RF calibration standards are just like Nintendo... - Page 1
Forum section: electronics / general

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 04, 2025, 11:02:58 pm
content: RF calibration standards are just like Nintendo.1. Always rebuy the same game: They just keep selling exactly the same thing to you over and over again on each new platform. Type N, Type BNC, GR-874, APC-7, 3.5 mm... "But our DUT is in another connector." Not to mention you need usually two copies for each due to the pin and socket split too!2. Incompatible PAL/NTSC editions: The 75 Ω vs. 50 Ω split is the RF equivalence.3. ROM Dumps: You can create multiple copies of working standards from an official standard by measuring your own DUTs, but you need to get licensed (lab accreditation or traceability certificate) for legally-recognized commercial uses, and that license is rarely granted.4. TAS: Using an E-Cal unit!

author: joeqsmith
author_rank: Super Contributor
timestamp: August 05, 2025, 12:55:35 pm
content: I would have keep your post under the same thread you already started.Quote from: niconiconi on August 04, 2025, 11:02:58 pmThat should be part of your study. We had to replace a Keysight short standard recently, to the tune of $1000 USD. Certainly part of the high cost of the metrology grade standards is the VERY tight mechanical tolerances they hold. They are tight enough that you can actually order a replacement part and not have to characterize it. With the lower cost standards, rather than hold those tight tolerances, they opt to characterize each standard sold.For the really cheap standards, the suppliers do not characterize them. Sure, you go through the process. Can you just replace one without from the same supplier without characterizing the new part? I doubt their mechanical tolerances allow it.Then again, in the context of the low cost VNAs, does it matter? At what point does it become a problem?Look forward to reading your published paper showing the results of your study.https://www.eevblog.com/forum/chat/out-of-control-solt-calibration-study-what-ive-been-doing-for-past-few-months/

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 05, 2025, 07:35:53 pm
content: Quote from: joeqsmith on August 05, 2025, 12:55:35 pmAchieving "absolute" accuracy or SI traceability is never the goal. We all know that getting VNA calibration with real uncertainty bars to SI is an extremely complicated problem even for NIST and METAS (so I won't elaborate for this reason). I would be satisfied by just a partial correction. In other words, I'd call it a win if I can meet these criteria:* VNA reading (working standard) = VNA reading (official vendor standard) + "small" error* "small" error < uncalibrated errorFor the working standard:* error (manufacturing variation) + error (characterization) < error (uncalibrated measurement)In other words, this is based on the assumption that an uncalibrated VNA has an error larger than the sum of other sources, either in the homebrew standard or the VNA.From the finding of my unfinished research, my current opinion is that:1. For a SOL kit, it's easy to purchase yourself a high-quality Load with SWR spec, so it's not a problem. Probably not for $5, but you can pay $50 and find many suppliers.2. The Short standard is easy to implement, at least for the SMA/3.5 mm pin connector. A regular shorting cap is nearly a zero-length flush shorting plane, with a phase error comes from the metal's surface impedance. According to the existing literature, a shorting metal plane's phase error is much smaller than 1 degree. (WARNING: the shorting cap pins often have protrusion that violates the spec, may damage precision connectors, use them only with sacrificial adapters!)3. The Open. This is the worst-performing standard, as I've already pointed out in another thread, and the exact value (with all parasitics taken into account) is still under my investigation (Note 1). But just let's assume the test port is a bare open-ended 3.5 mm coax into free space, the existing literature agrees that the parasitic capacitance is around 35 fF to 42 fF. This means a phase error of 8 degrees at 6 GHz. This scales approximately linearly with the connector diamater, so it's 79 fF for 7 mm connectors, etc. This is a SIGNIFICANT value, so I strongly believe any correction is better than nothing.QuoteI doubt their mechanical tolerance disallows it. It's extremely unlikely that any manufacturing variations (even standard-grade connectors) will create an electrical length or phase error of 8 degrees just from the millimeters tolerances of SMA.I'd like to also mention that open-ended coaxial probes are used for the characterization of dielectric mediums by material scientists. It's treated basically like a VNA open standard, terminated into something else than air. The simplest probes are just off-the-shelf coax cables (although sophisticated designs exist), and the published literature reports little fringe capacitance error from theoretical calculations. One paper measured RG402 and RG403 cables, the result agrees within 1 fF (0.001 pF).If we use connectors instead of cables, it should only work better.Note 1: In practice it's much more complicated. It strongly depends on the connector and the internal construction of the standard, so the actual value can be much smaller or higher. The fact that the connector transition parasitics is partially removed during SOL calibration also makes it impossible to directly measure, because Serror_connected_port is not S_err_unconnected_port. I believe there's a small step capacitance between the transition of both connectors, so the parasitic capacitance of an open port and the step capacitance of the connected port partially compensated each other (because we will calibrate the step capacitance out anyway during SOL), making the "effective" fringe capacitance error smaller, as small as 10 fF (2-3 degrees phase error at 3 GHz) for some SMA sockets. Note that the above information is NOT fully experimentally verified yet.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 06, 2025, 12:50:08 pm
content: Quote from: niconiconi on August 05, 2025, 07:35:53 pmBuying high quality standards is as easy as opening your wallet. I am using a Mini-Circuits ANNE for a load. Consider that I used a metrology grade set to calibrate my Agilent VNA, then used that setup to sort the loads. Found one part that was really good and another that was very reasonable.Quotehttps://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg3095447/#msg3095447Quote from: niconiconi on August 05, 2025, 07:35:53 pmI don't understand how. Do you have any photos showing what you are describing? At least with the standards supplied with my low cost VNAs, the pin is referenced to a flat surface.Quote from: niconiconi on August 05, 2025, 07:35:53 pmYou would need to provide that data. I think part of the problem is going to be the frequency range you are considering and the equipment you are using to make these measurements. I'm guessing all of that will be included in your published paper.***In your other thread (not sure why you decided to make a separate one) you wrote about using S-parameters to define the standards. If this is the case, do you really care about the how close the standards are to ideal?

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 06, 2025, 04:08:28 pm
content: Quote from: joeqsmith on August 06, 2025, 12:50:08 pmHow can you not understand this? What I meant is, if you plug the shorting cap into the SMA interface gauge (center pin), for the two shorting cap I've tested, the needle of the dial goes over 0. Whenever the connector gage dial goes over 0 it means an interference fit, meaning that bottom of the pin of the shorting cap is outwards from the ideal reference plane, which means the pin tip "protrudes" out of its ideal position, implying a potentially destructive mating.Quote from: joeqsmith on August 06, 2025, 12:50:08 pmIf I already have the data, I'll publish the paper already, instead of posting in the "Chat" section. I already said everything I've posted should be seen as unfinished draft. Even showing the data is not enough to convince anyone at the moment, because the scope of my experiment can only be fully covered by a series of perhaps 7 articles, each documenting a specific aspect, perhaps in 2026.Without any systematic measurement data, a simple physics-based argument can be to qualitatively (not quanlitatively) assesses the problem. A phase shift of 8 degrees at 6 GHz (approximate phase shift of naked 3.5 mm coax) is a time delay of 1.4 ps, which is 0.42 mm. Because this delay is mostly the result of a fringe capacitance, the value has an asymptotic behavior. As long as the shield has a reasonable thickness or if the hollow outer cylinder is sufficiently long, numerous simulations have showed that this fringe capacitance barely changes at all, perhaps 5 fF. Thus, the phase error can only come from the connector's pin and body sections. But if your pin is 0.4 mm too long, the connector is by all means out of tolerance horribly. MIL-STD-348A says 0.08 mm.QuoteI made two post, the first was about sharing my technical progress on calibrating the VNA, meanwhile, this post you're reading right now, started as a joke, comparing the headaches of VNA calibration process and Nintendo games (repetitive purchases, format split certification). Both threads ran in parallel, and has nothing to do with each other's focus. I don't understand why you would post your technical question in response to a joke, rather than replying to the "correct" thread.But to answer the question anyway. Yes, for two kinds of considerations.=== Priority ===If a random standard is already close to ideal by due to its "typical" construction, it means the uncalibrated error created by the "ideal termination assumption" is already small, so one should focus on characterizing the worst-performing standard to maximize the gain of correction.To repeat my current conclusion: I suspect a well-characterized Open standard is, potentially, all one needs for daily measurements. In standard SOL, the load is uncharacterized and assumed to be ideal, the scalar return loss error mostly comes from the non-ideal load standard. But as we all know, good Load standards with guaranteed spec are readily available for purchased (because it only needs a scalar measurement and widely used in impedance analyzers). So there's no problem for us to solve. Furthermore, a flush shorting cap's (the cheap construction, without delay line) deviation from an ideal short is so small, if the existing literature is to be believed, you can barely simulate or measure this shift. One research paper showed negligible phase shift of a 1.00 mm lossy coax shorting plane at 110 GHz - shorting planes are used in waveguides too for a reason. The connector pin uncertainty would be a much bigger factor. But good luck detecting it at sub-6 GHz - well, besides the fact that they might kill your connectors due to pin protrusion...Meanwhile, the Open standard has significant parasitic capacitance, which has been studied since WW2 (MIT Rad Lab) to the 21th century. More than 50 papers have been published on this exact topic. In academia it's common knowledge which has been demonstrated by many researchers that the fringe capacitance of an open-ended coax is (1) significant, (2) can both be calculated to high precision, (3) can be reasonably constructed if the geometry is simple. So I believe it's reasonable to focus the effort on the Open.=== Comparison ===In my original words, I said if you already have access to characterized standards, the problem "becomes much simpler". In other word, I'm implying the problem I'm working on is a more general problem with a higher difficult in comparison, so the raw S-parameters cannot be used.1. For characterizing unknown working standards using known standards with guaranteed standards for one's own use in your own lab, using the full S-parameters is always the best approach. The result is as good as your VNA, since each measurement is only a linear transformation of each other. Even if the data is "wrong", you can't get better data anyway.2. For assessing the general characteristic of a DUT of the same design (I said I plan to publish reference data online for the community to use), handing away your own raw S-parameters to someone else is not a good idea. If you do this, all the errors (including unphysical errors and noise spikes) in your particular sample and VNA is inherited by everyone else online, especially bad if your VNA is not much better than someone else's instrument. This is especially bad for reflective DUTs. So the idea is fit the S-parameters into the standard equivalent circuits. These equivalent circuit model (fitted from data) will not be exact, butA. They provide physical constraints, to filter out the unphysical artifacts like spikes. An isolated artifact of your own VNA is not transferred to everyone else's measurement.B. The phase uncertainty can be bounded to a reasonable range (e.g. Open standard, X degrees at Y GHz), just as Keysight can. Furthermore, these reported "typical" data can be independently compared by everyone else. Everyone including you can join the verification effort when the data is out. If I said I measured 3 samples of a connector model AAA and found a phase shift between 2 degrees, but you find a 3 degrees difference using a high quality VNA, this data point can be included, so the expected uncertainty based on the published "typical" AAA coefficients can be improved over time.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 06, 2025, 04:43:44 pm
content: Quote from: niconiconi on August 06, 2025, 04:08:28 pmI think with mine, you want a positive reading. Negative would be bad. The SMA male short does not have a step like a standard connector. It is flat from the reference to the pin, or should be. Of course, as I have shown there are countless poor machining examples. Again, maybe a photo of yours would help clear that up. The mechanical gauge standard is a flat surface and I would expect the short to basically read zero unless they machined a lip at the base of the pin that is catching on the gauge. I think I have 5 or so unused kits that came with the low cost VNAs that I could check if you like.***With my gauge, the pin's flat edge to the reference is specified to read between +0.01 and -0.000, assuming industrial standard. The more positive the number, the deeper the flat is recessed. Note the can of dust off used to blow out the part before measurement. You can see the one I use measures about +0.00025. Pretty close to 0 as expected. I did try to get a better picture of the pin to the flat surface.Surface finish is poor compared with some of the high grade connectors I've shown in the other thread. There's no way I would attach this to a metrology grade connector. Still, it's fine for the low cost VNAs I use it with.I assume for your published paper you will document the gauge and standards used so people can follow along with your work.