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How to meas Zo of FT caps?

Discussion in 'Electronic Design' started by [email protected], Jul 6, 2004.

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  1. Guest

    I have a bunch of small feedthru caps that I need to measure their Zo
    to assure it's 50 +/-10 ohms. Any thoughts on how to do it? I'm going
    to try to put a 50 ohm load on them and look at them with a network
    analyzer. I should be able to get it's Zo from the VSWR, no? Any other
    ways?
     
  2. John Larkin

    John Larkin Guest


    But a capacitor is, well, a capacitor; it doesn't have a Zo. What is
    has is capacitance.

    John
     
  3. Guest

    Yeah, the more I think of this, the screwier it is. It's a case of a
    lab manager (who doesn't understand past E=IR) dropping this on me on
    my way out: Wants to verify that Zo=50 on these. No freq, no
    capacitance...I thought they were some kind of coax signal feedthru,
    but they definetly are built like feedthru caps.
     
  4. John Larkin

    John Larkin Guest

    They might be signal feedthrus. Measure their regular capacitance and,
    if it's only a couple of pF, that might be the case. If it's a lot
    more, like hundreds of pf, it's a real cap.

    If it is a matched impedance feedthru, it wouldn't make sense to
    connect to it with wire leads, 'cause the wires certainly wouldn't be
    50 ohms and the whole concept falls apart. So the only way a 50 ohm
    feedthru makes sense is if it's fully coaxial on both sides, and then
    it's a connector.

    Does it have straight or hooked wires poking out both ends? What does
    the manufacturer's datasheet call it?

    Good luck explaining this to what's-his-name.

    John
     
  5. Guest

    It has straight wires protruding from either end. The body is about
    1/4" long at most maybe 1/8" dia. Center is ceramic. Outside is gold?
    metalized with small flange at one end, inserts into a hole. Looks
    like every other ft cap I've seen. I'll have to question why he wants
    it and what he really wants.
     
  6. Robert Baer

    Robert Baer Guest

    .....*NOT* what he wants; what circuit is it going to be in, and what is
    the part "supposed" to do!
    I believe there are some feed-thru parts designed as RF suppressors;
    basically a damping bead in series and bypass cap in parallel =
    equivalent to a distributed pi lowpass filter.
    And totally useless in DC apps (ie all signals below 20MHz).
     
  7. Guest

    Well here's what I did: I treated it like a peice of coax. I made a
    fixture to hold the "feedthru" with semi-rigid coax on one side and a
    50 ohm chip resistor on the other. I dropped in a short peice of coax
    and cal'ed to that. Then checked VSWR, was about 1.00something. Then I
    took out the coax, dropped in a few feedthrus and got VSWRs about 1.04
    - 1.06, which works out to about 53 - 55 ohms or so.
    How's that sound?
     
  8. Robert Baer

    Robert Baer Guest

    Really has no meaning.
    You must find out what it is really going to be used for and how
    (eXplicit info).
    Maybe the use is as mundane as a power feedthru in an audio app and
    the "requirement" is strictly off the wall.
     
  9. Guest

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