Out of Control SOLT Calibration Study - What I’ve Been Doing For Past Few Months - Page 1
Forum section: electronics / general

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 02, 2025, 04:52:12 pm
content: Original Plan:I wanted to evaluate the typical error of common no-name SOLT calibration kits on the market, I purchased one set of self-characterized kits from a vendor and several sets of no-brand kits. Initially, I just wanted to do a simple comparison and publish the calibrated parameters online to help users of these no-brand kits calibrate their network analyzers.What Actually Happened:1. During the first experiment, I found the data was inconsistent. I eventually traced it to poor contact on the VNA port - possibly non-compliant or damaged. So, I completely disassembled the VNA and replaced the connectors to continue testing.2. However, since the VNA had been disassembled, hidden faults might now affect data reliability. Also, its frequency limit of 4 GHz couldn't cover the entire sub-6 GHz range I needed. I bought a 6 GHz analyzer for the experiment.3. Realizing that non-compliant connectors might damage equipment (likely the cause of the previous instrument failure), I purchased a connector interface gauge. I studied IEEE, IEC, MIL-STD, HP, and Maury standards, compared tolerances across 8 standards, identified “safe” parameter values, and posted a tech memo.4. During the experiments, I discovered a potential algorithm called SDDL that can derive SOLT parameters without a full SOLT calibration - allowing open standard self-calibration. To verify its accuracy, I bought a used Agilent 85033E for comparison.5. To simplify parameter extraction from no-brand calibration kits, I developed a tool called EasySOLT that uses equivalent circuit fitting, allowing users to characterize unknown SOLT kits using a calibrated VNA.6. After fitting experimental data with EasySOLT, I realized equivalent capacitance and electrical length were nearly indistinguishable, producing many physically meaningless fits. For better rigor and physical interpretability, I decided to fix the reactance based on theoretical capacitance values and only fit the electrical length.7. While reviewing literature, I found conflicting methods for calculating the equivalent short standard. After comparing four papers and trial-and-error, I finally identified the correct method.8. I then looked for ways to calculate the parasitic capacitance of the open standard. I initially spent three days struggling with a classic paper that used a Bessel-Neumann function in a numerical solution - only to discover I had misunderstood a math table. Eventually, I got usable results, but discrepancies remained. I switched to a simpler polynomial fit from another paper, but results were still incorrect. After reviewing more papers by the same author, I found a printing error - one parameter was off by a decimal. Fixing that gave me the correct theoretical value.9. In the process, I realized that some “no-brand” open standards with lengthened shield body were not just arbitrarily made - they were actually based on classical electromagnetic theory of “coaxial-to-below-cutoff circular waveguide transitions” (from MIT’s Rad Lab), which once served as a 7 mm metrology standard and still studied in material science today as a dielectric measurement probe. This greatly boosted my confidence.10. I added the open and short models to EasySOLT and was ready to wrap up - until I noticed the model failed to fit. The theoretical and experimental open capacitance values were wildly inconsistent.11. Comparing various other papers confirmed my theoretical value was correct. So I turned my attention to the possibility that real-world open standards include non-ideal physical features (like hollowness, sharp pins, or step changes) not present in ideal coaxial opens.12. I wrote my own simplified connector simulation script based on openEMS for full-wave simulation.13. But soon, I encountered anomalous results. After a week of debugging, I found a bug in openEMS. As one of its core contributors, I fixed the bug and continued modeling.14. The simulation results varied inexplicably with different input parameters. I suspected I had misread the mechanical drawings in the standard. So I decided to verify with real hardware. Since the end of microwave engineering is always mechanical engineering, I bought a vise and destructively disassembled the cheapest SMA through-connector I could find. I also bought a contact micrometer and 1-micron pin gauges to measure its mechanical dimensions. It turned out my modeling was correct, and I had read the drawings right. Incidentally, I also discovered why these cheap through-connectors often jam and damage ports. While I had long distrusted these flange-less, torque-wrench-incompatible, easy-to-jam adapters, I finally learned the root cause: one end of the adapter was 0.05 mm below the standard diameter, in violating to spec - so much so that even the connector interface gauge couldn’t enter. Only a pin gauge could measure it. This was likely intentional, increasing friction to prevent both ends from rotating simultaneously.15. Because I had been using trial and error, it was hard to identify causes. So I redesigned the experiment systematically, ran dozens of simulations to compare parameter effects, and confirmed the simulation was accurate and close to literature values. The likely problem was parasitic effects from connectors.16. The new direction was to independently determine the open capacitance using an S21 resonance-based method found in literature, which simulation also validated.17. Wanting a reliable, independently verifiable method, I spent a week writing and debugging a Monte Carlo simulation script to calculate the error bounds. To my surprise, the method was highly sensitive to uncertainty - under worst-case assumptions, the experimental results were meaningless.18. I purchased a large variety of coaxial cables, connectors, and adapters to assess real-world error contributions. If results were consistent across different setups, it would mean real-world errors weren't that significant.19. While evaluating S21, I used this chance to apply the SDDL algorithm to measure open capacitance and reassess SDDL. The results were completely invalid - but only for open capacitance. Measuring DUTs, including calibration kits, worked fine. This strongly suggests that connected vs. unconnected states of the port introduce different parasitic effects.20. I then replaced the cheap SMA adapters with 3.5 mm ones and re-ran the initial experiment. Sure enough, results shifted significantly - much closer to theoretical values, but still off by as much as -40%. This further confirmed the critical role of connector parasitics.21. The research now shifted toward connector parasitic parameters. I placed a high-priced order for a Precision Slotless Connector (PSC) version of a 3.5 mm connector to continue experiments - still waiting for delivery.22. At the same time, I'm preparing to construct a simple air-line structure to test whether inserting the center pin causes mechanical and parasitic changes. This is to validate the hypothesis that “the parasitic parameters of a connected port (with pin) vs. a disconnected port (without pin) are not time-invariant.”23. By this point, I’ve read dozens of papers published on this topic, from WW2 to the 21st century, including both standard results and niche little-known niche papers.24. While writing this post, I suddenly realized the SDDL algorithm isn’t just usable in short-circuit mode (SDsDsL), but also in open-circuit mode (SDoDoL). By using a short and load to determine zero-length data at the port, and then two delay lines to derive delayed open data (Do1, Do2), I can keep the instrument port always connected and the open port always disconnected - possibly allowing error states to remain consistent. This might be a breakthrough for isolating center pin effects.What started as a simple personal project to measure and publish some parameters has ballooned in complexity, budget, and workload. So far, I've spent over $3000 - and may now be one of the world’s top experts on the seemingly utterly useless subject of “a coax connected to nothing.”Ironically, I never solved the original “simple” question. Instead, I’ve concluded: the open capacitance of no-brand calibration kits is heavily affected by the connector itself, and no strict calibration or analysis can be performed until the center pin insertion effect is fully understood. This might also explain why no commercial 3.5 mm or SMA open standards use circular waveguides as their open standard. (I even found a suspected early Maury Microwave open standard on eBay from the 1980s, which required manual pin insertion - possibly to avoid operation without a center pin, for the same reason identified by myself)Though costly in time and money, my work might one day make it feasible to make $5 calibration kits (by using open standards with pins, standardizing on one adapter model), potentially saving countless amateur electronics enthusiasts $3000 per capita - and maybe even phasing out mid-range calibration kits from the amateur RF scene. This research still holds great significance.---Wish me luck. If successful, I'll publish all my findings in a series of application notes, perhaps becoming the AN47 of amateur radio VNA measurements.

author: shabaz
author_rank: Frequent Contributor
timestamp: August 02, 2025, 05:59:29 pm
content: QuoteFor anyone wanting low-cost kits up to a few GHz, there are already cheap cal kits for under $100, for instancehttps://www.sdr-kits.net/Female-12-GHz-Kitis ballpark $80 where they have already done pretty much what you're proposing, which is to use standard off-the-shelf connectors/adapters, to reduce cost.It's not $5 since they need to turn a bit of profit, but it's not excessively high either.The people who would spend $3000 on calibration standards might not be all that interested in the $5 (or $80) kits anyway, because they are paying the $3000 for better-constructed standards with manufacturer-measured coefficients which the manufacturer stands by along with all the other benefits of the brand, and they don't get any of that for $5 or $80.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 02, 2025, 06:15:53 pm
content: Quote from: shabaz on August 02, 2025, 05:59:29 pmI wouldn't even start this project if the SDR-kit was available. Its status is "Sorry, currently out of stock" for months, before I even began. It's still showing "out of stock" right now. Otherwise I'll certainly order a set for cross-validation. I wanted to get Kirkby Microwave's kit for cross-validation too, but unfortunately Kirkby also stopped selling them in his one-man operation.QuoteSee my effort as a form of "independent replication" or "peer review".I'd like to point out that the characterization of this SDR-Kit kit is based on the research and measurement done by Kurt Poulsen OZ7OU (who published extensively on SOLT kit characterization, which I, of course, had read them all). But during my experiments, the technical soundness of the fringe capacitance parameter of the radiating open standard (1.35 ps delay) has been called into question. This is still pending further investigation.To show that I actually did my homework, this is the story behind SDR-Kit's equivalent 1.35 ps electrical delay. Basically, OZ7OU built his own VNA called VNWA, which needs SOL calkits to work. So over the years, OZ7OU kept refining his SOL characterization using better and better techniques.1. An early was documented in a PDF called "How to measure on SOL calibration kits_REV1.pdf (https://www.hamcom.dk/VNWA/How%20to%20measure%20on%20SOL%20calibration%20kits_REV1.pdf)", published in 2010. But one can clearly see some technical details are incorrect, for example it incorrectly claimed that the reference plane of the SMA pin connector is located at the tip of the pin, which is incorrect. In reality both 3.5 mm and SMA connector's reference plane are at the tip of the hole of the SMA socket, and at the bottom of the center pin (at the plane of the PTFE pielectric for SMA). This is documented in IEEE Std 287:2007 and IEC 61169-15:2021. You can also see that at this time, OZ7OU was relying on Heuermann's rule-of-thumb data of cheap Rosenberger adapters on Heuermann's company website (https://web.archive.org/web/20160328154953/http://www.hhft.de/index.php?page=competences&subpage=calibration). An attempt was made to shift the reference plane based on the adapter's physical dimensions from the center of the Thru adapter (because the Thru length was not de-embedded, so this is only an approximate assumption). But these were only good-enough measurements, not traceable to any calkit. The reference plane was also not rigorously-defined. You can also see that this limitation was carried over to other early measurement data of some characterized adapters.2. OZ7OU's next major step-up was characterizing unknown adapters using a R&S VNA calibrated with an E-Cal unit. This effort was partially documented in "How to derive the coefficients for an Open and Short Calibration standards.pdf" (https://www.hamcom.dk/VNWA/How%20to%20derive%20the%20coefficients%20for%20an%20Open%20and%20Short%20Calibration%20standards.pdf). Now it's based on actual measurement data, and it's using the standard numerical model (offset transmission line with polynomial fit). Well done. However, OZ7OU later said some measurement was not entire satisfactory.3. OZ7OU's third upgrade was the purchase of a Keysight HP 85033C calkit, in the document (How to use and verify HP35033C.pdf,https://www.hamcom.dk/VNWA/How%20to%20use%20and%20verify%20HP35033C.pdf). This was in around 2014. This measurement was the most satisfactory one, and reportedly improved upon previous characterizations and numerical values, so it has became the basis of all SDR-Kits offering. This is where the 1.35 ps originated from: OZ7OU plugged the APC-7 to 3.5 mm adapter on his test fixture, measured the Keysight SOL standards, then removed the standard to check the naked port (or with an empty thru with PTFE or pins, forming a coaxial waveguide beyond cutoff for repeatability). He found the phase shift was an electrical delay of 1.35 ps. This became apparently the accepted reference value of fringe capacitance by SDR-Kit's calkit, regardless of the adapters used.My current work is based upon OZ7OU's existing research (and many other published articles in the literature and online), and aimed to improve it, either in accuracy, scope, or replication. I was not joking when I said I've read everything "from the end of WW2 to the 21th century."For example, my opinion differs on the capacitance: 1.35 ps is only the effective value measured on a 3.5 mm air adapter, and the extrapolation to other SMA adapters need some tests and justification, which would make it more complete and rigorous. In fact, my tests have showed that this value can be replicated with 3.5 mm air adapters, but it's considerably lower on SMA connectors. The origin of this step discontinuity is one focus of my current experiment, using the method that I listed above.QuoteI don't plan to make or sell anything, my plan is simply to publish some reference data that would allow others to purchase $5 off-the-shelf DUTs as their own better-than-assumed-ideal standards.QuoteI explicitly said it's for "amateur radio", not metrology. Also, the $3000 to $5 number refers to the cost of equipment needed to characterize unknown $5 DUTs, it's not meant to replace $3000 calkits for $5 ones to those who *really* needs them.The idea to find a middle ground. The characterized no-name adapters will not be traceable of course, but the idea is that since their physical constructions should be largely consistent, we can settle on some DUTs that can be demonstrated to be consistent enough. Once measured by HP 85033/85052, the fitted parasitics coefficients would be accurate within a few degrees at 6 GHz for all common parts, which is an upgrade from completely unknown standards.I plan to publish raw S-parameters and fitted coefficients on GitHub. All commonly-used no-name SOL adapters that one can find on eBay will be included. The idea is to create a community-maintained database, characterized by someone else (myself currently) using known SOL calkits, so these unknown standards would perform better in comparison to the ideal assumption if those are what all you have. Anyone who questions the accuracy or stability of the published coefficient is free to challenge these numbers by contributing measurement data using a calibrated VNA, so the data would be improved for everyone.Quote$3000 is sometimes (often?) not enough to get individually-characterized calkits. Many calkits have a single-set of standard parameters, guaranteed-by-design, and they already cost that much.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 03, 2025, 05:12:01 pm
content: Using S-parameters to characterize some cheap standards vs using METAS to derive the coefficients for them, gained me nothing. I was splitting hairs with the standards where the real limiting factor in my case was the VNA.https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5656063/#msg5656063What about the wear with these low cost standards? When they need replacing, can you just grab another of the same low cost standard? My guess is no but maybe for the low cost VNAs, it's good enough.I dare say most of what I have presented used no calibration, or I used the ideal model.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 04, 2025, 08:36:32 am
content: Quote from: joeqsmith on August 03, 2025, 05:12:01 pmIf you already have access to good lab-grade standard, and you only need some working standards for personal use, the problem becomes much easier.1. Calibrate VNA via SOLT.2. Measure transfer standards.3. Export S1P/S2P.Next time, calibrate your VNA directly via the raw S1P/S2P of the transfer standard, METAS VNA Tools or scikit-rf can do it for you. Don't use any fitting. Prepare some absorbing and reflecting DUTs (even random circuit boards) for comparison. Ideally it should be limited only by connector repeatability. Optionally, monitor working standard drifts by remeasuring periodically. Most of the time, the data quality should be as good as the VNA itself, since the only difference between the reference and working standard is a linear transformation.But since I'm publishing reference data to the public Internet, I wanted to get higher-quality data, which was why I wanted to get matching simulations and measurements to show the data is self-consistent. So far I'm not be able to do this successfully for 3.5 mm connectors, I strongly suspect connector transition parasitics is the problem. More tests are planned using homebrew bead-less air lines to eliminate this factor.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 04, 2025, 02:00:03 pm
content: Quote from: niconiconi on August 04, 2025, 08:36:32 am3) Assumes software supports it. I had to add this feature.Using the metrology grade standards to characterize the low cost ones required some sacrificial adapters. There is no way I would connect a metrology grade standard to a low end VNA or cables.Mostly I run the VNAs with my own software. There is no need to post process data with METAS. I used the METAS tools to derive the coefficients for the low cost standards. I also use it to plot data.I will be interested in how you quantify the low cost standards errors with their use. Torquing them to the high end vs low end of the specs.For the mechanical metrology grade standard, they are returned to the manufacture. For the ECALs, we transfer the mechanical standards to them. This is what I did for home as well. I characterized them at a high and lower power using very low IFBW, two different bands to get some decent resolution.For the low end VNA, much of this work was pointless. The VNA was just not good enough to obtain good results. Provide a link to your paper once you have published it.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 05, 2025, 01:12:00 pm
content: Quote from: joeqsmith on August 04, 2025, 02:00:03 pmMe too. Personally I post-process all my measurement programmatically without using any calibration feature on the VNA itself, this way the uncalibrated raw data is always available and there's always a chance to reanalyze or recalibrate, if something is messed up.For example, old versions of NanoVNA-Saver in fact had a bug that caused incorrect Thru calibration during SOLT, all the data published in that time frame (a few years) were corrupted is subtle ways.QuoteIt's when a standard library becomes important, implementation difficulty is greatly reduced if someone else has already implemented the linear algebra. The software library scikit-rf has implemented 10+ calibration algorithms in the open literature (including rare ones that are not even supported by $5000 VNAs). Most are usable with just 20 lines of code to call its APIs.For example, to do an arbitrary-impedance SOL calibration, you simply do:Code:import skrfdef sol(ref_list, meas_list, dut):# calibrate "dut" using measured SOL standards in "meas_list" with respect to their# known S-parameters in "ref_list"algo = skrf.calibration.OnePort(measured=meas_list, ideals=ref_list)algo.run()return algo.apply_cal(dut)# define inputsdut = skrf.Network("dut.s1p")sol_known = [skrf.Network("open_ref.s1p"), skrf.Network("short_ref.s1p"), skrf.Network("load_ref.s1p")]sol_measured = [skrf.Network("open_meas.s1p"), skrf.Network("short_meas.s1p"), skrf.Network("load_meas.s1p")]# run calirationdut_corrected = sol(sol_known, sol_measured, dut)# write outputdut_corrected.write_touchstone("dut_corrected.s1p")But yeah, I do agree with you on the software implementation problem. The lack of a GUI is a major obstacle preventing their wider uses. Implement a full-featured GUI that supports all calibration algorithms in scikit-rf is far from an easy problem at all, since the possible usages are endless. You need a "GUI wizard" for each problem. It becomes even more problematic for multi-tier calibrations (use different calibration algorithms recursively for each reference plane).I think the calibration workflow management problem may be solvable using a "flowchart" based method. Instead of creating a wizard for each possible calibration, users are allowed to wire "calibration boxes" together like LabView or ADS/Qucs "de-embedding box" circuit schematics. Common use cases like conventional SOLT can be added as built-in charts...But of course it's easier said than done.QuoteI use a connector interface gage to check if the low-cost SMA connector's dielectric and pin depths are standard-compliant. If it is, it can be safely mate with 3.5 mm connectors. Unfortunately most of the time the answer is negative, so you need at least one sacrificial adapter per port. For the NanoVNA, NanoVNA v2, and VNA6000, I suspect even the connectors on the VNA themselves can be out of spec.My memo on SMA/3.5 mm connector interface gage and standards are available here:https://gist.github.com/biergaizi/2420faf4fa2dbb8ca4666c3a75918455

author: joeqsmith
author_rank: Super Contributor
timestamp: August 05, 2025, 02:26:40 pm
content: Showing some of the problems when gaugeing SMAs.https://www.eevblog.com/forum/testgear/dont-buy-sma-cable-assemblies-from-centricrf/I have not inspected any of the standards I have that were supplied with the low cost VNAs. The finish on the set that came with my first NanoVNA was damaged in short order. That may be another good metric for your paper.https://www.eevblog.com/forum/chat/rf-calibration-standards-are-like-nintendo/msg6001277/#msg6001277

author: shabaz
author_rank: Frequent Contributor
timestamp: August 05, 2025, 02:40:48 pm
content: Quote from: niconiconiI emailed, and they stated they are not stocking them currently due to lack of demand. They might consider reintroducing them next year.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 05, 2025, 03:15:06 pm
content: Quote from: niconiconi on August 02, 2025, 06:15:53 pmYou could have a look at Applied EM (now Antenx). They come with a case, M/F OLS standards, thru, attenuator, torque wrench, open end wrench, adapters, attenuator and USB stick. The complete cost is around what you would pay for a single component for a Keysight set. These are serial numbered and come with test data.https://antenx.com/product-category/measurement/vna-cal-kits/Below, showing supplied data for one of the standards. Also attached is a report with the coefficients. Note that there is no data below 500MHz and not many points. They only offer SMA (no precision connectors). We re-characterized a set with more data points over a wider range and used METAS to derive new coefficients for them.# Hz S DB R 50! HP 8720B Vector Network Analyzer! Start Frequency: 500 MHz! Stop Frequency: 6100 MHz500000000 -4.8632758182067057e+01 1.5150515120998764e+02675000000 -4.8378694326493672e+01 1.4345770608097547e+02850000000 -4.8007602925858265e+01 1.3823641118322914e+021025000000 -4.7541560918429326e+01 1.2943987604962206e+021200000000 -4.6774784976042355e+01 1.2005609644599100e+021375000000 -4.6272333233632239e+01 1.1570094174713294e+021550000000 -4.5350206616903186e+01 1.0975470797973357e+021725000000 -4.5057540605129212e+01 1.0221635758789442e+021900000000 -4.5164194499447419e+01 9.4249470798397041e+012075000000 -4.5232937173930878e+01 9.0788515750174710e+012250000000 -4.4639581728813759e+01 8.7489288350676546e+012425000000 -4.4337853313690658e+01 8.5353562662262803e+012600000000 -4.4079609158428468e+01 8.3514309638153861e+012775000000 -4.3658745537834882e+01 7.9969588353478471e+012950000000 -4.3447734501956923e+01 7.5073813381567348e+013125000000 -4.3429119702175356e+01 7.0873940473093256e+013300000000 -4.3539687018030591e+01 6.8383132727691489e+013475000000 -4.3790971806019897e+01 6.7692292392292885e+013650000000 -4.3796583020940943e+01 6.7704532356579961e+013825000000 -4.3659604188031729e+01 6.5813450903288782e+014000000000 -4.3731399053434707e+01 6.3652733836260353e+014175000000 -4.3690446056727723e+01 6.1242472982275586e+014350000000 -4.4247445436933610e+01 6.0452950231864577e+014525000000 -4.4799247179273145e+01 5.8916563766523630e+014700000000 -4.5295131388360403e+01 6.0639780262170291e+014875000000 -4.5883463405761091e+01 6.2910574572966290e+015050000000 -4.5769660725853868e+01 6.7596763357999961e+015225000000 -4.5746806120579336e+01 6.9970073213963659e+015400000000 -4.6349662777253158e+01 6.9858106148900575e+015575000000 -4.6757094421386412e+01 7.6395074279914880e+015750000000 -4.6657974813063923e+01 8.3233825385002334e+015925000000 -4.6374530119173954e+01 8.8836030691513074e+016100000000 -4.5983593323630849e+01 9.0136023155137195e+01

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2025, 12:26:01 pm
content: Definite rabbit holeSome reading here:https://www.eevblog.com/forum/testgear/kirkby-calibration-kit-alternatives/Member hendorog has already travelled this road to find sources for affordable VNA Cal standards and then properly characterises them before sale:https://vnalab.net/

author: tszaboo
author_rank: Super Contributor
timestamp: August 08, 2025, 12:46:57 pm
content: Quote from: niconiconi on August 05, 2025, 01:12:00 pmInteresting point about the calibrations. I'm wondering if using a TRL standard kit might be more reproducible than trying to get 50 Ohm terminators and dust caps to work as calibration standards.I think the various NanoVNAs don't support it out of the box but with these python libraries it's a different story. I haven't tried using my LiteVNA with python yet, you are definitely sending me down a rabbit hole.Like can we use a Mini-Circuits SMA to SMA bulkhead adapter as the L standard?For the GUI stuff, try Jupyter notebook for now. It's a python interpreter running in your browser, which allows inline plotting and other fancy things. It's not a full GUI, but you can publish results as a HTML results together with the code.

author: tszaboo
author_rank: Super Contributor
timestamp: August 11, 2025, 11:38:18 am
content: Yes this seems to work. It took me a while to figure out how to control the LiteVNA but it seems quite good.This is a 300 point RF sweep or an antenna, offline calibration.The interactive markers that are present in software are quite useful of course. But this is quite a bit more flexible than ie the NanoVNA saver.