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20190427 - Inductance Measurement

posted Apr 27, 2019, 6:39 PM by Onno Benschop   [ updated Apr 27, 2019, 6:41 PM ]
Denis VK6AKR shares this information on how to measure inductance:
  1. Direct and simple, use an inductance meter, usually combined capacitance / inductance meters since they both typically measure reactance and often ratiometrically so that even the cheap and cheerfuls can give good results when calibrated against a known capacitance / inductance value in the range of interest. Disadvantage is that inductor Q is not measured. Laboratory grade instruments are available, think $$$$, that do assess Q factor.
  2. Grid dip meter method. Create a parallel tuned circuit comprising the inductor under test and a known capacitance, or just the inductor alone if some idea of its self capacitance is known (rare I would say.) If using an external capacitor the external cap value should be large enough to swamp out any self capacitance. E.g. multi-layer winding and / or close turn spacing would have higher self capacitance than single layer and / or widely spaced turns. Use the frequency displayed on the dip meter and the known capacitance value to work out the inductance. Q can be estimated - check the width and depth of the dip - but relies on calibration of the dip meter and accuracy of capacitor.
  3. Series resonance method. Needs accurate capacitor and frequency source. Connect a series string of resistor, known capacitor and unknown inductor across the signal generator output. Inductor connected to ground. Measure the resonance as a dip in signal at the junction of the resistor and the capacitor. Advantage, the signal strength detector loading does not affect the resonance point. Calculate inductance from the known frequency and known capacitance. Value of the series resistor not critical, could be in the range 470 ohms to several thousand ohms. Depends on the (resistive) loading effects of the signal detector which might be an RF voltmeter or oscilloscope or SDR. If using SDR be mindful of their typical 50- to 75-ohm input impedance in which case the series resistor might need to be lowered in value. Same comments apply to external cap value as for method 2 - if high self capacitance in the inductor, that will cloud the results for low external capacitance values.
In general, accurate frequency counters would be more available (economical) than accurate signal sources so the combo of a good freq counter and so-so signal generator will achieve good results for method 3. A freq counter could be coupled to a grid dip meter coil but would need to be very sensitive as most gdo's have very low radiation.

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