Measuring low DCR coil inductance
Copyright 2017 © Troels Gravesen

I've had a couple of responses to the 18 mH C-coil used in the DTQWT. Using a standard LCR meter usually ends up around 14 mH. How can that be?

The instrument needs a certain series resistance to make the calculation right, and for the C-coil range we're often below 0.1 Ohm. This is too little. Not even my CLIO measuring equipment can do this. It measures ~14 mH as well.

To measure a coil with this low series resistance, you need measuring equipment that can do an impedance plot.
The method is described here:
http://daycounter.com/Articles/How-To-Measure-Inductance.phtml

There are several methods available and I use Method #2:

You add a precision (2% is OK) capacitor parallel to the coil and add a series resistor, e.g. 1.5 Ohm like seen above, value of the resistor is not important.

CLIO measuring equipment must be set to 48 kHz sampling frequency. I use the sinusoidal function to do the impedance plot.

Perform the measurement and you may get something like this, a peak at 1,278 Hz.

Next you insert the two values at the website (Method #2):
http://daycounter.com/Articles/How-To-Measure-Inductance.phtml
- and it's no problem finding a reference capacitor as mentioned in the article, use e.g. Superior-Z.

- and we get 2.77 mH, close enough to the claimed 2.7 mH.