Thursday, 5 January 2017

Measuring inductors from 10nH to 10uH

Here is a test fixture I use to measure small, down to nH size, inductors. I got the idea from the book Solid State Design.

Provided your power meter shows a loss of at least 20dB with no inductor connected then the capacitor between the bnc connectors and the floating pcb are small enough in value. When Cg is small then the peak detected is more pronounced and the measurement of Q is more accurate.

In my implementation I use 2 bnc sockets from a panel which are connected to a small piece of blank PCB supported above the ground plane.I glued the floating PCB down with some plastic spacers between it and the PCB underneath. My reasoning was this would reduce the impact of temperature variations on the capacitance by putting an air dielectric between the substrate and the groundplane. I haven't determined if it really makes any practical difference so you might find this unnecessary.

Each bnc connector is connected to this supported pcb via a gimmick capacitor. I used a 2cm or so length of miniature coax. I should have built this with the inner to the bnc connector, the shield to the floating pcb. You can see from the photo below I got one of the small pieces of coax transposed. You could use a gimmick capacitor from twisted wire. If you were using a larger known capacitor then you might find a really small value disc ceramic is suitable.

From this pcb to ground I have a nominal 100pf capacitor. The value isn't critical since you can measure the actual capacitance with a capacitance bridge or meter. I use my digital LC meter which appears suitable for measuring C but I can't rely on it when measuring L since it works at low frequencies. If you can't measure capacitance then use a tight tolerance capacitor and assume a few pf extra.

When I checked with my capacitance bridge I had 109pf from the supported pcb to ground. That forms the C in the well known f = 1 / (2PI sqrt(LC)) equation. I then sweep with my signal generator on one port, and my power meter on the other. At the peak response I solve for L. This allows me to measure nH inductors with ease. Did you know that capacitance is essentially constant with frequency whereas inductance changes with frequency? Measuring an inductor at 1kHz  gets you close, but I prefer measurements at RF frequencies.

By sweeping the frequency to find the points 3dB down I can estimate the Q! The unloaded Q is equal to the resonant frequency divided by the difference between the two frequencies where the response is 3dB down. I will post a separate blog soon about this aspect.

For inductors of 800nH and upwards I have an alternative instrument I can use which measures the capacitance at several megahertz using a clock module. It works well despite only providing a subjective measurement of Q.

The fixture described does work really well and since I strive to build things that do not need tweaking it reduces the uncertainty around measured inductance.

Regards
Richard

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