![]() With many inductors the DC + skin effect AC resistance is only a few Ohms so I'm either at the limits of what the instrument can do or my test setup / settings aren't up to the task. I see that the resistance starts to get way off at higher frequencies. The formula for the Q factor is 2*pi*f*L/R (=Z/R) where R is the real part of the impedance. I think somehow the measurement loses the accuracy required to determine the Q factor. ![]() From there the results are just plain wrong with what they should be in theory. Measuring the Q factor for the inductor I want to use seems OK up to 200kHz or something (which would suffice for the project at hand) and in that region the transfer measurement with the PI jig and the reflection bridge results are the same. I have put a PI network together using two pi-pad attenuators / impedance converters to make a 12.5 Ohm-ish PI network jig (mine is 13.2 Ohm because I didn't have 68 resistors and used 22+47=69 Ohm). Perhaps to keep the impedances at the source and input more closer to 50 Ohm. From that it should be easy to calculate the Q of the inductor, provided that a high quality capacitor (maybe an air trimmer) is used and the quality of the resonance circuit is almost entirely determined by the inductor.įrom the pictures it seems a PI network is measuring S12 but at a lower impedance. 20dB additional attenuation (= 40dB total) means a series resistance of 18 ohms, likewise 40dB (60dB total) would indicate a series resistance of 1.8 ohms. attenuation at the series resonance frequency. The quality of the inductor can be estimated by the max. The following graph shows the result for 100nH, 1µH and 10µH: With a reference capacitor of 25.33pF, resonances will occur at 10MHz for 10µH, 31.6MHz for 1µH and 100MHz for 100nH, which should match the usual frequency range for these inductor values, but can easily be adapted of course. It was intended to be used with a scalar network analyzer – in fact an SA with TG and out of my memory, it looked like this: Basically a 20dB attenuator that helps to keep the port impedances reasonably constant and a reference capacitor that forms a series resonance circuit with the unknown inductor. ![]() Unfortunately, I cannot answer that question either, but it reminds me on a test fixture I’ve built some 30 years ago in order to measure small inductors at appropriate frequencies, which happens to have a Pi structure.
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