I thought it might be interesting to start a post dealing with how to accurately measure components like inductors, capacitors and resistors on a NanoVNA. As an RF engineer I have made mistakes over the years using trial and error methods. By reading technical books/articles and posts by from others skilled in this area I learned about some of the pitfalls one can easily make and how to avoid them. So I hope some of you will jump in and share your tips and knowledge.
It is easy to make a mistake or draw a false conclusion about a component's characteristics when using a VNA. This can be due to many factors including the following:
- limitations of the NanoVNA hardware and software
- limitations of the test jig
- quality of calibration load and calibration method
- excess lead length
- misconceptions about the component under test
- insufficient technical knowledge
Rather than discuss each of these individually it might be more interesting to do some tests on actual components and point out the pitfalls and things to watch out for when making these measurements.
Here is a test I did on a 47 pF SMD capacitor using a NanoVNA-H4 with the DisLord 1.0.45 firmware. The PC software used was the NanoVNA app by OneOfEleven. Both of these individuals have done an excellent job developing this software and thanks for sharing it with the user community.
The test fixture used was a female SMA connector with a small header row attached. Cal loads were made using male pins and a SMD 49.9 ohm cal load. The idea originally came from a post by Owen Duffy on his blog. A annotated photo of the setup is attached.
A sweep from .05 to 900 MHz. was done and the reactance plotted (graph attached). A graph converting the reactance to "apparent capacitance" is also shown. A pitfall made by those new to VNA's is to assume this is the actual capacitance vs. frequency of the part. This is not the case because a physical capacitor also has some inductance associated with it as shown in the simplified capacitor model. This results in positive inductive reactance adding to the negative capacitive reactance and a faster rise in total reactance with frequency than just the capacitor alone. In the graph below note the rapid reactance increase with frequency as we get close to the self-resonant frequency (SRF) of the capacitor.
Now one might think it is reasonable to measure at a very low frequency in order to get the actual capacitance but this is not possible because the capacitance reactance is so high at low frequencies that it cannot be actually measured by the VNA. By looking at the markers you can see that we can only start making accurate measurements around 2 MHz. for this part. Another instrument in my lab a DE-5000 can make these measurements at low frequencies (below 100 kHz.) and the part measures close to 47 pF in this range. The frequency at which accurate measurements can be made will be based on the capacitance of the device under test. Larger capacitors can be measured at a lower frequency and smaller ones like 10 pF at a higher frequency.
If we wish to determine the frequency of self resonance we can look for the frequency where the reactance is zero. This is when the capacitive reactance and inductive reactance are equal and opposite in sign and result in zero at the SRF. This is also the frequency where the S11 phase angle abruptly changes from - degrees to + degrees for this example. For those familiar with Smith charts this is a data point on the horizontal line of the Smith chart.
Roger
Roger Need
