Accurate Measurement of Small Inductances

Posted on Feb 7, 2014

How does one measure small inductances with a precision of at least two, and possibly three, significant digits A search of the literature produced scanty results. One interesting circuit was described in The 2001 ARRL Handbook on p 26. 22. I built a replica, and duplicated the results. Measurements with accuracy of ±10% can be made with that clever

Accurate Measurement of Small Inductances
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device from about 3 to 3000 µH, and that is good enough for many ham applications. A more accurate and direct reading device was described by Robert Vreeland, W6YBT, in QEX (May 1989); but an expensive Fluke DMM is required. Evidently, accuracy in the 3-5% range can be expected with W6YBT`s elegant method. Nonetheless, 1% accuracy and measurement of smaller inductors (down to about 0. 5 µH) was the goal, even if elusive. I attempted numerous approaches, including various unsatisfactory bridges and the old standby grid-dipper method. By forming a parallel resonant circuit with a known-value capacitor and finding the resonant frequency with a grid-dip meter, the value of the inductor can be estimated. At best, this yields ±10% accuracy. A serious shortcoming of this method is that coupling to toroidal inductors is nearly impossible since the magnetic field is concentrated in the core. (Did I miss something Has someone found a way to do this without influencing the unknown tuned circuit`s resonant frequency ) Finally, a simple solution occurred to me: the converse of the grid-dipper method. If we put the unknown inductor in parallel with a known capacitance in an oscillator and measure the frequency of oscillation with a counter or accurately calibrated receiver, we can calculate the inductance from the known tuned circuit capacitance and oscillator frequency. This method works very well, and I am convinced that it yields...

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