LCR impedance measurement repeatability (Agilent 4285A)

[mtwieg]

I made a setup to characterize litz wire performance, and am having some issues with measurement repeatability I can't explain. I'm using an Agilent 4285A (uncalibrated, but in good condition).

The DUT for now is a simple air core solenoid wound on an acrylic former. This DUT is just something I threw together to refine the measurement setup/procedure to my satisfaction. My intended frequency range is 100kHz-10MHz, measurement mode is Ls-Rs.

To make the connections from to the DUT as robust as possible, I use the 16047A test fixture instead of the kelvin clips. I also created little adapters boards which fit nicely in the "jaws" of the test fixture (see attached photo). These adapters are made of double sided copper clad, with the bottom/top layers (force/sense terminals) connecting at the far edge, which is also where I solder the litz wire for the DUT.

This setup seems much better than sticking the litz wire directly into the jaws of the 16047A, but I'm still seeing odd variability on the Rs result. For example, I plug in the DUT+adapter, and at f=1MHz I measure Ls=15.98uH +/- 0.05uH, Rs = 0.251 +/- 0.015ohm. But if I then wiggle the adapter a bit in the jaws, the Rs measurement will settle at a very different value, like Rs = 0.331ohm, but measurement noise is still +/- 0.015ohm.

I'm fairly certain the only thing moving is the adapter itself (the DUT coil is mounted rigidly and there's nothing nearby for it to couple to). The contact resistance on the four terminals is probably varying quite a bit, but my understanding is that so long as all four terminals of the instrument make "decent" contact with the adapter (i.e. impedance low enough that it doesn't cause clipping in the current drivers, or cause phase shift in the sense signals), it shouldn't affect the final measurement.

Another explanation is that strands of the litz wire are intermittently breaking as I wiggle the adapter. But if this were the case I would also see it affect DC resistance, and I've verified this isn't the case (used a 4263B for that).

[KungFuJosh]

My guess is that your adapter is at fault, but you should also be using a guard plate beneath the DUT.

If you can use two solid pieces of copper instead of what I'm assuming is copper tape, that might improve the connection quality with the fixture.

If you have good kelvin clips, I would also consider properly adjusting them, and seeing how it looks with that when using a guard plate.

[TimFox]

How did you terminate the two ends of your Litz wire?

[mtwieg]

My guess is that your adapter is at fault

That's my guess too, but not sure why or how to improve it. Except just giving everything a thorough cleaning with IPA and polishing.

but you should also be using a guard plate beneath the DUT.

I had come across this in keysight's impedance measurement handbook. My understanding is this helps reduce the effects of stray capacitance, but if stray capacitance were an issue I would expect that to affect the Ls measurement more than Rs... but a large ground plate will likely interact strongly with the coil's magnetic field. Still worth trying, just to see.

If you can use two solid pieces of copper instead of what I'm assuming is copper tape, that might improve the connection quality with the fixture.

The adapter is made of double sided copper clad. The copper clad is what makes contact with the jaws, and I just use a sliver of copper tape at the other end of the adapter to connect the P and C sides together (also soldered).

I wonder if the large contact area of the adapter is backfiring by reducing the contact pressure, making it less likely to break up oxides on the surface. Going to try cleaning the surfaces, and maybe narrowing the legs of the adapter to reduce the contact area. Wish I could put gold plating on the adapter legs. Maybe should order an actual PCB with ENIG plating...

If you have good kelvin clips, I would also consider properly adjusting them, and seeing how it looks with that when using a guard plate.

I do have a set of 16089B kelvin clip leads, but typically they don't work very well with a DUT like this (unless the clips are mounted somehow). Clipping them to the adapter plate might help though.

I'm not aware of any adjustments I can make to these, aside from maybe replacing the springs.

[mtwieg]

How did you terminate the two ends of your Litz wire?

Pretty carefully. I tinned them with a solder pot and a bit of flux.

The measured DC resistance per length is just about 6% higher than what I expected based on the wire's "equivalent" AWG specification. To validate my tinning process, I cut off the tinned part, re-tinned, and re-measured Rdc several times on the same length of wire, and I get at most 1-2% variation in resistance. And no observable change when bending the wire. So I think that's good.

For attaching the wire to the adapter, I just used the minimal amount of solder and heat. Don't want to disturb the arrangement of the strands (I intend to play more with this later, but not until I get my measurement setup figured out).

[KungFuJosh]

If you can use two solid pieces of copper instead of what I'm assuming is copper tape, that might improve the connection quality with the fixture.

The adapter is made of double sided copper clad. The copper clad is what makes contact with the jaws, and I just use a sliver of copper tape at the other end of the adapter to connect the P and C sides together (also soldered).

I wonder if the large contact area of the adapter is backfiring by reducing the contact pressure, making it less likely to break up oxides on the surface. Going to try cleaning the surfaces, and maybe narrowing the legs of the adapter to reduce the contact area. Wish I could put gold plating on the adapter legs. Maybe should order an actual PCB with ENIG plating...

I've found copper tape unreliable with this type of setup. This is probably partly due to the lack of conductive mass. The short bars are thick for a reason.

I think the PCB with ENIG is a cool idea and worth trying. I'm tempted to make one too just for entertainment value. Might come in handy in the future.

If you have good kelvin clips, I would also consider properly adjusting them, and seeing how it looks with that when using a guard plate.

I do have a set of 16089B kelvin clip leads, but typically they don't work very well with a DUT like this (unless the clips are mounted somehow). Clipping them to the adapter plate might help though.

I'm not aware of any adjustments I can make to these, aside from maybe replacing the springs.

I meant cable/open/short adjustments/corrections in the LCR. I assume it at least has open/short correction. Sometimes it's called adjustments, or correction, or calibration.

Thanks,
Josh

[mawyatt]

Try and use a physically large resistor, with a very high resistance to not effect the measurement. Carefully wrap the Litz wire around the resistor leads near the body and solder, then use the Standard Leaded LCR Meter Fixture for measurement.

Best,