NanoVNA Test Fixture - Details

Here is a picture of the fixture I am using. I had allowed for several fixture styles. At present I am using two of the fixtures, both single sided. The unused fixtures are double sided but I may never get around to trying them. The results to date suggest single sided is more than adequate.

The 50 ohm chip resistor fixture is shown on the top right hand corner. A tricky soldering job since one pad is underneath, and the other wraps around. Clearly too much solder but the OSL calibration process is hopefully negating the additional stray capacitance that blob is contributing to the measurements. Not obvious is the manual removal of the ground trace that runs under the chip resistor by scratching it off.

To calibrate the fixture I use the chip resistor version. As shown it is open circuit. I use a piece of foil squashed onto the SMA pins with my thumb as the short. The piece of foil is used to connect the pad adjacent to the ground side of the 50 ohm resistor to the untinned areas as the load calibration.

I then use the same calibration settings for both fixtures. Using the calibration for the chip resistor version with the non-chip resistor version has proven to be acceptable.

To use the same fixture without the resistor I sweat soldered a piece of tinned copper foil as shown. The SMD then is placed where the pad for the chip resistor was used to connect to the center pin of the SMA connector. That tiny chip inductor is I think 0201. It was the smallest physical and electrical value I have tested to date, with good results. But more on that later.

I hold the test device in place by clamping it with a wooden clothes peg. Sometimes  I use a piece of bare fiberglass, or other benign material like the fingernail on my thumb, and I have found no material difference in readings by doing this.

Experience with this fixture suggest I should have made the pad slightly larger to more easily accommodate some of the SMD inductors just larger than 1206 size parts. And maybe shaped the ground pad to be closer to make placing the 0201 parts a little easier.