How to create VNA calibration coefficients (delay, C0..C3) by measuring devices? - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 22, 2025, 09:44:39 pm
content: I'd like to "derive" (for lack of the correct term) cal coefficients for a set of "standards" (open, short, load of both genders, so six pieces) which currently have none (and weren't intended as calibration standards.)Is this possible and practical (from a DIY perspective), and if so, how would I go about this? Detail follow:I'm thinking start with a known cal kit (HP 85033 series), measure the devices on (HP 8753 series) VNA, then use the resulting measurements (presumably as S-parameter listings) to generate the delay (and in the case of opens, the polynomial fringing capacitance values C0..C3) that would become the open and short devices' calibration coefficients. At that point, with some added (and for my purposes, inconsequential) added uncertainty, I'd have a second cal kit "derived" from the first. Load standard(s) might be taken as "ideal", given that in the HP 8753 series cal kit specifications, that seems to be the assumption.If this is possible and practical (as a DIY project at home, not involving an outside cal lab or otherwise), what is the process that would take the measurement data (of the new devices, from the 85033-calibrated VNA, possibly as S-parameters) and transform that data into the delay and (in the case of opens) fringing coefficients?If this is not possible and practical, any ideas on what the impediment(s) would be?Thanks for any comments, suggestions, or requests for clarification.

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 28, 2025, 09:11:46 pm
content: It can be done. Measure the standard on a golden VNA and save the S1p file. Measure the S11 delay to the short or the open. Half of this value is the one-way delay. Pull the data into Excel and create a calculated reflection based on the known delay and guessed values for C0 through C3 or L0 through L3. Use the Solver to optimize the coefficients for best fit. That is, you'll create a column which is the squared difference of the real part and the other the imaginary part. Add the sum of both error columns together to get a total error term and minimize it by changing the coefficients. Using Excel to solve multivariate problems is pretty cheesy, but I don't have time to mess around and it works. You may find that the delay term alone is close enough. The effects of the capacitance and inductance are quite small at moderate frequencies.Keep in mind that negative coefficient values are allowed.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 28, 2025, 10:28:46 pm
content: Thank you so much for the approach information!I'm already using the derived cal kit just with delay (that was easy to determine) and a guess for the C0 parameter (the HP 8753 series assumes L0=L1=L2=L3=0.) The cal kit need only work to 3 GHz.I'll get started per your outline as it looks like it will be an outstanding learning experience and provide good utility.It's very likely I'll come back to ask for "what do I do now" next step (the first challenge will be to get S parameters out of the VNA and into a spreadsheet, as it has no floppy or other media and I don't have GPIB attached.)Thanks again!

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 30, 2025, 02:40:53 am
content: Hello Virtualparticles,I'd like get your comments on my first steps towards deriving a cal kit.The experimental test set-up is a HP 8753A VNA with 85044A test set. The "source" (reference) cal kit is an HP 85033C type. The APC-7 connector on the test set is adapted to 3.5mm/SMA female using a Wiltron APC-7/3.5mm female adapter. All connector surfaces were pre-cleaned using isopropyl and lint-free swabs. This is my "golden VNA" as you described in your initial response to my posting.The "destination" (target) of the exercise will be a pair of SMA-male PCB ("vertical" type) connectors. The short "standard" has been shorted with a blob of solder on the pins. The open "standard" has had all pins filed flush and cleaned of metal particles as well as I could.The 8753A was left at default "reset" settings except for the following: cal kit was selected as 3.5mm (the 85033C is one of the factory cal kits available.) The number of points was changed from 201 to 11 (mainly so that I could eventually transcribe a smaller amount of data when time comes to make up the initial spreadsheet.)The 8753A was calibrated S11 using the 85033C standards (the open standards have small center contact extensions that are tedious to use, so that's the principal reason I'd like to derive a less-golden, but mechanically friendlier cal kit!). I then installed my target male cal component and used port extension to find the offset closest to a "dot" on the left (0-ohm) end of the smith chart (that's the photo attached "short-smith.jpg", showing the round-trip delay with the male component installed; the round-trip delay on this component differs from the 85033C male short/open components as expected.)The information of interest at this point would be the round-trip delay with the short target standard installed; I'd expect that the same delay would apply to the open standard as the two male connectors used for the project are identical (until they were made into the short and open standards as described.)With calibration and port extension unchanged, I then replaced the male target with it's female counterpart (that's the photo attached "open-smith.jpg", leaving the delay unchanged.) I then used copy->list to display the 11 test points as R/X components. The 8753A doesn't have a built-in option to display S parameters, but I believe I can convert the R/X components to their corresponding S parameters as needed.I'd expect at this point that I want to measure the artifacts of fringing capacitance of the open standard, with the delay (to the mechanical open point) removed from the S-parameters that I end up curve-fitting C0..C3 coefficients.My question is this - am I getting started in the right direction?Thanks!Dave(edited to replace the three screen images as the original three were incorrect due to calibration procedure error.)

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 30, 2025, 05:36:14 pm
content: Yes, you're headed in the right direction. Keep in mind that the measured S11 delay is round-trip but the cal kit definition is one-way, so you'll need to divide by 2. At the end of the day, all you need is something that mathematically recreates the curve on the screen. The VNA will perform the calculation and compare its results to what it is measuring in order to create the correction vectors.For more on how that calculation is done, see this article by yours truly:https://www.mwrf.com/technologies/test-measurement/article/21152010/copper-mountain-technologies-one-port-vna-calibration-a-look-under-the-hood

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 30, 2025, 05:58:49 pm
content: Thanks - understood re the one-way and two-way delay usageArticle found and archived.If I get the open's delay exactly correct, then is what's left over in the S11 measurements only a result of the residual capacitance I'm trying to model with the C0..C3 parameters? Put another way, does an imperfect delay "taint" the S parameters potentially throwing off the rendering of C0..C3?Is my assumption that the delay (of the short) is (practically) same as delay (of the open), given that both are built on exactly the same underlying component (a male SMA PCB-mount connector)?Is the unit-of-measure of the coefficients C0..C3 something like Farads-per-Hz^N where N is the exponential factor specific to each of the four coefficients (so then if I plug a given frequency in and specify C0, C1, C2 and C3 the result is a capacitance in farads?Thanks again

author: TheSteve
author_rank: Unknown
timestamp: January 30, 2025, 06:31:38 pm
content: My process is to calibrate with a known good cal kit/ECAL and then save the S1P files for the unknown open/short/load. At that point I load the files into Metas VNA Tools and use the builtin function to calculate the parameters(this isn't quite fully automatic as it is good to know some of the values before hand, or have a ballpark idea anyway). If you get values that seem logical you create a cal kit file and test them out.I've posted about some of my adventures here.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 30, 2025, 06:46:45 pm
content: Thanks, Steve - I'm aware of METAS tools but didn't think that it could be used to solve this sort of problem.I'm working now to get registered for download of Metas tools.Metas registration procedure appears to be an e-mail with a few bits of info in the body, so hopefully my hand-crafted version works.As for the C- parameters, I have a pretty close guess as a starting point - C0 = 50, C1 = C2 = C3 = 0. That's what I've plugged in (based on the value of C0 used for the HP 85033C cal kit opens) and even with that guess the "cal kit" appears to behave reasonably.Anyway, thanks for the clue and suggestion - learning (getting started with) the METAS tools should be worthwhile in any event!Dave

author: joeqsmith
author_rank: Super Contributor
timestamp: January 30, 2025, 06:57:16 pm
content: I piggy backed on Steve's thread about METAS. I ran several experiments using synthesized data before using it. It does a fine job and in the end, using a cheap VNA, I could not see much of a difference between the coefficients METAS picked and the actual measured S-parameters for the standards.https://www.eevblog.com/forum/rf-microwave/calculating-correction-data-for-an-unknown-vna-calibration-kit/

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 30, 2025, 08:04:16 pm
content: While waiting for my registration response, I realized I had long ago (2018) downloaded the METAS tools installer but never installed; that is now installed and the machine has version 2.0.6667.33608 running.One little step - converting the R and X listing (posted previously as a photo of the display of the HP 8753A in LIST mode) to S-parameters (discovered on-line OCR tools are completely useless dealing with "green screen" type character display photo, so hand-transcribing into a spreadsheet is next step there.)One big step - how to use METAS Tools for this task??! Found documentation on their website (current version).Pressing on....Thanks joeqsmith, good to know there's user(s) with hands-on experience here!Dave(EDIT - thanks as well for URL pointing to a long discussion 2022-2024 timeframe, looks like it will fill in many blanks as well.)

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 30, 2025, 08:36:42 pm
content: Nice, Thanks Joe and Steve, I was completely unaware of this capability in the METAS tool.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: January 31, 2025, 03:23:39 am
content: Coming up for air (gasp!)I've re-done the cal-and-measure procedure to use 26 points (across the 300 kHz - 3000 MHz range), as this is the amount of data that will fit on one screen (of the VNA) - I'm not equipped to capture the data electronically as I have no GPIB connection (which is the only way HP intended to get data extracted.)My goal is to transcribe this data and convert it into Touchstone S11p format, which I've not done before.Attached are four screen photos and a small spreadsheet I've started. The 8753A was calibrated with 85033C cal kit and then had the "unknown" open attached in lieu of the female cal standards (switched from male cal standards to female cal standards so I could avoid use of a female-female adapter following my 7mm/3.5mm adapter.)26-smith.jpg is the open device in smith chart format26-smith-list.jpg is the smith chart data shown using the 8753 COPY->LIST function26-polar.jpg is the open device in polar format26-polar-list.jpg is the polar format data shown using the 8753 COPY->LIST function26-data.png is print from a spreadsheet on which I transcribed the list data from both display formats.I need help understanding what I'm seeing in the 26-smith-list.jpg (and on the 26-data.png image, the columns called "smith" that are transcribed by hand.)In particular, I'm puzzled by the first S11 column in 26-smith-list.jpg. What is it saying?Any suggestion as to how I can convert either of these to Touchstone S11p format?Thanks!