Inductance Test Jig

Recently, I was winding some toroids for a low pass filter I was building.  I needed two different values: .28uH and .48uH using T37-6 cores.  These required winding 10 and 13 turns respectively, according to my toroid winding calculator.  Once they were completed I measured their values with my AADE L/C meter.  The “.28uH” toroid measured .36uH while the “.48uH” toroid measured .60uH, so something wasn’t right.  I tried spreading the windings further apart to lower the inductance and that helped a bit but the readings were still too high.  I began to suspect something was up with my meter.  I needed another way to confirm the values so I started looking for a circuit that could be used to measure inductance.  I stumbled across a simple test jig from a YouTube video (w2aew - measuring coil inductance) where the author used a circuit described in a September 1990 article from 73 Magazine.  A picture of the circuit is shown below:

To use, simply insert the part to be tested into the circuit, hook up an oscilloscope to measure the resultant resonant frequency and then do a simple calculation to determine the unknown inductance value.  I altered my circuit slightly by using a jumper for S1 and substituting J310s for the FETs.  The jig was assembled using “ugly construction” techniques on a small copper board.  

Once completed, I tested the jig by inserting an inductor with a known value (marked as 3.9uH) and found that the resonant frequency was 4.848MHz.  

Resonant frequency of 4.8478MHz

The calculated value of L was determined to be 4.07uH.  My L/C meter measured 3.93uH  giving a difference of <4%, well within
acceptable limits for me.  This confirmed that the meter was performing as it should.

So, it appeared that the toroid cores I was using were to blame.  To compensate, I reduced the recommended number of turns by 1 (or 2) and re-checked the readings.  This seemed to do the trick as seen in the table below:

                                               Test Jig (uH)     L/C Meter (uH)
                                                    .299                    .286           
                                                    .294                    .288           
                                                    .489                    .483           

An added feature of this test jig is the ability to measure the resonant frequency of a tuned circuit (like an IF transformer).  Simply remove the jumper to the left of the sockets, insert the device and read the resulting output frequency.

 Admittedly, for general purpose use as a way to determine an unknown inductance, my L/C meter is quicker and more convenient but its much more satisfying to use something you built yourself.