  11. Nov 20, 2015 #10

    CWatters

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    The problem is that at high frequencies the physical shape of wires and cables affects their electrical properties. You have to treat them as transmission lines. Its quite a complex subject but there are lots of books and articles on the subject.
     
  12. Nov 20, 2015 #11

    CWatters

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  13. Nov 21, 2015 #12
    The main issue for me is that, despite the high frequency, this is an electrical circuit, and yet you do not have continuity of the
    coaxial shield through the joint.
     
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