On Tue, Oct 15, 2019 at 03:42 PM, Rune Broberg wrote:
This looks very interesting, Bryan! I am happy that my little piece of
software has turned out to be able to do things like this ;-)
I am a little curious to see Smith chart measurements of your open, load
and short after doing this calibration adjustment - just to see how much it
changes how they look.
I have wondered a little whether allowing a pure resistive value for the
short would also be relevant?
I'm still looking for a good reference for the calculations for how to
compensate for a "load" standard that has a capacitive component: How does
that look? Is it just like the inductive phase shift with the sign
reversed? Could a load capacitance reasonably be represented by a negative
inductance?
--
Rune / 5Q5R
Rune,
Yes, your software can do a lot of things that I seem to need. Thanks, again, for continuing to work on this effort. You and your efforts are greatly appreciated.
I will cover the changes in the open, short, and load separately below. To get the plots shown here, I averaged 3 (or more) copies of 5 scans from 50 kHz to 900 MHz. The total number of frequency samples was therefore 505 samples or about every 1.78 MHz. Averaging is very useful and important in nanoVNASaver, especially when looking at these kinds of measurements, especially above 300 MHz.
First I'll talk about the open.
On the Smith chart plot little change is observable after the tuning is applied because the changes being made represent small changes in the phase of S11. In fact, to see the changes one has to look at the phase of S11 on a scale of about +/- 2 degrees. The S11 phase of the open is shown in the file "S11-Open-Phase-NoTuning.png". The change is such a small change in the phase, it is almost impossible to see anything on the Smith chart. The data is all clustered near the right side of the Smith Chart as it should be after an effective calibration. However, it appears to me there is real data in this phase plot, even after calibration but before applying any tuning to the parameters in the "Calibration standards" form. When I changed C0 in the "Calibration standards" form of nanoVNA-Saver to 1200 and the offset delay to -55 ps. This change produced the phase plot in the file "S11-Open-Phase-with-Tuning.png". Notice two things about the plot after tuning. 1) There is no "quadratic looking" drop in the phase as we move from low frequencies to higher frequencies as there was before tuning. 2) Although I did not do this, a linear fit to the data in the phase plot after tuning will be a horizontal line near zero degrees. This is the definition of an ideal open. Thus, while you cannot readily see such a small change on a Smith chart, there is some significance to the changes because we removed frequency dependent artifacts of the open I used in the calibration process. Please also notice from the amplitude charts in the first post in this series show some improvement in the trend of S11 in dB when measuring the open RG-213 cable. This is not very compelling, but read on.
Now let's talk about the short.
The short I used produced a nice dot on the left side of the Smith Chart after the normal calibration; however, there was significant ripple remaining in the S11 amplitude as shown in the plot below "RG-213-WithOpenOnlyTuning.png" I suspected, but did not know that could be the fault of the inductance in the short that was used in the calibration. By modifying the L0 parameter in the "Calibrations standards" form we can remove the inductance that may be present in the short I used. The resulting S11 amplitude plot is attached to the first post above and called "S11-RG213-WithCalTuning.png". Also, please refer to my post to Kurt in this thread that shows the vastly improved Smith chart for the RG-213 cable. The circles are now circles and they are much more closely centered on 50 ohms.
So, now to your question: What does the actual short used in the original calibration look like on the Smith chart once the L0 correction for the Short is inserted? The result is shown in the figure "Smith-OriginalShort-WithCorrections.png". This result should be exactly like a 1.2 nH inductor which is what I tried to remove using the L0 term. In fact, nanoVNA-Saver confirms this inductance after this correction is applied by indicating 1.2 nH is the "parallel X" when measuring the actual short. When the VNA is properly calibrated for a short, it shows the actual inductance of the physical device I used, equivalent to 1.2 nH.
In the case of my BNC short, I don't think I have the ability to reliably measure a resistance that small. The ohmmeter I have won't go lower than 0.05 ohms even when shorting the test leads together. When measuring the short I used, it still measures 0.05 ohms. I doubt that many folks doing hobbyist work can measure the resistance of their short either. Such small resistance values have little impact in a 50 ohm system. Therefore, I don't think there is benefit to including a resistor as a part of the cal correction for a short.
A discussion of the load follows.
I won't go into the details about the tuning I did for the load. A similar procedure to what was used for the open was used for the load. Here, again, averaging a lot of sweeps is important in order to measure the trends in the phase of a signal with a return loss of some 40 to 50 dB. In order to flatten the S11 phase of the termination I used for calibration I did need to add a very small amount of inductance (50 e-12) in the calibration form. This is the equivalent of trying to remove 0.05 nH, a small amount indeed. At 900 MHz that is a reactance of about 0.28 ohms. Certainly not huge in a 50 ohm system. Of course, this small change is unobservable on a Smith chart using the nanoVNA but it is clearly visible on the S11 phase plot.
Long papers have been written on all of the possible issues with trying to model various loads and what to do to compensate for various types of loads at various frequencies. I think there are too many topologies to deal with in this type of software. The capacitance can be in series with the 50 ohm resistor or it can be in parallel or there might be capacitance in series and in parallel.... The same can be said of the inductance. In my opinion, you could leave what you have in place and let folks work with it so long as you allow negative numbers everywhere. Then folks can think about things in whatever way they must as long as you are clear about what you are doing in the software with the non-ideal values entered in the various boxes of the "Calibration standards" box.
By-the-way - When I reset some of the values in the Calibration standards box to zero after putting in and applying values, nanoVNA-Saver 0.1.2 immediately crashes. You might want to look into this. I am sure it is reproducible. Right now, I don't recall which ones do it.
I hope this helps answer your questions.
Again, thank you Rune for making the nanoVNA-Saver software available. I find it particularly valuable and useful.
--
Bryan, WA5VAH
