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Hi Bryan

Congratulation. Tweeking parameters in NanoVNA-saver is great fun. You missed to show us a sweep of the coax cable after tuning to celebrate the victory ?

I think there is a misspelling in so far you used F for the inductance and H for the capacitance, anyway it does not matter it is the numbers and the sign in front which provide the results.

Kind regards

Kurt



-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af bryburns via Groups.Io
Sendt: 15. oktober 2019 23:28
Til: [email protected]
Emne: [nanovna-users] Am I Fixing my BNC Calibration using Calibration Standards Adjustments in nanoVNA-Saver 0.1.2



Folks,



Because of convenience and several other factors I usually use my nanoVNA through SMA to female BNC connectors. As a result, I calibrate both the nanoVNA and nanoVNA-Saver through the SMA to BNC connectors. One reason I do this is to avoid changing the connector mating to the SMA connectors on the nanoVNA.



I realize that BNC calibration kits are notoriously poor. Mine is no exception. The open I use is the female BNC to SMA adapter that is connected directly to the nanoVNA. Also, the BNC short that I use is likely to have some inductance because there is no insulation around the shorting pin. And, the shorting plane cannot be very close to the center pin of the female BNC even when it is properly connected to the mating connector. The 50-ohm load I am using is 50.0 ohms at DC based on a precision ohmmeter measurement. It does compare quite favorably with other terminations that are rated up to 3 GHz when measured on wider bandwidth VNAs.



With this cal kit, and doing both the nanoVNA calibration and the nanoVNA-Saver calibration, I get a nice clean dot in all of the right places. The short does register on the left side of the Smith chart, the load in the center and the open at the far right side as it should be after the OSLT calibration. However, we know that issues remain. The big question is: How to fix them or determine what errors may exist?



Some of the issues can be most easily observed by connecting a fairly short length of good, low-loss 50-ohm coax on the CH0 port and observing S11 on a Smith chart or looking at the return loss in dB over the frequency range from 50 kHz to 900 MHz. The cable used in the measurements shown in the attachments is a 20" (~0.5m) length of RG213 which has SMA connectors on both ends. It was connected to the BNC through a male-male BNC adapter and a female-BNC to female SMA connector. Yes, this is maximum adapter usage to connect the SMA cable to the SMA connector on the nanoVNA. But, the task here is to determine what is going on with the BNC calibration. My experience says that at frequencies below 900 MHz all of the connectors I am using should be pretty good. The first 2 plots I have attached show the result. I also used a commercially manufactured (surplus) piece of RG-233 with BNC male connectors with very similar results. So, this is definitely ans issue and most likely with the calibration.



Taking a careful look at the figure below contained in "Smith-RG213-WithoutCalTuning.png" I observe at least a few problems. 1) The plot should stay within the Smith chart but does not. 2) The circles which should proceed around the Smith chart in a counterclockwise direction are a) not circles and b) not centered on 50 ohms. Similar issues are observed when looking at the plot in the file "S11-RG213-WithoutCalTuning.png". Clearly their should be no positive values in the plot but there are. Furthermore, there is significant ripple in the plot caused by issues that are not likely the short cable.



So, I asked myself the question: "Can I manipulate the "Calibration standards" parameters in the nanoVNA-Saver application to fix what is wrong with the calibration?" I hasten to point out that I do not have access to high-quality measurement equipment and therefore I am left to figure out what is lacking in my BNC calibration kit. Here is what I did.



I began to try first of all to fix the open calibration by continuing to observe the Smith chart as I changed the "open" parameters C0 and the offset delay. Also by blowing up the scale on S11 phase I could see that using the BNC female as an open there remained a negative going and somewhat quadratically increasing phase ramp at frequencies above 500 MHz which could be due to the capacitance in the BNC female I was using as an open. I found that I could get rid of this negative going and quadratically increasing phase ramp by setting the "Open C0" value to 1100 e-15 H. With this setting the "hook" was removed and a linear phase trend remained. However, with that correction in place, the entire phase had a linear phase ramp. Adjusting the time-delay offset to -55 ps I found that I could then level the phase and get less than 0.4 degrees of phase deviation from zero degrees all the way from 50 kHz to 900 MHz. This seemed to me to be a big improvement. Clearly this is an empirical result but it could be a real description of the difference between a female BNC connector and a real open circuit. After these two adjustments the curve shown in "S11-RG213-WithOpenOnlyTuning.png" was collected. This is an improvement in that the trend in the S11 data is now approximately correct; however, large ripples remain,



During the adjustments of the open parameters, I continually looked at S11 in dB and S11 phase to make sure that what I was adjusting was making the phase more constant over frequency and S11 amplitude move in a correct direction (downward at the high end of the sweep.)



I then began to adjust the tuning parameter "L0" for a short. After a few guesses, I arrived at an adjustment for the short parameter of 1200 e-12 F. Said another way, this is like 1.2 nH of inductance is in the short I am using when compared to a real short. This is plausible for the short which is clearly not at the reference point which I consider to be the end of the center conductor for the female BNC center pin of the adapter to SMA. After this additional adjustment I created the chart shown below as "S11-RG213-WithCalTuning.png". I observe the following 1) There are no longer positive values - a definite improvement. 2) The trend is fairly monotonic out to 900 MHz but not perfect. 3) I haven't included the data for this, but the return loss out to at least 300 MHz is now almost exactly (within <0.02 dB) twice the measured S21 loss of the RG213 cable. Before the adjustments listed here, this was clearly not the case. Because there seems to be a limitation of 4 attachments in this forum, I did not include the S11 Smith chart below; however, the plot is now completely within the Smith chart, the circles get smaller as the number of times around the Smith chart increases, and everything is much more nearly centered on the Smith chart. These are all what I expect for this fairly simple measurement. I can put the Smith chart in a separate post if there is interest in it.



Is this really an improvement or am I kidding myself by tweeking parameters to improve the results of this simple measurement? My opinion is that it is a definite improvement and an approach that other may want to explore. I welcome your comments.



--

Bryan, WA5VAH