Wilkinson resistor test
  • UnknownEditor
    4
    We've all used Wilkinson power dividers... the network shorts out the resistor. So how do you know the resistor value is within specified tolerance? OK, maybe the tolerance could be +/-10%, so it would be hard to screw up a thin-film job. Or maybe you are attaching 0402 (or even 0201) resistors to a circuit card. How would you know if one shorted out, or became an open. Or during a high-power odd-mode event, you might want to know if you blew the resistor without opening up the box.

    Here's an idea, but I am not smart enough to know if it is practical... what if you added some type of electrical loop through the Wilkinson, and tried to measure the "Q" of the network by coupling AC power to it. A resistor would behave differently than a short or an open, right? Seem like a job for an antenna engineer.

    Thanks for your thoughts, even if you tell me it is a stupid idea (but explain why).
    Steve
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  • madengr
    1
    I assume you are trying to do this in-situ without access to any of the ports?

    I was messing around several months ago with non-destructively measuring dielectric constants of curved plastic radomes. You can cut a piece of 1/2 wavelength copper tape and lay it on the plastic, then take two small coax loop probes and magnetically couple near the center of the dipole, looking at Q and resonant frequency. It easily tells the difference between ABS and anti-static plastics.

    Maybe you can couple to the output traces, as the isolation will degrade with the resistor open or shorted.
    madengr Options
  • Desert Sage
    0
    Geez, we just printed identical resistors to nowhere adjacent to the "real" resistors and measured those. Did this for space products. The ohms per square of the resistor material is set and stabilized with a high temperature bake. The alternate resistors verify the process.
    Desert Sage Options
  • UnknownEditor
    4
    That seems like a very good idea! My concern is for a combiner where they are assembled with eentsie chip resistors, thinking that the solder process could short them out or leave them open...
    UnknownEditor Options
  • UnknownEditor
    4
    and yes, madengr... no access to the ports....
    UnknownEditor Options
  • madengr
    1
    Maybe use a TDR. This guy has some lower cost pulse generators and probes. In this video he demonstrates probing some traces:

    https://youtu.be/kXpYIczdta8

    Probe across the resistor to measure the impedance at the probe tips. Of course you could probably just use the probe with a VNA too in TD mode, and gate out the response just after the probe tips, but in essence you can isolate the measurement just around the resistor, limited by bandwidth or rise time.

    Even if you can’t probe directly across the resistors, if you can probe the splitter outputs differentially, you’d see the resistor further down the lines. This is assuming the resistor is closer to the probe than the stuff the splitter is feeding, as the pulse would travel in both directions.
    madengr Options
  • Desert Sage
    0
    It seems to me if you are worried about shorted resistors after a solder operation that you have a design and/or process problem. Fix those and testing becomes less important. Just sayin'.

    (And by the way, solder shows extremely well on xrays.)
    Desert Sage Options
  • UnknownEditor
    4
    Totally agree. Combine a good process and good people and there is nothing to worry about. This was more of a thought experiment gone awry! Thanks Desert Sage.

    I did not consider TDR, thanks madengr!
    UnknownEditor Options
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