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Inductor Q measurement

Discussion in 'Electronic Basics' started by Philip Newman, Oct 1, 2004.

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  1. Does anyone know how to measure the Q value of an inductor, using
    Agilent's ADS? Or any simulation package?

    I have a inductor from the SMT library, and it seems to have a higher
    Q value than the data sheet says it should have. I want to measure
    the Q value and see what I am getting. I am using these inductors to
    build a high-order filter and the Q value is important as I need to
    obtain a flat pass-band (0.2dB ripple).

    If anyone can point me in the right direction, or give my any help in
    measuring Q of an inductor, then I would be most grateful.

    Thanks

    Phil
     
  2. john jardine

    john jardine Guest

    Believe the data sheet. The real inductor is tested under real conditions.
    The library inductor can only be an idealised version of the real component.
    Many physical design aspects such as loss and skin effect factors can not be
    modelled with any accuracy, if at all.
    Also, the makers usually supply Q values at only one test frequency. your
    filter will no doubt be working at some other frequency and the Q will be
    different. Maybe best to grade the components on a commercial Q meter. 2%
    ripple is not a lot, and a simple bench Q meter setup (generator, series L,
    parr C, AC voltmeter, would not easily offer up the required precision.
     
  3. BobGardner

    BobGardner Guest

    Does anyone know how to measure the Q value of an inductor,
    ============================
    Its just wL/R isnt it? Or resonate it against a known C, measure hi and lo 3dB
    down freq points, then compute f0/bw
     
  4. john jardine

    john jardine Guest

    Yes it is wL/R but the damned R will refuse to be easily pinned down. A
    simple way is to series resonate the inductor and capacitor. That way there
    is fewer complications from shunt loading effects dropping the measured Q
    value.
    Use a signal generator with a 50ohm output. Feed the output voltage to a
    47ohm 1ohm potential divider. Feed the circuit from across the 1ohm
    resistor. Measure the voltage feeding the circuit (across the 1ohm) and the
    magnified resonant voltage across the L or C. Voltage ratio is the Q value
    at that test frequency. It's not perfect but it'll get you up and running.
    regrds
    john
     
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