NanoVNA - Inductor Measurements

Starting with a 1.2uH inductor results were immediately satisfying. The measured L and variation with frequency matched the curve shown in the TDK datasheet. And the self resonant frequency was readily measured as 180MHz.

1.2uH Inductor

Let's try an unknown but small 0603 wire-wound chip inductor. Measured as 22nH.

How small can we go? I have some 12nH 0201 chip inductors with factory measurements (sample size, variance etc). Wow. 12nH measured.


Previously I have used a signal generator and test fixture with a measured parallel capacitance
( 112pF from memory ) to measure the resonant LC frequency, then back calculate what L was. This worked very well down to nH sized inductors. With a power detector I was also able to estimate Q by sweeping around the resonant frequency for the -3dB points.

The NanoVNA is now my preferred approach. I prefer the fixture without the 50 ohm resistor. The curves are easier to see and therefore interpret. Either fixture is fast to use and easy to see how L changes with frequency. I can't see how Q can be measured, but A/B comparisons show slight differences. The higher the inductor Q the closer to the edge of the Smith Chart. 

Go on, give it a try. 

NanoVAN Test Fixture - Capacitor results

Here I present two results: measuring a supposedly high Q 110pF capacitor with and without the 50 ohm chip resistor in series.

110pF capacitor in series with 50 ohm chip resistor

Immediately some questions are posed, and some pleasing results. VNA suggest 110pF capacitor is actually 112pF. This is true over most of the HF range but by 54MHz the effective capacitance has already fallen to 109pF. It gets progressively worse after that. Worth knowing in the design of filters and matching networks.

Overall the fixture with a 50 ohm series resistor appears useful.

110 pF capacitor with no series resistor

Keen readers will notice that the 110pF capacitor, tested in this fixture, measured 110pF, not 112pF. That could be because I lost the first capacitor in the shed floor. Sorry, but the time taken to present these results didn't warrant re-measuring the capacitor in the first fixture. ( Seconds to measure and view on the nanoVNA screen, many minutes to collate and present. )

What is interesting is that while the nanoVNA display showed a trace around the edge of the Smith Chart, the software did not. Is this evidence that at the extremes there are errors as data is transferred and manipulated to re-display the trace, or underlying accuracy issues with the NanoVNA? Loss of one too many decimal places? The S2P file I exported with "nanoVNA partner" went to 9 decimal places, however that does not mean the software worked at that level of precision. Displaying answers to 4 decimal places is meaningless without knowing the precision of the underlying calculations.



Viewed on the nanoVNA it suggested by the software the capacitor is useful up to 2m before it departs from being a 110pF capacitor.

At this point both test fixtures have, within reason, results and findings consistent with each other.

I then examined in turn capacitors of 100nF, 10nF, 1.5nF and 150pF. The nanoVNA screen clearly showed where these capacitors departed from being the stated value and confirmed my understanding of why the value and type of decoupling capacitor requires thought. Unfortunately, with no series resistor once the readings are transferred to the PC the data translation/transfer/rounding issues prevent a meaningful display of the results when the S parameter file is later opened with "nanoVNA partner".

However, all is not lost.  The exported S2P file, viewed in RFSIM99, is a close representation of what the nanoVNA screen was showing.

100nF capacitor, no series resistor

The 100nF 1206 capacitor measured 93nF at 50kHz. At 9MHz it was already a 1nH inductor. Not what I expected from a 1206 part. A trip to the workshop to check. Indeed, this seems correct. While the 50 ohm resistor fixture showed the transition was at 18MHz, the difference between the two is within the accuracy I wanted.

For decoupling purposes the series impedance at 36MHz is 0.05R+j0.65R. Above that the decoupling gets worse.

How does 10nF look? A 10nF 1206 "recovered" capacitor was tested. The series impedance at 72MHz is 0.09R+j0.63R, getting worse as frequency rose beyond that.

The 1.5nF capacitor demonstrated why two values of decoupling capacitor are sometimes shown. It would be poor at decoupling until the frequency rose to 72MHz. Above 300MHz the response was erratic so this needs checking.

The 150pF capacitor was unaffected by parasitics until the frequency rose to 234MHz.

I overlaid the results of 3 39pF capacitors of different construction, mica clad, 1206 ceramic and a SMD mica? film capacitor. There was no real difference until the frequencies rose above 300MHz. After this all displayed a similar erratic pattern though it was shifted.

3 39pF capacitors of different construction.

Advantages of 50 ohm test fixture

There is some suggestion in published papers that this fixture should have greater accuracy. I see nothing in the results to date to support this. Perhaps my test fixture for SMD parts was better than hte authors test fixture.

In any event, since you have to build one of these to calibrate the other fixture ( OSL ) then for now I put aside the "accuracy" debate. 

Advantages of the test fixture without 50 ohms

A/B comparisons of decoupling applications at spot frequencies gets easy since for the same frequency A is worse than B if it is further away from the 0+j0 point of the Smith Chart.

Conclusion to date

Either fixture appears useful at characterizing capacitors down to very small values. The lowest I tested was 1.5pF and the results gave me confidence in the value and presence of parasitics.
Accuracy appears degraded above 300MHz but for my purposes will still be adequate.

For A/B comparisons I would use the test fixture without the series resistor since the deviation is easier to spot on the small display.

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.

NanaVNA - Test fixture for measuring passives

Recently I've been exploring what the NanoVNA can do. It really is an impressive piece of test equipment for such a small sum. There has been lots of discussion about measuring passives at HF but I was troubled by the lack of evidence that some commentators had actually done what they advocated. Just like fake Chinese transistors, too many online experts write on the basis of what they have read, not what they have done.

And there was very little online information I could find about using the NanoVNA to measure very small values of passive components. What information I could find was at times inconsistent. So I set out to discover for myself what worked. Over the next few weeks I'll post what I found.

But first the starting point has to be the text fixture. I had a PCB house fabricate a few boards so I could experiment. A variety of single and doubled sided layouts were contemplated, some would use a 50ohm chip resistor in series with the passive being tested, some would simply place the passive component across the SMA.

My biggest challenge was presenting the test results. Taking a picture of the screen was impossible given the screen reflections.  "nanoVNA partner" is a small download and worked well on both XP and Windows 10. I recommend it.

As far as using the NanoVNA, at least one published paper would appear to advocate a different approach to what I found. Could it be the NanoVNA was better than the equipment used in that published paper?