VNA for cable characterization - Page 1
Forum section: electronics / rf-microwave

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 03, 2024, 10:09:01 pm
content: Hello,I am new to RF measurements and would like to perform some high precision cable impedance measurements to characterize some short(0.5m to 2m) 50Ω BNC cables of different types in the MHz and low GHz range.I read that cable impedance measurements of the required accuracy can be done with a TDR sampler like the Tektronix 7S12 with a respective pulse generator and sampling head. A setup like this should be able to measure the impedance along the cable with an accuracy of +/- 0.1Ω or +/- 1 mρ and a time resolution in the sub nano second range. It should also be possible to do frequency dependent impedance measurements with a suitable milli-ohms-meter like the R&S URV5 with a suitable insertion unit. Unfortunately, both vintage setups are extremely difficult to acquire and I do not have the required bench space and operating knowledge for a suitable sampling oscilloscope. So, I tried to dig into the topic in order to maybe find some kind of measurement setup that could somehow fulfill my needs and stumbled onto vector network analyzers.It seems that high quality VNAs, like most kind of test equipment, can get ridiculously expensive, but there are also affordable solutions, like the liteVNA 64, which might or might not be sufficient, for what I want to do. However, until now I could not figure out what measurement precision can be expected from a device like the liteVNA 64 with respect to cable impedance and TDR measurements. This is where I hope I can get some answers here.I already found the very feature packed Solver64 software from forum member joeqsmith, who has put tremendous amounts of work into providing countless features like TDR measurements for the inexpensive VNAs. I read that the performance of these devices depends heavily on the usage of high quality calibration standards, which might easily cost more than ten times the price of a budget VNA. So my question would be: does anyone here know what TDR measurement performance in terms of impedance deviation can be achieved with a liteVNA 64 or similar device, combined with some affordable calibrations standards? Or am I mistaken and a VNA is not the right tool for this task? Any help in this matter would be appreciated.

author: jwet
author_rank: Frequent Contributor
timestamp: July 06, 2024, 05:09:59 pm
content: I saw that you got no responses so I'll give some guidance to get a conversation started perhaps. I'm not a renowned expert at all but there are some on this board.I think a standard VNA, cheap or fancy is not a great high precision tool for your application. The problem is that since VNA's depend on reflections and the reflections will be very small (loads close to 50 ohms) , relative errors will be large. The dynamic range of a decent VNA is 80+ db, cheap VNA's are about 70 dB. Flatness of a good VNA is .05 db which will turn into fraction of an ohm.There are RF impedance measurement devices that use an V/I technique. One is the HP4396B that I'm familiar with. It looks and acts like a VNA but in impedance measurement mode, it uses the I/V technique. HP has an impedance measurement handbook that is pretty good and goes through their solutions and limits of accuracy. Good standards are expensive and will be required to get accurate absolute results.Good Luck.

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 06, 2024, 10:36:03 pm
content: Thank you for the kind and helpful reply. So, it might be as I already feared: my best bet is to lurk on ebay and wait a few years for a chance to get some vintage TDR equipment. Unfortunately, this stuff is much harder to get here ein Germany than in the US. In any case, I will have a look at the HP impedance handbook you mentioned. Until now my main literature source about VNAs wasThe VNA Applications Handbookfrom Bonaguide and Jarvis.Quote from: jwet on July 06, 2024, 05:09:59 pm

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2024, 03:26:42 pm
content: You never state your requirements, what your goals are, or the reasons behind them. Reading your post, I get the impression that you feel that 50 vs 50.1 is a big deal for your application, like the quote below taken from the linked article. I want to understand why?Quotehttps://www.dsinstruments.com/support/understanding-characteristic-impedance-vswr-reflection-coefficient/

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 07, 2024, 07:40:52 pm
content: Thank you for the link and for pointing out that I did not provide enough information for the question to be answered. I tried not to overload the question with irrelevant information, to prevent readers from thinking: Too long didn't read. Thankfully, the situation is not as bad as in your quote. I have mainly two goals with this:The first and less critical goal is that I want to be able to verify the quality of BNC cables I buy or make. In the past, I have bought cables from sources I thought I could trust, only to discover later, that the connectors were of terrible electrical quality or the cables had abysmal frequency dependent return losses. Also some cables may go bad with time unnoticed, e.g. foam dielectric coax cables which have been bend too much. This made me believe several times that certain measurement gear was bad, when in fact it was only a matter of bad cables. So being able to ballpark my cables would be nice to find out, which cables to throw away and which not to use for precision measurements.The second goal is that I have repaired some vintage calibration gear. For a correct calibration of the gear and the devices that will be calibrated with it later on, there is the requirement of using a precision 50Ω +/- 1% cable with a length of 36 inches. These cables are then paired with the calibration device in order to later be able to make precise statements about the calibration accuracy of devices calibrated with this combination. Unfortunately, the original cables used for this e.g. the Tektronix 012-0482-00 are near to being unobtanium nowadays and I know from people that worked with these cables on a daily basis back in the days, that they had a tendency to go out of spec after a while. So, original vintage cables cannot be trusted without testing and a way is needed to validate them.Due to the bad availability of original cables I tried to get information on the precise specifications of the cables to see, if maybe there was a way of making those using parts that are available today. According to my current findings, the validation of a cable should at least include verifying that the impedance stays within 50Ω +/- 1% below 1GHz and that the VSWR below 3GHz is less than 1.3. A few years ago, Dennis Tillman did an in-depth TDR evaluation of an original cable (link) to reverse engineer its specifications using a Tektronix 7S12. This evaluation suggests that an impedance measurement accuracy of at least +/- 0.1Ω or +/- 1 mρ is possible and also allows a precise look at what is going on in the used BNC plugs (given a high quality 50Ω termination is used). So, 0.1Ω will certainly be no big deal for my application as long as the 1% spec is met, but I had the impression that this resolution may be the minimum required to see IF the 1% spec is met.Based on this, I managed to identify several combinations of available parts(BNC plugs and cables of different types) that could be suitable for creating respective cables. However, the the quality of the cables not only depends on the parts, but also on the manufacturing process. The latter has some potential to be screwed up by me. So, just buying several combinations of parts and making different types of cables would be useless without a way to measure and compare the cables. At the moment, I do not have sufficient gear or knowledge to reliably take the required measurements. So, I am now trying to find out what I need to know to take the required measurements and if there is a way to acquire the respective measurement gear. After all, just owning measurement gear usually does not mean that you know how to use it correctly. So, time for some research. At the end, if possible at all, it would be nice if I could come up with a recipe, that can reliably be used by others to create calibration cables of this type, optimally for a reasonable price.Quote from: joeqsmith on July 07, 2024, 03:26:42 pm

author: pdenisowski
author_rank: Super Contributor
timestamp: July 07, 2024, 07:42:41 pm
content: There are many different ways of measuring the impedance of cable, but if I had to choose a methodology that's a good compromise between accuracy, simplicity, and cost, I would use a VNA and the methodology I describe in this video:

author: gf
author_rank: Super Contributor
timestamp: July 07, 2024, 10:05:41 pm
content: In order to preserve the leveled output of the SG503 generator, wouldn't it be sufficient to verify that the cable's |S21| is flat enough (say +/- 0.1dB) over the generator's 250kHz...250MHz range?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 09, 2024, 03:13:29 am
content: Quote from: gf on July 07, 2024, 10:05:41 pmIn OP long post, they fail to mention anything about the actual test equipment, only the need for a Tektronix 012-0482-00 cable. Was the SG503 Tektronix's only use case for it? Sounds like a higher performance cable may not be better for this application. They setup the generator for that one specific cable, what ever it is.It does appear you can buy them new. For a standard, $100/ea seems like a no brainer.https://www.testequipmentdepot.com/tektronix-012-0482-00-bnc-male-to-male-precision-1-coaxial-cable-50-ohm-36-in.htmlBecause OP mentions the LiteVNA and I had mine out, ran a quick test using the following cables. All cables are fitted with a 50ohm BNC and SMA. All connectors are different. All cables are within a foot of each other. Using my BK RLC meter to measure each cables capacitance and inductance to derive the impedance.TEFLEX60.1PasternakRG58C/U56.5MIL-C-17G THERMAXRGS-40056.8All read high using this method with my meter. Using the LiteVNA w/ TDR, calibrated using the V2Plus standards with Mini-circuits ANNE for load (sorted for RL), and the ideal model. Sweep range set to 6GHz. Should give you some idea of what to expect. Maybe using good standards that have been characterized and using those coefficients you could improve your measurements enough to meet your goal. Of course, that may cost more than just buying a new cable.If my only goal was to test cables I have, I would physically move them around while measuring them to make sure they are stable, but would consider the LiteVNA even with poor standards and ideal model, good enough.

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 09, 2024, 08:19:01 pm
content: @gf:According to my current knowledge, regarding the special case of the SG503, the answer to your question is a clear "maybe". The output flatness seems to be one of the main design features of the 012-0482-00 cable. This is achieved by using a precision cable with custom made 50Ω BNC connectors with a reasonable return loss. In addition to that, a cable with a solid center conductor and a solid dielectric is used, most likely to increase the robustness against bending and kinking. According to the manual, deviating too much from the intended cable length of 36 inch can cause amplitude variations of up to 4%, but getting the cable length right is a trivial task.Lets assume it would be sufficient to make a cable from high quality BNC connectors and any low loss high quality 50Ω cable (e.g. RG233 or H155), that has a flat enough $$|S21|$$. As I understand it, for this to meet the original specs, the required flatness in dB has to be derived from the allowed impedance deviation of 1%.I read through some of the stuff you linked and some additional documentation from Keysight.With $$Z_{t}$$ being the transmission impedance of the cable and $$Z_{0}$$ being the system impedance which equals 50Ω, I found the following equation to compute $$Z_{t}$$ from $$|S21|$$.$$ Z_{t} = Z_{0} \frac{2(1-|S21|)}{|S21|} $$Since S21 seems to be a complex number, I assume its magnitude has to be used here, which should be a linear value.To get to and from the logarithmic dB values you mentioned, the following two terms can be used according to my sources:$$|S21|_{dB}= 20\cdot log_{10}(|S21|)$$$$|S21|=10^{\frac{|S21|_{dB}}{20}}$$I will math-fumble with that next weekend.@joeqsmith:Thank you very much for going through the trouble to make some example measurements of different cables. From what I can see there, the liteVNA, combined with your very nice solver64 software should be more than enough for my basic cable quality testing application. It looks like it will be able to give me a good idea of the cable impedance and also the return loss, to evaluate and compare my bench cables. This capability alone will most likely make me buy a liteVNA. Although it will not produce high accuracy measurements, it will be enough for that case and has a very compact shape and competitive price.As for the 012-0482-00 cable: the SG503 was probably the most common application of this cable, although it appears in several other Tektronix calibration gear service manuals. The SG503 was optimized to perform best with a cable of this type an then each individual generator is matched to a specific 012-0482-00 cable during calibration.The thing with new 012-0482-00 cables is that all the sources I could find up to this date only deliver to locations in the US or Canada, but not to Europe. Also, some sources who list the cable as "not in stock, but can be ordered" will call ridiculous prices (>500€) for something that can most likely be made from parts between 30€ and 50€.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 10, 2024, 01:06:49 am
content: I'm not sure what a good set of BNC connectors and cables would cost. The SMAs were shown towards the start of my data, not the BNCs. Then you have the crimp tool. Once you have all that, you still have to verify it meets what ever your requirements are which maybe you could rent the equipment? Get all of that sorted and you still need to get the cable working with what ever equipment you have (which I am still not sure of).If the end goal is just to have a leveled output, maybe there are better options. Maybe just a good splitter, detector, PC and some software to control your RF generator or attenuator? I played with some of those dirt cheap log amp eval boards which seemed to be fairly accurate at the 50MHz I was playing with them. Still, tough to verify. It may be time to think of a good used Watt meter and send it off for calibration?

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2024, 08:39:42 am
content: Quote from: joeqsmith on July 09, 2024, 03:13:29 amCalibrating Tek 1502 TDRs, both checking the impedance and adjusting for length.The 1502 (not A B, 1503 or any combination thereof!) has a 140ps system risetime, so you can resolve impedance changes 1-3cm apart. Lovely devices; I have been known to cart them around hamfests to check the quality of the cables(BTW, don't but one without seeing it working; it requires a functional NiCd battery before it will turn on)If the OP has a suitably fast scope, they could make their own TDR. Use one of Leo Bodnar's 50ps step generators, or even a few 74LVC1G* gates can push 2.5V steps into 50ohms with 250-300ps risetime. Need to be rather careful about the layout, though.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 10, 2024, 07:22:51 pm
content: Quote from: pdenisowski on July 07, 2024, 07:42:41 pmAttempt to use this technique with the Pasternak RG58C/U cable. This cable is 3' or 0.9144 meters long. Using their 75/length in meters for fstop, call it 75MHz. They talk about start not critical. 100kHz or lower? I set it to 300k in case I decided to try and replicate with my Agilent (limited to 300k). We are down into the muck, so set the IFBW to 1.3kHz. I had to enable the magnification for the Smith chart so we could see something. Then had to add two more gain ranges.Again, using low grade standards and ideal model. Yellow showing the load, red is cable + load. I am not sure why they care about the stop frequency rather than maybe just wanting the first cross over? Maybe more data points? If we use the second cross over of 50.5, sqrt (50.5 * 50) - 50.25. The first cross over is at 50.1 giving us 50.05.Guessing better standards and VNA would help but I think we are just asking too much from the low cost setup.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 10, 2024, 07:24:39 pm
content: Also note the amount of drift at the low frequency end. This was after about an hour of warmup.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2024, 03:33:16 am
content: After several more hours of running. This particular LiteVNA64 is the latest hardware revision.Still, for the $120 we paid for these, they are excellent.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 11, 2024, 10:52:43 am
content: Quote from: joeqsmith on July 11, 2024, 03:33:16 amI have several "hobbyist" VNAs and they are indeed an excellent value for the money.I also think that the availability of these hobbyist VNAs is good for the "professional" VNA industry as well. Historically, it was hard to get hands-on experience using a VNA since they were priced well above what a hobbyist could afford. The Nano, etc. VNAs provide a great opportunity to both build up VNA skills in the "community" as well as perform basic (or "undemanding") measurements.The cable impedance methodology I posted is something I've only ever done with a "professional" VNA, so it's perfectly possible it might not work well (enough) with a "hobbyist" VNA.Incidentally, Alan (w2aew) did a video on this same methodology using a NanoVNA

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2024, 12:03:22 pm
content: Skimmed it enough to notice how he takes the easy route and used 75. Would have liked to have seen him try it at 50. That said, the original NanoVNA may do a better job than the LiteVNA in this case.I assumed OP wasn't interested in professional systems, but with you having both, maybe they would like to see how a couple of cables would compare using your professional vs low cost VNAs/standards. Because they are asking about a 50ohm cable, maybe stick with that.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 11, 2024, 01:11:56 pm
content: Quote from: joeqsmith on July 11, 2024, 12:03:22 pmGood idea. I'll do it both ways in the next day or so and post the results.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2024, 11:32:55 pm
content: If you have a standard (air line) with a known impedance to use as a reference, it would be really interesting to see how close the various setups are to the published values.

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 12, 2024, 08:46:20 pm
content: Quote from: joeqsmith on July 10, 2024, 01:06:49 amA pair of good quality BNC crimp connectors starts around 16€ and the cable will add another 4€ to 10€, not including shipping. Very high quality connectors can cost double the price. I have an acceptable crimping tool available, but it does only work for some RG58 and RG223 and H155 cable connectors. I may have the chance to access some expensive equipment at my university, but that would be complicated to get and of course require that I can prove that I know how to use it. You wouldn't let someone drive your Ferrari if you weren't sure they were up to the task? So even if I could lend an expensive VNA or TDR test device, I would not do it before having learned how to operate it properly.In another post you mentioned that the original NanoVNA may do a better job than the LiteVNA for the cable testing case. Would that be because the NanoVNA has a lower noise floor in its fundamental based range below 300MHz? Because, otherwise the liteVNA seems to be much better spec'ed: fundamentals up to 6GHz, more sampling points, etc.@tggzzz:Yes, the tube based 1502s seem to be nice tools for TDR applications. There is a common hack to fake the presence of the battery and make them run on wall power. I already have one of those on my watch list, but did not find one in acceptable condition yet. Unfortunately, I do not have any gear qualifying as suitable for high frequencies. My fastest self-repaired vintage scope has only lousy 50 MHz. Once I have the necessary bench space available that will change. I already made some experiments with stacked Schmitt triggers for a pulse generator in the past, but my gear was not up to the task to validate the results.@pdenisowski:Thank you for your helpful input and the offer to maybe take some hobbyist vs pro VNA measurements. Those would be quite interesting.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 12, 2024, 08:50:34 pm
content: Okay, so I measured an approximately 1.5 meter long SMA cable using both a R&S ZNL and my (ancient) NanoVNA. Both were cal'ed using the NanoVNA cal kit.As expected, using the measurement procedure I talk about in the video on a 50 ohm cable leads to a "dot" in the middle of the Smith Chart in both cases. I zoomed in very close on the ZNL and got a "first crossing" impedance with a real part of 50.287 ohms. Plugging into the equation give a cable impedance of 50.143 ohms.I had trouble figuring out how to zoom in on the NanoVNA, either on the device itself or using NanoVNA Saver - mouse scroll wheel doesn't work. Without sufficient zoom, I can't accurately measure the first crossing of the resistive axis.I'll look into this some more, just FYI for now.Edit: I was a little sloppy with the marker placement, etc. - just comparing procedures at this point

author: G0HZU
author_rank: Super Contributor
timestamp: July 12, 2024, 09:46:45 pm
content: Don't forget that the Zo of 'real' coaxial cable will tend to rise (and become increasingly complex) at lower frequencies due to the resistive losses in the copper.Therefore, I'm not sure how valid the test is at 1.7MHz assuming you want to (accurately) know Zo up at VHF or UHF. There are other ways to measure it using a VNA or an impedance analyser. Also you can explore how much the Zo increases at lower frequencies for a given cable type.Skinny cables tend to be lossy so the Zo climbs rapidly at lower frequencies. Try repeating your VNA test using a skinny cable like RG174 to see what I mean.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2024, 09:56:15 pm
content: Quote from: Pinörkel on July 12, 2024, 08:46:20 pmI find the battery hacks are unreliable.If you want to run them from mains without a battery, the simplest way is to have a 12V 1.5A PSU, and use 4mm banana sockets to connect to the battery terminals down in the case. Make sure you get the polarity right! Alternatively replace the transformer with a small 240/110Vac->12Vdc converter, and attach that to the battery terminals.You can use NiMH cells, but the 1502 is very rough on NiCd cells, and NiMH are less robust and their capacity is often "questionable".

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2024, 09:59:57 pm
content: Quote from: pdenisowski on July 12, 2024, 08:50:34 pmCheck whether your NanoVNA has a "TDR mode", i.e. an IFFT on the the frequency domain data.

author: EE-digger
author_rank: Frequent Contributor
timestamp: July 13, 2024, 02:10:39 am
content: Can't contribute much but here is an interesting comparison between an original Tek cable and H155 cable. The writer claims that the H155 is within tolerance limits (for the SG503 output ?). (translations with Google)https://amplifier.cd/Test_Equipment/Tektronix/Tektronix_500/SG503.htmAlso, in either groups.io or the Tek wiki, there is mention of Tek's quest for a true 50 ohm BNC connector and the fact that they had to fabricate their own to get it (at the time).

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 13, 2024, 10:12:50 am
content: Quote from: EE-digger on July 13, 2024, 02:10:39 amThank you for the reply. I was already aware of that and even tried to order two of these cables from the ham radio shop mentioned in the associated discussion because they were very cheap and worth a try. However, the cables were delivered with 75Ω BNC connectors instead of 50Ω ones. So, I send them back for replacement, but it turned out that the shop insisted on them being 50Ω connectors and they did not even know the difference. That was when my trust in them dropped to zero. ;-)There is an interesting analysis on mixing 50Ω and 75Ω plugs and sockets on cables and devices (link). Although it is only available in German it should be translatable with one of the free online pdf translators.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 13, 2024, 10:22:33 am
content: Quote from: tggzzz on July 12, 2024, 09:59:57 pmThank you for putting "TDR mode" in quotesI still don't understand why the NanoVNA (software) developers didn't just call this "FDR" (or better yet, "DTF") mode. I just did a couple of videos on DTF mode on our instruments and I have a "technology" video on DTF coming out next week and TDR vs FDR is one of the things I talk about.That said, I'm not sure what relevance "TDR" mode has to measuring cableimpedance(rather than length) or zooming in on the Smith chart display. Sorry if that's a dumb question - I don't use my NanoVNA very often these days.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 13, 2024, 10:28:13 am
content: Quote from: Pinörkel on July 13, 2024, 10:12:50 amDankeAs always, a great article from HF-Praxis. I like the fact they mention ("for the sake of completeness" - lol), the old ARCNET cables. I actually picked up one of these when I was experimenting with measuring cable impedance for the video I mentioned above

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 13, 2024, 07:47:22 pm
content: Quote from: pdenisowski on July 13, 2024, 10:28:13 amAh, good old 300Ω "UKW-Antennenkabel" on an SMA connector and THT termination, a classic.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2024, 02:59:30 am
content: Quote from: pdenisowski on July 13, 2024, 10:22:33 amI used TDR to measure the various cable impedance in my post. Z=ZO(1+S11)/1-S11).I am guessing they referred to it at TDR as it seems the industry refers to it as TDR and TDT. Still though, I didn't want to see this but rather the method you show in your video that you feel was the better choice. If you want to include other methods, great but don't leave out the one you recommended.Sorry, I don't know how to use zoom, or most features of the NanoVNAs firmware. Seems I was able to get their standard return loss or something to zoom in and it was a nightmare. I wrote about how poor it was. It really was this poor UI that drove me to use the PC and wash my hands of it. Because I already had a decent code base to work from, it wasn't a big deal to support the NanoVNA. The LiteVNA's protocol was much better IMO. It's too bad they didn't just settle on that for all the low cost VNAs.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2024, 12:23:55 pm
content: RS paper on TDR. Describes measuring impedance.https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez44/1ez44_0e.pdfIf you plan to publish a new paper/video, you may want to mention that Copper Mountain Tech uses the chirp Z rather than DFT/IDFT. You may want to cover the pro's and con's of each.https://coppermountaintech.com/inverse-chirp-z-transform-for-vna-time-domain-processing/

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 16, 2024, 02:02:05 pm
content: Whoops, looks like FieldFox defaults the TDR y-axis (for rho measurements) to start:0 stop:2 rather than center:0 span:2!Firmware build is A.12.64, 2023-11-30.19:26GMT

author: pdenisowski
author_rank: Super Contributor
timestamp: July 16, 2024, 07:47:53 pm
content: Quote from: joeqsmith on July 16, 2024, 12:23:55 pmThat's a great paper, but from 1998 -- all of the products listed on the first page are at least two generations old.I'll look and see if we have something more modern

author: tggzzz
author_rank: Super Contributor
timestamp: July 16, 2024, 08:29:36 pm
content: Quote from: pdenisowski on July 16, 2024, 07:47:53 pmHave the fundamentals changed since then?If only the details have changed, I would hope people could make the appropriate adjustments.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 16, 2024, 08:43:38 pm
content: Quote from: tggzzz on July 16, 2024, 08:29:36 pmNoBut measurement methodologies sometimes become "obsolete" over time in the sense that they are no longer implemented in T&M instruments. And of course there are types of instruments that are no longer made.For example, you can still measure antenna impedance (well, resonance) using a grid dip meter or a noise bridge - I've done these myself many times - but no one manufactures them anymore. The general idea of generating a signal and observing how much is "coupled into" something at a given frequency is not that different from a "modern" S11 measurement on a VNA, but you can have a very successful career measuring antenna impedance without knowing anything about how a "dipper" works.

author: tggzzz
author_rank: Super Contributor
timestamp: July 16, 2024, 09:40:42 pm
content: Quote from: pdenisowski on July 16, 2024, 08:43:38 pmAh yes, the "you must buy this months/years new shiny toy" argumentSoftware sales (mis)practices invading the electronicengineeringworldQuoteOh, drat. A solid well-argued counter-caseYou might add wavemeters as a similar example. (I still regret never using a slide rule in a business meeting, just to unsettle the opposition)Mind you, using a dipperwithoutunderstanding what's happening really would be a case of just following established procedures, "monkey see monkey do". While there's a place for that (e.g. military field operations, while under fire), I think someone at the start of their career should be considering aiming for more than that.Another point is to consider what oscilloscope you should use to test and verify the world's fastest oscilloscope. Or how to test the linearity of an 8.5-digit DVM. Or how to build a 100GHz transceiver. When operating in such domains, you have to be inventive, and will often end up using variants of very old techniques.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2024, 11:21:24 pm
content: Quote from: pdenisowski on July 16, 2024, 08:43:38 pmCertainly possible to use that meter below 1MHz.Goal though is for you to use it to measure the coax's impedance.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2024, 11:23:10 pm
content: Quote from: tggzzz on July 16, 2024, 08:29:36 pmHas the math changed ...? Nope.

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 21, 2024, 12:57:36 pm
content: My testing of a liteVNA64 for basic cable validation will take a little bit longer than expected. I got one delivered yesterday, but unfortunately, that one had a broken 3-way switch and corroded contacts. So it immediately got sent back. Not a great first impression of quality.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 21, 2024, 01:09:54 pm
content: We have bought 5 Lite's now with no quality issues. Did you buy it direct from the OEM?

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 21, 2024, 02:10:45 pm
content: Quote from: joeqsmith on July 21, 2024, 01:09:54 pmKind of. Since the Aliexpress offers of Zeenko are seemingly blocked for customers in the EU, I took the Zeenko recommended EU distributor, with which I have made only good experiences in the past. If there is an issue, they are very helpful and customer-friendly, so I have no doubts that this will be resolved without problems.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 23, 2024, 01:28:01 am
content: Comparing the same three cables measured with Agilent E8357A, different cables, adapters, standards, but using the ideal model like I had done with the LiteVNA. I would call it a wash. Also shown is a home made 25 ohm 70mm Beatty standard. It is an airline but made from hobby brass.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 23, 2024, 01:35:29 am
content: Looking at the Beatty standard on the Smith chart, zoomed in 20X, not sure this method would work well. Using a real Beatty standard would be another good addition for Pdenisowski's video.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2024, 04:43:25 am
content: Except for its bulk, a coax line with a (mainly) air dielectric using a separate outer tube, with the inner supported inside it, could be designed &/or "tweaked" to be a very precise 50, & would be unlikely to change to any significant degree over time.The larger diameter 31/8inch, or the like in air dielectric coax lines would probably be the easiest to configure, but, at the same time the bulkiest.As an aside, back in the day Marconi preferred 51.5coax & the 1959 vintage TV transmitter setup at the ABW2 site, duly used that coax standard for the 31/8inch rigid cable sections involved in all the "plumbing" involved in getting the TV signal "up the spout".These early installations used discrete Vision & Sound transmitters, the outputs of which were combined in "filterplexers", which as well as combining V & S, carried out a respectable amount of the sideband shaping of the VSB signal.This particular installation used 2 Vision & Sound Transmitter pairs, with the output of the appropriate filterplexers being combined in a further device called a combining unit.For the colour modifications in 1974, all of the equipment between the transmitters & the antenna feeder had to be replaced by tighter spec stuff, which was duly supplied by Amalgamated Wireless Australasia Ltd.All the new stuff was 50, so AWA also supplied, at great expense, a number of 1/4 wavelength impedance matching sections sections of 50.7445.Of course, being curious, we opened one up, & found that the weird impedance was achieved simply by turning the centre conductor down in a lathe!

author: joeqsmith
author_rank: Super Contributor
timestamp: July 23, 2024, 10:46:43 am
content: Good luck with that one on the bench and getting it adapted down.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2024, 11:58:59 am
content: Quote from: joeqsmith on July 23, 2024, 10:46:43 amThe length would definitely need a large bench as the Velocity Factor would be close to that in free space.!On the other hand, 31/8inch to "N" adaptors are a standard part from Bird, RFS & others.To SMA might be a tad more difficult, though!

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2024, 12:00:08 am
content: What's the cost of the adapters?

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 24, 2024, 12:04:53 am
content: Quote from: joeqsmith on July 23, 2024, 01:28:01 amThank you for taking additional measurements. Would you mind detailing that assessment of yours? Looking at your two measurements, and ignoring the values before 0.1m, my inexperienced eyes would see comparatively consistent impedances of the three cables, also maintaining their relative values. The only exception is the Pasternak cable between 0.1m and 0.4m which reads about 1.5Ω higher on the Agilent. Considering the Agilent device has at least a 110dB noise floor(if I got the datasheet right) and thus should be much better than the liteVNA, I would conclude that the liteVNA measurements seem to be comparatively accurate keeping in mind that it is a low cost device. However, your comment suggests that some part of the conclusions your second measurement allows when combined with the first one, are rather disappointing.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2024, 01:11:40 am
content: If we really wanted to improve the measurement, to start I would want some sort of reference standard (along with better cal standards). Then take the time to use the correct coefficients and torque the connectors. Maybe then we could get some idea. Right now, there are too many variables (including the cable).I could at least try and lay the cables flat, torque them, then overlay the data. May be better off with the homemade Beatty standard as that should be more consistent than the cables. Not sure if it would be helpful but I am willing to run it if you like.What I was attempting to show is that depending on the cable, the method shown in the video may not be a good choice and that TDR can indeed be used to measure the impedance. I am hoping pdenisowski has access to proper standards and can actually run a good test for us as it sounded like they were going to make a video on the subject anyway.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 24, 2024, 03:39:28 am
content: Quote from: joeqsmith on July 24, 2024, 12:00:08 amProbably a King's Ransom!People who installed TV transmitters & the like could quite happily buy them as they could be used & reused over decades.Such things do turn up surplus, at a much lower cost.Obviously, smaller diameter fully rigid line could be bought back in the day.I have a short section of "smallish" diameter 50rigid coax out of an old Siemens TV transmitter, but it is terminated in old style "Spinner" connectors, which I don't think even "Spinner" makes these days.Semi-flexible "Heliax" is probably a good 50, too, with the advantage of reasonably usable sizes & better availability of connectors/adaptors.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2024, 12:28:52 pm
content: Are you thinking that the OP would source the airline and pay a kings ransom to replace a $100 cable? Or are you suggesting a standard could be made from it that would out perform a Beatty standard from a name brand?I think they still have the problem of how to verify their results. Then, I am still not sure that what ever they come up with could be integrated with what ever test equipment they have, and aligned to meet their requirements.QuoteThat paper they posted seems like a good starting point to making your own but without any real references, I don't see how they would verify it.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 01:16:57 am
content: Quote from: pdenisowski on July 12, 2024, 08:50:34 pmLet's try repeating this experiment also using my supplied cal kit using the sorted Mini-Circuits ANNE load. Both VNAs using the idea model. All connections were properly torqued for each measurement. Both VNAs were swept from 300kHz to 9.0GHz for each test. The LiteVNA used was the latest hardware revision: 64-0.3.1, using the supplied cables. Both VNAs were allowed to warmup a half hour.With the same standards, math, and torque, we have isolated some of the differences. There is still the cables, connectors, VNA, DUT placement, other....This first test is showing the RG400 cable. Green trace is the LiteVNA, White is the Agilent.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 01:19:25 am
content: The TEFLEX is a really old cable. The connector repeats but the cable is poor. Move it, things change.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 01:24:50 am
content: I have two of the Pasternak RG58 cables that are roughly the same. They were purchased at the same time, same connectors. I think I swapped them when I ran the PNA last time which is why you see the difference.Green (Lite)/Gold (Agilent) is the cable 1Yellow (Lite)/White (Agilent) is cable 2Note just how different the two SMAs behave.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 01:33:01 am
content: And finally, the homemade 70mm 50-25-50 Beatty airline. Green is the LiteVNA, White the Agilent.Even with the Beatty standard which doesn't flex, there is a fair difference between the two setups. If we used my better standards with proper coefficients, we could remove some of that ripple and get a better absolute measurement with the Agilent. Still, the standards I have are not really good enough for what you are asking. It's that hunting for perfect coax article I linked for you.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 01:17:49 pm
content: I wanted to give you some idea on how the movement of the test cables matters at the levels you are asking about. I repeated the test after four ties were wrapped around the TEFLEX cable in an attempt to stabilize it. This is also why I suggested when you use the LiteVNA to test your cables, you move them around a bit and see what effects that has.White is the Agilent, Green is the LiteVNA.https://www.youtube.com/shorts/2iLCt-u6M2MCMT showing their TDR feature. At roughly 7:30, they change the scaling to display ohms.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 25, 2024, 02:52:25 pm
content: Quote from: joeqsmith on July 25, 2024, 01:17:49 pmYes, phase (and amplitude) stability of cables is something that is often overlooked. In my experience, the difference between cheap cables and expensive cables is often primarily the difference in phase stability.When you're constantly connecting, disconnecting, bending, moving, etc. cables (like VNA test cables), this becomes even more important.Or you could just buy a big roll of tape** This is a joke - taping cables to the bench is not a substitute for good quality cables

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 03:25:54 pm
content: At 3:00 in, notice how they move the cable to check the stability. Their cable is a much higher grade than my TEFLEX example. I had seen Brian's video showing the stability of the cables they supply but wasn't able to locate it.Another one I suggest OP watch on accuracy.Gore's demonstration of their test fixture:

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 03:44:05 pm
content: Quote from: pdenisowski on July 25, 2024, 02:52:25 pmThose are four coffee bag ties. I like them because A) they are thicker and much more heavy duty than garbage bag ties, B) they are free.QuoteFrom that original paper, they talk about the original Tektronix cable not only having in-house developed connectors but also the stiffness of the cable. The RG400 cable I show is very stiff compared with the TEFLEX which is reflected in the data.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 04:46:35 pm
content: You can get high flex cables with decent specs but you do pay for them. How much?? Looking at Digikey to get some idea on cost from various suppliers that would be used with a VNA. They carry many of the name brands.Maury Microwave offers the SV-185-FF-48 for $5,678/ea.https://store.maurymw.com/cdn/shop/files/2Z-002.pdfLooking at Pasternak (I think Fairview Microwave makes their cables for them with custom blue anodize), you get an idea of how the connector options effect the price:PE3VNA1801-24 SMA Male to SMA Male, 24", $1130/eaPE3VNA2603-24 3.5mm Male to 3.5mm Male, 24", $2020/eaPE3VNA4004-24 2.4mm Male to 2.4mm Male, 24", $2730/eaLooking at Keysight, their PN# 85131E 3.5mm cable is $5342.LiteVNA with cables and standards is now $150ish. That sorted load cost me under $100 and time to sort them.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2024, 07:24:49 pm
content: This was the webinar I was thinking of.https://coppermountaintech.com/webinar/time-domain-measurement-and-time-domain-gating/There is another that OP may be interested in on measurement uncertainties.https://coppermountaintech.com/webinar/effects-of-random-factors-on-the-metrology-of-vna-measurement/

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2024, 12:18:35 am
content: Same setup but measuring a section of RG59. I don't have a good SMA to 75 ohm female BNC. There is an SMB/F-BNC/M to BNC/F-BNC/F to test cable.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2024, 03:48:40 am
content: Same RG59 section/adapters, using the crossover point. Measured terminator at 54.0 ohms. Sqrt (98.67 * 54) = 72.99 ohms.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2024, 12:50:00 pm
content: I wanted to try and measure the cable using the RCL meter. Test cable is 6'2". RG59 is 22.1pF per foot. Or roughly 130.1pF. Measured 126.2pF, so we are in the ballpark. Installing an BNC to SMA to SHORT, I measured 0.806uH. So 79.9 ohms. As with my 50 ohm cable measurements, readings with the RLC meter are again roughly 5 ohms higher compared with using the VNA.***Measuring a 220pF cap, RLC meter reads 221.7pF. A 100nH measures 0.105uH.

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 26, 2024, 06:37:54 pm
content: Thank you for the many interesting measurements and reading suggestions. I already started digging into that. The more I read, the more I get the impression that one needs to get at least a 15000€ VNA bundled with the guy who build it, just to tell you that if you did not spend 10 times more on a calibration kit and another fortune on adapters, you could as well have tried just eyeballing your measurement using scaleless calipers. (Owning the company that does the local weather forecast might also help.)Nonetheless, my replacement liteVNA64 should be on the way. For now, I only managed to get some very cheap BNC-F to SMA-F and SMA-M adapters to go with it. The supplier of the liteVNA had only those and I wanted to have something to evaluate what types of adapters I would need and which work best. Strangely there seem to be only two types of SMA-adapters on the market: very cheap china sub-3€-stuff and ridiculously expensive adapters that cost well over 50€ up to infinity, nothing in between. At first, I wanted to get some ofthisnice looking compact type to directly screw onto the liteVNA ports. Then I noticed that the center pin would rotate every time they were screwed on or off and that can not be good for the gold plating of the liteVNA port contacts.The following two from Fairview seem to be of at least acceptable quality (they have a datasheet).https://www.fairviewmicrowave.com/sma-male-bnc-female-adapter-sm4721-p.aspxhttps://www.fairviewmicrowave.com/sma-female-bnc-female-adapter-sm4709-p.aspxAt the moment, I am not sure what the best type of connection would be - directly to the VNA ports or with the short supplied SMA cables in between. As far as I see it, the liteVNA can only be calibrated right to its ports when using the supplied calibration kit and minimizing the amount of required adapters sounds like a good idea.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2024, 07:43:59 pm
content: As shown, some errors can be reduced just by using good practices.Quote from: Pinörkel on July 26, 2024, 06:37:54 pmOf course you can calibrate with what ever you want between the VNA and the standards. Obviously the calibration can not overcome a bad interface. Software can't correct for a bad cable!!For all of the data I have presented here, I have used the cables supplied with the LiteVNA. I am using a female to female adapter at the end of the cable. I am always calibrating at the end of the adapter. I tend to leave these parts attached to the VNA to avoid the wear.I may try and calibrate the Agilent and see what difference it makes. Another test would be to try and characterize the standards I use with the LiteVNA. I've seen people post about doing this but I have never used anything besides the ideal model with them.***One thing to consider if you do decide to not use a cable between the VNA and your load, you may end up putting undue mechanical stress on the connector and damaging the VNA. You also have the added wear of the connector. I had seen some of the hams mount SMA to UHF connectors on theirs. They would use panel mount types and bolt the entire assembly down to remove the majority of stress on the SMAs.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2024, 01:54:59 am
content: Searching for that Tektronix cable, appears there are others that have looked at it as well. In this first link, they were trying to use some version of my software with the cable connected between the two ports. I'm not sure why.https://physik.co-i60.com/2022/01/new-addition-to-the-lab-nanovna-v2-plus4/https://groups.io/g/TekScopes/topic/my_tdr_evaluation_of_the/31377831

author: Pinörkel
author_rank: Regular Contributor
timestamp: July 31, 2024, 10:13:40 pm
content: OK, time for some feedback. My liteVNA64 finally arrived yesterday and I had some time for trying to fiddle out how to make it work.Getting the Solver64 software to be able to communicate with the device via any COM port was a four hour nightmare. Manually installing the correct CDC driver was easy, but some parts of the NI-VISA or Labview components that are responsible for serial communication repeatedly refused to install right on all three machines, I tried this on. I finally got it working by beating the heck out of some stubborn DLLs.Then I tried to find a suitable measurement setup for the liteVNA, the supplied calibration kit and the four el-cheapo BNC to SMA adapters. Good thing, I ordered those. I put the supplied short SMA cables on the liteVNA and tried to connect everything else there. The calibration was performed with the supplied SMA-F to SMA-F adapter between the SMA cables and the cheap calibration standards. For cable test measurements, I left the adapter in place and connected an SMA-M to BNC-F adapter to attach the BNC cable as the DUT. At it's end, I used a BNC-F to SMA-F adapter to connect the calibration kit load as a 50Ω termination.Measurement setup:Then I played with calibrations for different start and stop frequencies, with connecting different cables, repeatability and SMA connectors. Insight number one: Some of my equipment and my RF measurement skills are crap. Improving the latter is the key to identify how to improve the first.For testing, I used three self made RG58-CU cables, made from the same cable roll using Telegärtner BNC connectors. Two of the cables are of similar length of about little more than a meter and the third one is about 1.5 meters. I measured all three cables in the ranges of 50k-100M, 50k-3G and 50k-6.3G. I am still not sure, which ranges would be the best to use here. At least the measurements were somewhat consistent.50k-100M:50k-3G:50k-6.3G:Could the large spikes at the beginning and the end of the traces be an indicator of the really bad quality of my SMA to BNC adapters or is this just an expected effect of a BNC connection?Regarding the repeatability, I redid some measurements after disconnecting and reattaching the DUT cable from everything. I also repeated some tests with just a redone calibration. I discovered that I had to tighten the SMA connectors with quite a bit of force by hand, to get some repeatability. Just disconnecting and reconnecting the cable sometimes lead to impedance differences of multiple ohms with the whole impedance curve shifted up or down(see image). I am still not sure what caused this. If, for example I had not tightened the double SMA-F adapter at the end of the SMA cable sufficiently or it came loose because of reattaching the DUT, I would have expected to see some kind of local impedance spike, but not a complete up- or down shift of the whole curve. Similarly for the experiments with three redone calibrations, I had some measurements with the same effect while other measurements were very close to each other. Then again, there might have been SMA connection issues on some calibrations.Reconnection repeatability:Recalibration repeatability:If I am correct, this maybe hints at using a 0.5Nm torque wrench to tighten the brass SMA connectors. Someone told me, it should be quite difficult to over-tighten an SMA connector by screwing it on by hand without any tools. So maybe I can get around investing over 350€ in a good quality SMA torque wrench by just tightening the connectors very hard without using any force amplifying tools?I will do some further testing in the next few days.

author: tggzzz
author_rank: Super Contributor
timestamp: July 31, 2024, 10:34:36 pm
content: Quote from: Pinörkel on July 31, 2024, 10:13:40 pmif you only want to do One or two, make your own; it is easy enough.Get an open end ring spanner where the open end fits the SMA. Measure the distance to the ring end, and calculate the force needed for the specified torque (or torques).Get a spring scale with an appropriate FSD, and use it to pull on the spanner's ring end at 90degrees.After using that a few times, you will have.sufficient "muscle memory" that the spring scale isn't so necessary.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 31, 2024, 11:15:56 pm
content: Quote from: Pinörkel on July 31, 2024, 10:13:40 pmHaving used LabView for several years at various companies and different PCs and OSs, getting it to run hasn't really been a problem. I've ran into people downloading random files from NI somehow expecting them to work. Once in a while people have had a different VISA installed and run into problems. I don't know what "beating the heck out of some stubborn DLLs" would mean but sounds like you were able to sort it out on your own. That IMO puts you far ahead of most people who contact me about it. So good job!! There is a newer version that I have not uploaded that would automatically replot the data when you change the TDR memory and attributes. It also changes the average function from a LIFO to a running average. If you want it, let me know and I will upload it.QuoteWelcome to the club.QuoteHigher frequency = increased the length resolution. More data points = longer length that can be measured. (you can see this in your graphs, 6GHz is clearly higher resolution). At 50k, the LiteVNA has some issues but should be alright. Looking at at the data I provided for you, I used 300k to 9GHz, 401 data points, 4kHz IFBW. I suggest that you use my settings as a starting point. You should at least be able to replicate what I have shown you.*** Bonus to you for not telling me how 10 data points increases the length resolutionhamsQuoteThese are your connections. Every discontinuity will cause a disturbance in the impedance. A 20GHz VNA would do a better job showing what is going on there but with the Lite, we can at least get some idea which are better. Yes, BNC connectors are poor. Guessing that is why Tektronix made a custom one.QuoteI couldn't tell you how many times people have written me about how pointless torque wrenches are and how they damage connectors. Oh well.QuoteThose 3D printed brakeover wrenches we made worked well after calibration. The plastic wears fast and you would need to continually check it but they don't cost a lot to print. Used is also an option. You can always check it for calibration and adjust if needed.

author: pdenisowski
author_rank: Super Contributor
timestamp: August 01, 2024, 03:28:18 am
content: Quote from: joeqsmith on July 31, 2024, 11:15:56 pmOh, you absolutely can damage connectors with torque wrenches if you use them incorrectly.But used properly, a torque wrench will help avoid many common types of damage.Quote from: joeqsmith on July 31, 2024, 11:15:56 pmThere are cheap Chinese torque wrenches available from Amazon for about 30 USD. No, they are not metrology grade, but they are definitely better than using a non-torque wrenchOr you could buy quality like these (just posted today, in fact)

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2024, 04:37:53 am
content: Quote from: pdenisowski on August 01, 2024, 03:28:18 amNot as fancy as yours, video showing different printed plastic wrenches with my home made calibrator. Hard to beat the price.Quote from: pdenisowski on August 01, 2024, 03:28:18 amYes, you can damage them as I demonstrate here.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2024, 04:44:12 am
content: CDI's website:http://www.cditorque.com/Pasternak's wrenches:https://www.pasternack.com/click-type-rf-torque-wrenches-category.aspx

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2024, 11:28:42 pm
content: Quote from: pdenisowski on August 01, 2024, 03:28:18 amYou can damage cylinder head studs with a torque wrench if the signalling device fails.Been there, done that, got the T-shirt.

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 02, 2024, 10:45:43 am
content: I 3D-printed a flex-style indicator-needle-type like torque wrench(the thinnest one with the least amount of torque) just to compare the resulting SMA nut tightening with my hand tightening. The results were surprisingly close. Based on that, I also had the impression that it should be quite difficult to over-tighten an SMA connection by only using hand-tightening. I think, I will get an adjustable ~40€ class SMA torque wrench, once I have some more expensive connectors and stick to hand-tightening for now.Apart from that, I did additional experiments regarding my poor repeatability and played around with the sweep settings. With the settings proposed by @joeqsmith (300k to 9GHz, 401 data points, 4kHz IFBW), I got a kind of repeatable offset of ~1.5Ω when compared to my initial 50k to 6.3G settings, which I cannot really explain. If I repeat the calibration with either of the two settings, I get kind of repeatable results with small deviations, but just loading a calibration of the other sweep parameters type (and not even moving anything else) will shift the whole curve up or down starting with the peak produced by the first adapter.Result differences between sweep parameter types:Next, I had a closer look at the SMA-M to BNC-F adapter between the SMA cable and my DUT. Swapping it with the second adapter I have of that type, also produced a nearly 2Ω offset. Additionally, I discovered that with one of the adapters, applying even a little bit of lateral tension on the connection will shift the curve around. The image shows a measurement of the two adapters in green and red and the white curve is the result of applying a small amount of tension on the connection. I would take from this that both adapters have an inaccurate impedance, bad coupling, and one of them is even mechanically defective and will not be used anymore. The respective adapter also had a kind of mushy end stop when screwing it on.Result differences between different adapters:Looking at available SMA adapters at Mouser, I noticed that nearly only the ones of Rosenberger had datasheets with VSWR specifications and torque recommendations for their products resulting in over 40€ per piece(e.g. parts 476-32S151-K00L5 and 476-32K151-K00L5). So, those might only be suitable to buy in small numbers for calibration work or high precision measurements. Then again, I would need some adapters for daily less critical work to protect the expensive ones from unnecessary wear. Maybe the ones of mini-Circuits could be an option, because they at least specify VSWR (parts: 139-SF-BF50+ and 139-SMF-BF50+)I also found this description on how to use several Rosenberger parts as an affordable calibration kit, suitable for sub 5GHz VNAs:https://hhft.de/sma-cal-kit

author: joeqsmith
author_rank: Super Contributor
timestamp: August 02, 2024, 03:57:04 pm
content: Quote from: Pinörkel on August 02, 2024, 10:45:43 amThinnest doesn't tell us much. Materials will play into it. Even knowing that, it still offers nothing. You need to measure the torque then provide that detail.QuoteI wouldn't think you could over-tighten by hand, but you could certainly under-tighten. Want to know, then measure.QuoteIf you like to guess, by all means, fingers work fine. You mentioned early on you have one of the books I read. Have you taken the time to read it? It's a pretty good introduction into how to use a VNA, and is filled with tips. Sadly, I have not found a way to absorb the materials outside of taking the time to read them.QuoteAs I mentioned, the upper frequency effects the resolution of the distance measured. I doubt you would measure a 1GHz clock with a 1MHz scope. You may find the slow scope greatly attenuates the signals amplitude. Its similar when you try and measure very small distances without a high enough frequency. You may find your 60ohm connector measures 50ohm as you lower the upper frequency. You saw this effect in the very first test data you uploaded. At that point you were unclear what was causing the spikes but I explained it was the connectors and that the LiteVNA is not good enough to measure them because it is such a low frequency device. This is explained in that book you have.The lower frequency will get you a better DC measurement. 50k vs 300k shouldn't make too much difference in the context of these tests. As I mentioned, I used 300k because that is the lower limit of my old Agilent and I wanted to provide you with some comparisons between the two. Also as I mentioned, the LiteVNA may start to have some problems at 50KHz. Should be alright but again, I don't think you are gaining much for this particular test by running it this low.QuoteWelcome to the club. I didn't tiewrap that TEFLEX cable to make it look all nice and neat and take up less space. I wrapped it to try and mechanically stabilize it to get a repeatable measurement and to show YOU how important these details are. I am not surprised that different connectors would be more stable. Compound that with hand tightening.... based on your accuracy goals, you have some work ahead of you.QuoteWell, how about we say maybe the connectors will help. I think there are other variables you could tie down before introducing your 3.5mm precision adapters into the mix.I calibrated the LiteVNA64 (latest revision) using three different settings, changing both the start and stop frequencies. The SMA connector measured a peak of 55 ohms with the two 9GHz tests and 53.5 with 6.3GHz. Matching your 1.5Ω difference. Again, maybe it is 57 ohms. We really can't tell without having a better VNA. Consider that home made Beatty standard. It's 70mm!! We can just see it settle. The LiteVNA64 with harmonics can still provide some small amount of data. Why not use it?The coax of course is MUCH longer than the connectors. So now the upper frequency has less of an effect on the impedance and we should get the same values if there were no other variables. The fact yours changed so much tells me there are other errors with your setup.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 02, 2024, 05:39:38 pm
content: If we calibrate using a stop frequency of 4 and 2GHz you can see how the measured impedance of the connector is less. We just don't have the resolution to show what is actually going on but 9GHz is certainly going to provide a much clearer picture than 2GHz.For the two added data sets, I hand tightened the standards during calibration and hand tightened the cable to measure it.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 02, 2024, 05:45:49 pm
content: No change other than loosening the test cable's connector and re-tighten by hand (of course, the VNA cables moved, along with the test cable). That foil tape and sticking things to the table don't seem so silly anymore....

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 11, 2024, 12:39:29 am
content: Sorry for the late reply. I could not access my computer for the last week.Quote from: joeqsmith on August 02, 2024, 03:57:04 pmYes, materials play a role, but the information I was after was not to determine the exact torque of my hand tightening. Over-tightening can damage connectors, under-tightening can only mess up measurements. I just wanted to use the 3D printed torque wrench as an upper bound to check, if my hand tightening force was likely to over-tighten and that information was easy to get by using the weakest version and applying only 2/3 of the force the inventor of this design did.Quote from: joeqsmith on August 02, 2024, 03:57:04 pmI read about 100 pages of the book. But since it was kind of late that night, I will have to re-read it selectively.Quote from: joeqsmith on August 02, 2024, 03:57:04 pmYes there definitely seems to be some kind of error in my setup. The longer I play with this, the more repeatable the issues get. I am suspecting some systematic errors, but systematic errors should be killed by the calibration, according to the book. So, I started to check the basics like instrument warmup times and checked the calibration results.I found the time of 30 minutes that I let the liteVNA warm up to be too short. The S11 noise floor of my device seems to transition into a steady state between 60 and 90 minutes. While checking that, I noticed something strange. The S11 trace on the display of the liteVNA and in the Solver64 software did not match when using the exact same sweep parameters. What I mean by that are not some small noise differences, but qualitative shape differences of the curves and an approximately 10dB lower noise floor on the liteVNA display. I redid the calibration many times to rule out calibration issues to cause this difference, but re-calibrating had no visible effect on this. For cross checking, I downloaded the VNA View Qt software and found that the display there matched the one on the device. So I could simply switch the software while leaving everything else untouched to compare the VNA View and Solver64 software output. The differences were still there, and then they somehow mostly disappeared for the newer calibrations and I could not find the reason for this. From there on the noise floor looked like this:VNA View noise floor:Solver64 noise floor:Another thing, I noticed while using the up to 9 GHz range, proposed by joeqsmith, was that the noise floor went up significantly above 6.3 GHz, which is of course to be expected on this device. However, when checking the validity of the different calibrations, I found the ones going up to 9 GHz to be affected by a lot of noise, when viewing the short, open and load conditions in the Smith chart. With the calibrations up to 6.3 GHz these result in a small dot at the expected positions for short, open and load. With the ones up to 9 GHz the result is more like an angry ball of wool. This can be reliably reproduced by re-calibrating and just changing the upper sweep bound between 6.3 GHz an 9 GHz.Calibration test with load 300k-9G 1x magnification:Calibration test with load 300k-9G 10x magnification:Calibration test with load 300k-6.3G 1x magnification:Calibration test with load 300k-6.3G 10x magnification:

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2024, 01:30:38 am
content: Quote from: Pinörkel on August 11, 2024, 12:39:29 amI tend to move things around when using the equipment. This means the cables can end up with a rotation force. If the connectors are not tight and the cable rotates, the two center pins can have rotation. For the cheap cables and connectors, maybe you don't care too much. Maybe if you start using precision parts, you care a little more. I posted some photos from another website showing some high grade HP connectors that were damaged this way. My advice is form good habits.Quote from: Pinörkel on August 11, 2024, 12:39:29 amDepending how stable you need it, it may not ever be good enough. There was a reason I modified the one LiteVNA to use an external low phase noise clock.Quote from: Pinörkel on August 11, 2024, 12:39:29 amI have not used QT and don't run the LiteVNA stand alone. That said, it's just some basic math and I would not expect there to be much difference if at all. Of course, this assumes that in all three cases, the errors due to the three calibrations are ZERO. My guess is it is a user problem in not knowing all the fine details, some of which is documented in that large software thread. I'll take a stab at it and say you have not changed Solver64's IFBW to match the LiteVNA's default. I mentioned this in that thread as well as a few videos why I use the wider bandwidth and sacrifice noise for speed. Then there is the isolation.My advice is take some time to read more about some of these details so you have a better understanding of what is going on. If you really feel my software has some math problem compared with the LiteVNAs firmware, I can take the time to run a few tests using their CF card (no software). QT I know has at least one math bug as I changed my software to handle importing their Touchstone files.Quote from: Pinörkel on August 11, 2024, 12:39:29 amOf course, you could use a lower IFBW before running the calibration to try and improve it. This is really up to you. The 9GHz is really to squeeze that little bit of extra resolution from the time domain measurement. The dynamic range at 9GHz is maybe 10dB or so. I've shown some data using the LiteVNA in the X-band compared with my PNA. You can see something, sort of, that kind of makes sense.Anyway, good to hear you are making some progress with it.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2024, 03:14:28 am
content: Using the same settings as Solver64 with the LiteVNA64's firmware running standalone, then saving Touchstone to SD card and importing with METAS. I'm not seeing these big differences you claim.I've been told I have math errors before. The last time was when using it with waveguides. Again, it is certainly possible there are problems with it. Actually, I know there are as some features I started to work on never panned out. But that basic software is almost two decades old and is used on a regular basis. I've also frequently compared the results with a completely different VNA, cables, standards, software. Even in this thread. What you are describing would be a major problem.If you do feel there is a problem, you will need to provide enough details for me to reproduce it.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2024, 03:41:11 pm
content: This is the article I mentioned showing some damaged precision connectors.Quotehttps://www.microwaves101.com/encyclopedias/how-to-not-trash-a-calibration-kitLink to the discussion on calibration errors the last time it came up. I ran several tests, comparing the LiteVNA64 with Solver against my Agilent PNA (standalone).https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4797743/#msg4797743

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 11, 2024, 10:03:32 pm
content: Quote from: joeqsmith on August 11, 2024, 01:30:38 amMy measurement setups usually don't go for a walk. I read through most of the interesting stuff you linked onmicrowaves101.com. Now I see why some calibration gear connectors are so delicate. I think, on the cheap liteVNA calibration kit, the most critical factor why not to rotate the pin during insertion would be gold coating abrasion. The female center contacts there consist only of two simple halves. However, I was surprised about the good quality of the two SS405 50cm SMA cables that came with the device. I already tried to find where to get more of those, but had no luck finding the exact type. There are some that look really similar, but the connector quality is much better on the supplied ones, than on the ones I found online.Quote from: joeqsmith on August 11, 2024, 01:30:38 amBingo. I found no way of changing the IFBW in the liteVNA or the VNA View Qt software, and was not able to find documentation about the liteVNAs default setting, but setting the IFBW in Solver64 to 1k produced very similar results.VNA View Qt result:Solver64 result with 4k IFBW:Solver64 result with 1k IFBW:Quote from: joeqsmith on August 11, 2024, 03:14:28 amI did not intend to imply that there was something wrong in Solver64. I just had the impression that the signals were processed in another way before being displayed. This turned out to be true: I did not set the right IFBW values to get comparable results. Regarding repeatability, I think I am on a good way to finally gain some trust in my measurements.The next step will be to decide what additional gear I need to make some useful cable measurements. At the moment, I have two main construction sites:1. I have only cables of the exact same materials, regarding cable and connectors. I will need to build some different cables for making comparisons.2. My current set of adapters is bad. To fix that, I will need to reduce the amount of adapters to an absolute minimum and find a calibration setup that eliminates almost everything except my BNC cable DUT from the setup. If I use the current calibration kit, I will have at least one unnecessary SMA-F to BNC-F adapter in my soup. To eliminate that, I could get a SMA-M to BNC-F pigtail for putting it between the liteVNA and the test cable. Calibrating that out would require me to build a BNC-M calibration kit, although I already found anarticleproving that BNC connectors make poor calibrations standards, it may perhaps be sufficient for my application. The alternative would be to use the liteVNA cal kit or some better SMA calibration standards, that have the disadvantage to force the inclusion of an additional adapter into my final measurement. The last component to get would be a good quality BNC-F 50Ω terminator to put at the end of the cable. The Fairview ST3B-F looks quite decent. Too bad I could not also use that as the calibration load without using an additional adapter.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2024, 02:12:14 am
content: Quote from: Pinörkel on August 11, 2024, 10:03:32 pmThey may not be gold. Anything of value would be very thin so expect a short life. I check these cheap connectors under the microscope before using them. One little defect and that rotation becomes more than a wear problem. Again, good habits = longer life = repeatable results.You may be able to buy the OEM cables direct from them.Quote from: Pinörkel on August 11, 2024, 10:03:32 pmSolver performs all of the processing and expects raw data from the VNA. For better or worse, it typically does what you program it to. Because you are playing with at least three different ways to collect the same data, my advice is not to fall into the trap where you expect Solver to work like any other software. That's caused a few people to have problems.***Quote from: KedasProbe on December 31, 2023, 05:27:58 pmhttps://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5249997/#msg5249997

author: pdenisowski
author_rank: Super Contributor
timestamp: August 12, 2024, 10:29:37 am
content: Quote from: Pinörkel on August 11, 2024, 10:03:32 pmThe quality of the pigtail (and especially the BNC part, as you mentioned) then becomes a potential issue: I'm not sure how phase (or, frankly, amplitude) stable any commercially-available SMA-BNC pigtail would be. This is where tape becomes handy

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2024, 01:38:24 pm
content: QuoteThey are available along with a gage kit.https://www.maurymw.com/wp-content/uploads/2023/11/2Z-069.pdf***One option may be to replace the custom connector from the equipment with something more common.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2024, 05:52:20 pm
content: LiteVNA64 (latest hardware) with supplied cable. Cable fitted with SMA F/F Amphenol PN# 132169. Cal standards applied to the end of the adapter. Again, ideal model was used. When testing the BNC adapters, the standard load (sorted Mini Circuits ANNE) was attached to an Amphenol PN# 24103 SMA F to BNC M. This device is rated to 4GHz. The IFBW set to 2k, sweep range 50k to 4GHz for all tests. Three different combinations were measured. From the photo, A+B, A+C and A+D. B is made up of two Kings adapters. The BNC F/F is silver plated. C&D are Amphenol adapters.I would expect you to have similar results even with your cheap adapters.

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 16, 2024, 07:58:04 pm
content: At the moment, my current lousy set of stuff unfortunately only allows to compare two combinations of adapters this way: The liteVNA calibration load on the calibration set SMA F/F and an ultra cheap SMA-M to BNC-F with a fitted ancient BNC 50Ω termination of unknown quality. I need to get at least a few half decent adapters BNC-BNC, SMA-BNC and some 50Ω terminators. Unfortunately, Mini-Circuits seems to be kind of the only reasonably priced supplier that is not afraid to provide datasheets with electrical properties like return and insertion loss. However their BNC related stuff is mostly rated only up to 2GHz.SetupMeasurementbtw.: If you ever come across wanting to visualize Solver64 data with LibreOffice you may find this monster useful, to convert the postfix letter style decade format into something, Libre Office can understand:Code:=IF(ISNUMBER(VALUE(RIGHT(A2)));A2;LEFT(A2;LEN(A2)-1)*10^(IFERROR(FIND(RIGHT(A2);"kMGTP")*3;IFERROR(FIND(RIGHT(A2);"munp")*-3;0))))

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2024, 07:41:41 pm
content: I have two BNC 50 ohm terminators. All of the ones I have inspected use an axial part. Shown after SOLT, then attaching SMA / BNC adapter and BNC loads. Similar results to what you measured with your setup.I imagine that the cables mating connector on the test equipment is also custom made to get the return loss they were looking for. I assume also that you have no plans to swap it for a standard BNC or different type all together. I wonder if mixing the parts will cause other issues.

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 17, 2024, 09:57:08 pm
content: Quote from: joeqsmith on August 17, 2024, 07:41:41 pmWhile your terminators and mine seem to behave extremely similar, the datasheets of the Mini-Circuits BTRM-50+ and the Fairwave ST3B-F seem to perform much better (I have drawn the curves into my measurement sheet for comparison).Quote from: joeqsmith on August 17, 2024, 07:41:41 pmThere is indeed something special regrading the BNC female connectors on the critical equipment ports. They were made by Specialty Connector Co. and do not have a solder connection at the back. Instead, a miniature Tektronix Peltola coax connector can be fit directly into the back of the BNC female in order to have a fully shielded RF path with minimal impedance changes. However I do not know, if they also optimized the impedance value in some way. Those connectors can be found in a lot of Tektronix 500, 5000 and 7000 gear.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 18, 2024, 01:00:05 am
content: I understand the problem with swapping it out now.You show a M and F terminator. I assume you want a female. This part is spec'ed to 4GHz with a return loss of 20.82. So slightly better than the Mini-Circuits part at roughly 3X the cost.https://www.pasternack.com/images/ProductPDF/PE6256.pdfThe male version is also spec'ed to 4GHz with a return loss of 23.15.https://www.pasternack.com/images/ProductPDF/PE6TR008.pdfOf course, there was the Maury Microwave kit. Or if you just want a load:https://www.fairviewmicrowave.com/content/dam/infinite-electronics/product-assets/fairview-microwave/product-datasheets/FMTR1064.pdf***Not seeing something like this being any value but it is inexpensive.https://www.ebay.com/itm/155393728567

author: Pinörkel
author_rank: Regular Contributor
timestamp: August 31, 2024, 10:43:27 pm
content: Sorry, the purchase of a calibration kit costing several thousand euros is unfortunately off the table, as it is completely disproportionate to its potential future use. I did not know that there are pre-assembled china calibration kits like that. However, I already thought to buy some of these PCB BNC connectors, since they seem to have a comparatively well defined reference plane and could be a good base for experimenting with building an own BNC calibration kit.Apart from that, I got disabled by a corona infection and I am simultaneously loosing my mind trying to get test equipment here in Germany. It is apparently near to impossible to buy specific stuff from Pasternak, Fairview Microwave and other suppliers here, because there are no distributors that deliver to Germany. Or, they do not sell to consumers, or only if you buy part counts larger than 100... In addition to that, German ham radio shops seem to be a concentrated accumulation of incompetence. I tried to buy some cabling and connector stuff: The first shop tried to sell me custom made H155 50Ω cables with 75Ω connectors, insisting that they were 50Ω. The second one sold me crimp connectors with missing center pins and shipped significantly less cable length than what I payed for, the third one packed the wrong cables and stuff I did not even order... long story short: It will take much longer to get hold of a few test cables and adapters than I originally imagined.At the same time I was able to play with the Solver64 software a little more. I stumbled onto some things, I am still trying to understand.One thing is the step and impulse diagrams in the time domain. As far as I understand it, those should be convertible into each other by means of integration and differentiation. I had the impression that the step response had kind of a delayed integration reaction to the impulse response, as if there were a horizontal offset, but that may well just be my imagination, playing a prank on me.Regarding calibration accuracy, I found some calibration kit constants for the liteVNA calibration kit on the web an tried to enter them into Solver64, but I could not really match the information with the Solver64 input fields for that. For the male load, I could e.g. not find input fields for R, L and C in Solver64.https://groups.io/g/liteVNA/topic/calibration_kit_data_for/102476618For checking calibration quality, I read that I could maybe have a look at how to do a T-check. For this I would most likely only need an SMA all female t-piece.Also in the main tab of Solver64 I potentially stumbled upon a minor GUI bug (or maybe this is intentional). In the graphical views, there is the possibility to deactivate auto scaling for the X and Y axis via the context menus, which I find quite useful for looking at zoomed portions of the graphs. While this works perfectly in the time domain graph in the advanced tab, for the graphs in the main tab (like Impd Rectagular and Reflection Coeff), the context menu is missing the check marks in front of the auto scale entries. They also do not appear when switching auto scaling on and off for the vertical axis. At the same time, horizontal auto scaling is reset to auto for every sweep. So, for looking at a horizontally zoomed portion of the graph (e.g. 0 to 500M out of 0 to 4G), one needs to disable the sweep first, or all manually entered values will get sweeped away.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 12:30:34 am
content: I would have thought in Germany you would have access to all basic components like cable and connectors.Solver 64 uses the same coefficients HP uses. My attempts to make a T-check have not worked out.You don't want a T with an added stub and connector.I think I have documented some of that in the NanoVNA custom software thread.The main graph does allow you to disable the vertical autoscale. I normally just set the sweep range for what I need. So, locking out the horizontal manual scale was intentional but it could certainly be changed.***Future Joe here. From on-going tests using a standard OTS coaxial T adapter and load to create a T-check, I am seeing good results to 9GHz. I have no reason to believe this technique could not be used at higher frequencies.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 01, 2024, 01:49:00 am
content: Quote from: joeqsmith on September 01, 2024, 12:30:34 amIt usually is, IF you try to get stuff as a company or if you want to get jellybean stuff. Getting specific high quality stuff from outside the EU, especially from the US or UK can be very troublesome or be associated with ludicrous shipping costs, if you are just a lowly non-commercial customer. We have a really well sorted supplier for all kinds of of custom-made cabling and cable components here. It just does not sell to private customers.Quote from: joeqsmith on September 01, 2024, 12:30:34 amInteresting paper. The inconsistencies I see are most likely due to the tendency of my brain to reject math stuff without a good visualization. Nonetheless I have captured an image of what I mean:To my understanding the white step response should be largest, where the gradient of the red pulse response is the highest, which in this picture is clearly not the case. Looking at the horizontal sampling point distance of the curves I have the impression that the red curve should be shifted right by one sampling interval which could be an off-by-one index error in the software or just my brain hallucinating bogus. ;-)Quote from: joeqsmith on September 01, 2024, 12:30:34 amAh, you mean the PCB-style T-check fixtures? I just read from someone that a T-check could be performed with really simple materials and you do not even need a high quality load. However, I do not know whether that is true.Quote from: joeqsmith on September 01, 2024, 12:30:34 amAh, ok. I will try that, but does this not require frequent re-calibration, because the sampling points are no longer at the same spots?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 02:13:58 am
content: QuoteAttached was taken from the paper I previously suggested.I had included a through termination on the cal boards but these can't be used for very high frequency. The last one I made was built without any circuit board. It performs better but still was not very useful.I keep things fairly well organized and typically reload previous calibrations. The software does provide interpolation as well.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 01, 2024, 02:38:00 am
content: Quote from: joeqsmith on September 01, 2024, 02:13:58 amYes, your picture shows exactly what I mean: there, the peaks of the pulse response are exactly at the horizontal positions where the slope of the step function is maximal/minimal. In the screenshot that I took, this does not seem to be the case.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 03:35:21 am
content: Quote from: Pinörkel on September 01, 2024, 02:38:00 amSadly, as I have stated previously the LiteVNA does not have enough BW for decent resolution. Shown is looking at the Beatty standard Touchstone file included.

author: G0HZU
author_rank: Super Contributor
timestamp: September 01, 2024, 12:20:13 pm
content: Quote from: joeqsmith on July 10, 2024, 07:22:51 pmHaving read through the thread again in detail, it looks like joeqsmith and pdenisowski tried to measure a ~50 ohm cable with a 50 ohm load at the far end.This is not wise, as you will just end up exploring the uncertainties of the test setup if the test load is very similar to the Zo of the cable. It looks like they both used short cables so the first alarm bell should have rung when the crossover point was not at the quarter-wave frequency of the coax cable. They both measured at frequencies that were way too low. 1.7MHz and 8MHz? This is obviously not the quarter-wave frequency of the test cable unless they actually used a really long run of cable?You are meant to use a test resistance that is some way away from the Zo of the cable. If this means trying several resistances until you find a suitable resistance then so be it.Maybe read up on a bit of theory about how transmission lines can act as impedance transformers and focus on the special case of the quarter-wave transformer.For a 50 ohm cable, try using something like a 25R test load at the far end. You can also use something like 100R and just look for the first crossing point on the smith chart.Normally, if someone wanted to match R1 to R2 they would use a quarter-wave transmission line that had a Zo of sqrt(R1*R2) ohms. However, the idea behind the Zo test method is to just exploit the equation to give Zo because you can define R2 as your test load and then use the VNA to measure R1 at the first crossing of the real axis of the smith chart.A typical RG58 cable or RG223 cable that is about 3ft long should be a quarter-wave long at about 55MHz. This is roughly where the first crossing should take place for the special Tektronix cable.Note that the Zo will change with frequency, and if you are trying to measure it to nerdy levels of accuracy, then the Zo will be different at (say) 1MHz, 10MHz, 50MHz and 250MHz. The Zo for a typical 5mm diameter coax cable might be about 52 ohms at 1MHz and the Zo might be closer to 50 ohms up at 250MHz for example.Note also that the quarter-wave method will have a slight error because of the finite loss in the cable. You can minimise this to some degree by using a test resistance closer to 50 ohms. If you use 39 ohms as the test resistor, then I think the Zo result will be more accurate. However, if you use a test load that is too close to 50 ohms, then you will end up exploring the uncertainty of the whole test setup and you will get flawed results.

author: G0HZU
author_rank: Super Contributor
timestamp: September 01, 2024, 02:31:07 pm
content: To show how Zo changes with frequency, I put together a model of a short run of Suco141 cable and measured the Zo from 100Hz through to 100MHz. I've plotted it using log scales as the Zo increases quite a bit at low frequencies. It gets as high as about 830 ohms down at 100Hz.The model is a physical model that contains the dimensions of the conductors and also the resistivity of the conductors and the details for the dielectric material used. From this it is able to model the skin effect and the loss in the coax vs frequency.The graph below shows Zo and also the real and imaginary parts of Zo vs frequency. To get it all on one graph I've taken the absolute value of Zimaginary. It's actually negative (capacitive) but it fits nicer on the graph if I convert it to the absolute value.At 1MHz Zo is just over 52 ohms and it gradually reduces to about 50 ohms by 100MHz.

author: G0HZU
author_rank: Super Contributor
timestamp: September 01, 2024, 02:39:49 pm
content: In the plot below, I've measured Zo for a very short run of suco 141 cable using an original 2.8" nanovnaH.I measured Zo from 500kHz to 50MHz and the results look OK to me. It looks to be very similar to the model. The nanovnaH loses some performance above 50MHz, so I stopped the test at 50MHz. However, it probably would have given good results to 100MHz or so. I rarely use the nanovna above 50MHz.I've plotted Zo and Zreal on the left scale. Zimaginary is on the scale on the right this time and this shows that it is negative (capacitive).Zo at 1MHz is just over 52 ohms and it drops to about 50.4 ohms by 50MHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 04:09:23 pm
content: Quote from: G0HZU on September 01, 2024, 12:20:13 pmI can't agree more with your post. My goal was to repeat what was presented in the two videos produced by Tektronix and R&S, right or wrong. No doubt they should have stated using a mismatch but instead specifically call out 50 ohms. I am not sure why. Having the mismatch would easily show the rotation and crossing vs working in the muck. As you said, there is a limit. Using an open for a load is going to be difficult but for a 50 ohm cable, even 25 ohms would be much better than the 50 they call for.Yes, Zo changes with frequency. It should be obvious at DC we are dealing with very high impedance's.Thanks for posting.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 08:50:59 pm
content: For the case where the load is a short, Zl = 0. Zero times anything = 0. The square root of 0 = 0. So for a short, Zo = 0.Considering Zl = infinity where infinity times any non zero = infinity. The square root of infinity = infinity. So for an open, Zo = infinity.I would think we could measure the cables actual impedance using these two extreme cases but rather than Zl, we would use the impedance at the lowest test frequency.Showing both extremes with the section of TEFLEX cable, with the coffee bag ties, using the impedance at 300kHz and the first crossing point.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 08:59:18 pm
content: Using the a 25 ohm terminator at the end of the TEFLEX cable rather than the 50 mentioned in both videos, we can now see the actual crossing point. We measure 50.69. Much closer to the TDR measurement using the Agilent (white) and LiteVNA (green).

author: G0HZU
author_rank: Super Contributor
timestamp: September 01, 2024, 10:36:08 pm
content: By doing a bit of mathematics with complex numbers, it's possible to estimate Zo by calculating Zo = sqrt(Zoc*Zsc) at each test frequency. That's how I did it using the nanovnaH and also with the physical model of sucoform 141 cable. This method is ancient and dates back over 100 years.However, when measuring real cables it's best to use a really short cable if you want to do this for VHF or UHF frequencies as it isn't possible to fit a perfect short and open at the far end of the cable. Because of this, there will be a blip or glitch in the results at the quarter-wave frequency and also at higher multiples.A typical 50 ohm coax cable that is about 3 feet long will give a glitch up around 55MHz for example.

author: G0HZU
author_rank: Super Contributor
timestamp: September 01, 2024, 10:56:15 pm
content: It ought to be possible to predict the performance of the Tektronix cable if it uses (double screened) RG223 cable. The insertion loss of a 1m section of genuine RG223 is about 0.22dB at 250MHz according to the datasheet.This just over 3 feet long, so if you factor in the connector losses the insertion loss of the cable is probably going to be about 0.25dB at 250MHz. It might be a bit higher than this but it will probably be less than 0.3dB.A similar cable made with good quality RG58 will have a loss about 0.05dB greater than this. Some of the loss is due to the inferior screening and some will be due to the stranded centre conductor. Lower cost versions of RG58 (eg for Ham or CB radios) will have a bit more loss. A cheapo cable that is quite flexible will have poor screening and increased metal losses. It might show 0.35dB loss or even 0.45dB loss at 250MHz for example.Good quality connectors will be needed, and even then there will be some risk of variations in output level if testing something old like a 1970s scope because of the connector wear on the BNC ports of the scope.The harmonic distortion from the SG503 will also contribute to uncertainty in the output level (especially odd order harmonics). For example, a third harmonic at -40dBc can cause up to about +/- 0.09dB uncertainty in the output level. This is about +/- 1% in terms of Vrms.The accuracy spec of the SG503 isn't that great either, especially above 100MHz where it is spec'd at 3%. So it probably isn't worth getting too serious about this stuff.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2024, 10:59:56 pm
content: Getting back to your questions,Quote from: Pinörkel on August 31, 2024, 10:43:27 pmBeyond not having enough data points, looking at my software I have been using a trapezoidal integral rather than Simpon's which would cause some small errors. I doubt you would see a difference but I have gone ahead and changed it.Quote from: Pinörkel on August 31, 2024, 10:43:27 pmI have a note in my software about this. Originally both the X&Y autoscaled. After I made the software public, someone was measuring attenuators and was wanting to disable the vertical autoscale. I had added that for them but not the horizontal. I have gone ahead and added that while I was looking. Let me know if there is anything else you want me to look at. If not, I'll go ahead and release this.***New release is available with the mentioned changes.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 02, 2024, 12:41:45 am
content: Quote from: joeqsmith on September 01, 2024, 10:59:56 pmAh, so the step and impulse responses are not both calculated independently from the S parameters, but one is calculated from the S parameters and the other one derived by numerical integration. As far as I remember it, trapezoidal really shines for periodic integrands and odd numbers of function evaluations and Simpson is better on even number of function evaluations.Quote from: joeqsmith on September 01, 2024, 10:59:56 pmThank you very much for addressing this. This will be really helpful. I better do not start looking for GUI issues. If I do, I will find a lot of odds and ends (its one of my occupational diseases, related to my computer graphics and data processing origins), and it is not your duty to play repair monkey for someone else. You are incredibly helpful without that already.Edit: I just tested the new release and turning off the horizontal auto scale works in the main tab. :-) However, the check marks on the menu entries in the axis context menus are still not updated. They initially show no check mark, when the auto scaling is on and do not change, even when switching back and forth multiple times. Also, the time domain graph now defaults to horizontal auto scaling being switched off with some strange initial values. However, here the auto scale status is correctly reflected by the check marks.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 02, 2024, 01:13:27 am
content: Correct, the step is derived from the impulse by performing the integral, then its just Z=Z0(1+S11)/(1-S11), where Z0 is the normalized value (50 ohms).https://unacademy.com/content/gate/study-material/chemical-engineering/integration-by-trapezoidal-and-simpsons-rule/Yes, the checkmarks will not work with the main XY graph like they would with all other standard graphs. You basically have a toggle mode as a work around. Thats the nature of the beast. I've had a few people ask about scaling the entire program to support different screen sizes. Part of the problem with this is how that front graph works. Way back when I wrote this thing, Labview had no support for Smith charts and such. While they now have some support for Smith which will autoscale, I don't like how slow it is. So for now, you are stuck with something I put together 20 or so years ago.I could have very well changed some of the starting defaults. I'm pretty bad about doing that. Some of the defaults I have saved to the defaults file, but not all.

author: G0HZU
author_rank: Super Contributor
timestamp: September 02, 2024, 12:59:48 pm
content: Quote from: joeqsmith on September 01, 2024, 08:50:59 pmI'm afraid that you aren't going to get much joy if you combine data at different test frequencies because of skin effect for one thing. Skin effect will result in different conductor resistances at each frequency. Also, it's risky to trust the quality of an open (or a short) up at the quarter-wave frequency up at VHF. So combining data at 300kHz and 45MHz isn't going to give useful results.It's much easier and better to measure the cable twice with a VNA set up for an S11 measurement. Once with a decent short at the far end and once with an open at the far end. It's best to add a cal kit open and short in each case as this will give very close to the same electrical length in both cases.Then at each frequency point compute mag(Zo), re(Zo) and Im(Zo) using Zo = sqrt(Zoc*Zsc)This should give valid data up to frequencies that are just below the quarter-wave frequency. Close to the quarter-wave frequency, the data will have errors because it isn't possible to have a perfect open or short at the far end of the cable. So the equation will give poor results here. However, it is possible to add a delay to one set of results and this delay might be a picosecond or so. This can account for any delay offset between the open and short. This can mute the glitch in the results at the quarter-wave frequency and give a smoother Zo response here.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 02, 2024, 02:46:48 pm
content: Quote from: G0HZU on September 02, 2024, 12:59:48 pmTime domain combines several test frequencies. Plus we introduce other concepts like gating.Quote from: G0HZU on September 02, 2024, 12:59:48 pmThe point of that little exorcise was mention what happens with the formulas presented in the two videos at the endpoints. Of course, you could never achieve these conditions as obviously nothing is perfect, open/short or anything else. Even thinking the standards would have the same delay is flawed. While using the impedance at 300k does get us much closer than using infinite and 0, its obviously not something that would be used.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 02, 2024, 03:07:42 pm
content: Quote from: Pinörkel on September 02, 2024, 12:41:45 amI have gone ahead a set that back to auto and changed the default start and stop frequencies to the range of the LiteVNA.***Member RHB introduced me to ego pimping and padding. I put both to use.Around this post, you will find more details on the time domain:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg2682576/#msg2682576

author: gf
author_rank: Super Contributor
timestamp: September 02, 2024, 08:39:19 pm
content: Quote from: joeqsmith on September 01, 2024, 10:59:56 pmYou can of course zero-pad in the frequency domain (before doing the IFFT) to interpolate in the time domain. With increasing resolution in the time domain, the integration method becomes less and less important.EDIT: That results not only in a higher resulution in the time domain, but also in a proper sin(x)/x interpolation, while e.g. trapezoidal integration calculates the integral as if there was linear interpolation between the points, and Simpson is not equivalent to sin(x)/x either.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 08, 2024, 06:55:35 pm
content: @G0HZU and @joeqsmith: Thank you very much for all your incredibly interesting and helpful input on this topic. At the moment, I do not understand all aspects of your posts, but I will try to reproduce some of the measurement ideas, once I understand how and I have the materials to do so. Non-50Ω-loads are kind of difficult to get here, so I will try to build some DIY ones. Those will not be of good quality, but maybe they will be good enough to learn something. Unfortunately, I am still waiting for my ordered experimentation parts, which seem to have gone lost during shipping.Apart from that, I tried to get something useful out of playing with the liteVNA calibration standard characterization values, I found online, which I linked earlier. Regarding this, I have a question: Assuming the liteVNA standards have parameters different from the ideal standards, and one would precisely measure and enter those parameters in Solver64 and then perform a SOLT calibration. What would be the expected effect on a reflection coefficient measurement with just the calibration load connected, like the one I made inthispost(green curve)? How would that possibly differ from a result obtained with faulty calibration standard parameters?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 09, 2024, 12:49:00 pm
content: Quote from: Pinörkel on September 08, 2024, 06:55:35 pmQuotehttps://www.microwaves101.com/encyclopedias/how-to-not-trash-a-calibration-kitIf you use a bad standard and/or wrong coefficients to cal the VNA and you measure that same part, I am expecting very good return loss. The VNA doesn't know the difference. If you wanted to measure actual return loss of an unknown device, you need to cal the VNA with a known standard.***https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5206677/#msg5206677

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 12, 2024, 09:40:39 pm
content: Quote from: joeqsmith on September 09, 2024, 12:49:00 pmThat is approximately what I would have expected, based on your good explanations in this and other threads. However, I got a kind of unexpected result, when entering the liteVNA calibration standard characterization values, I found online. I know that those could as well not be closer to the real parameters of my liteVNA calibration set than the ideal parameters Solver64 assumes by default. The effect I get is the following:I calibrated my warmed-up liteVNA in Solver64 using the supplied calibration set multiple times and did so with the ideal calibration values active and the set of values, I found online for the liteVNA calibration set active. In both cases, the results in the smith chart and the reflection coefficient measurement were practically indistinguishable, when conducting the measurements while the ideal calibration values are active in Solver64. I consequently got the impression that the calibration does not depend on the calibration standard values that are active in Solver64 during the calibration (I may be wrong there). Interestingly, if I switch to the set of non-ideal calibration values for the measurements, the reflection coefficient looks, like it is getting linearized to a target value of -40dB (see image). The better(less noise) the calibration is in terms of the smith chart when checking with the short, open, and load conditions, the closer the curve gets to -40dB. Now, regardless of whether this measurement is nonsense, I would like to understand the reason that is causing this. Is there any obvious explanation for the observed effect?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 12, 2024, 11:34:26 pm
content: Post a screen shot of your settings. Not the report. I want to see exactly what you are doing.QuoteThis is correct. During calibration, Solver64 disables calibration and collects the raw data for each standard. You can average but raw meaning no coefficients are applied. The raw data is then combined with the selected cal term coefficients to form the error terms. These error terms are then applied to each subsequent measurement.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 13, 2024, 06:16:03 pm
content: QuoteFrom:https://www.kirkbymicrowave.co.uk/Support/FAQ/How-do-I-verify-the-calibration-kit-is-working-properly/

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 13, 2024, 07:02:36 pm
content: I just identified the value I entered, which triggered the observed -40dB linearization behavior. I thought, I had to enter the precise resistance of the load in the Z0 setting of the load (which is of course wrong, since it is an impedance, not a resistance). So, I put 51,06Ω there, which then triggered the described behavior. Just entering e.g. 53 as the Z0 parameter for the load in the Custom Ideal Standards set can trigger the same effect. The more the value differs from the other Z0 values for the open, short and thru, e.g. 50Ω, the worse the return loss will get.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 13, 2024, 07:35:13 pm
content: I take it that you don't expect this. Maybe you could explain what you are thinking.***Check out the following recent thread discussing using a terminator as a load standard.https://www.eevblog.com/forum/rf-microwave/mini-circuits-karn-50-18-as-the-reference-termination-for-a-vswr-bridge/

author: gf
author_rank: Super Contributor
timestamp: September 14, 2024, 01:14:37 pm
content: Quote from: Pinörkel on September 13, 2024, 07:02:36 pmI'm not sure I understand what you are after.1-port SOL calibration basically works as follows: You have three standards withknownS11(f). The VNA's calibration procedure mesures their response, and you tell the VNA the known ground truth S11(f) values of your standards. Then the VNA can calculate the error correction terms for subsequent real measurements. It does not matter if the load standard is exactly 50 ohms or not -- you just need to know itsactualS11(f). The same applies to the short and open standard as well. They do not need to be ideal (you cannot realize ideal standards anyway), but you only need to know their actual S11(f).If you tell the VNA that your standards are ideal (although they are not), then you have to live with the resulting error. This may be good enough at low frequencies if the load standard is very close to 50 ohms, but at UHF or GHz frequencies every millimeter of length begins to matter, and parasitics play more and more a role.Instead of dealing with tabulated S11(f) of the standards directly, usually a parametric model with a low number of parameters is fitted to the measured S11(f) of the standards in order toreduce dimensionality, which also aids noise reduction and enables re-calculation of S11(f) for arbitrary frequencies from the model parameters. In this case you have to tell the VNA the (known) model parameters of each standard instead of S11(f).EDIT: If you calibrate the VNA using the actual model parameters of the standards and then measure the load standard (with calibration applied), then the VNA is expected to display the actuall S11 of the load standard.OTOH, if you tell the VNA that your standards are ideal (although they are not) and then measure the load standard (with calibration applied), the the VNA is expected to display S11=0, even if the true S11 is different. Btw, note that you'll never see S11=0 in practice, due to noise and uncorrectable residual errors. IMO you can interpret your green curve as S11=0 + some residual errors + noise.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 14, 2024, 06:40:20 pm
content: @gf: Thank you for your good explanations. This is approximately what I already figured out based on the input I got in this thread.Quote from: gf on September 14, 2024, 01:14:37 pmIt seems like I am not very good at explaining my intentions behind my little experiments in this matter. So, I'll try again: For using the liteVNA, I have only the supplied calibration standards, which seem to be of bad quality, according to the most likely well founded opinion of several people here. Buying a good set of calibration standards would be nonsense in this specific case, since those cost a small fortune and my application case and usage frequency would in no way justify spending that much money. Additionally, I am after measuring BNC cables, which are known to not have very good HF capabilities. I also do not have the possibility to measure my crappy calibration standards on a good and calibrated VNA to overcome this issue. Then I found someone online who did exactly that and measured the liteVNA calibrations standards with a good VNA. Said person also provided the measured offsets and error correction coefficients for free. So I thought it could be a good idea to use those values as my calibration parameters, since I have the same type standards. I further thought, that most of those parameters (except the load resistance) should result from the physical geometry of the standards and should consequently be very similar in my actual calibration set. At least more similar, than just assuming ideal parameters. So I tried to match the provided values with the input fields of the Solver64 software, made an error while doing so, and got this, at least for me, unexpected effect of linearization, which I could not explain. However, I was curious what exactly caused this.At the moment, I have the theory that a deviating value of the calibration constant for Z0 of the load somehow causes the frequency invariant noise floor to be amplified proportional to the deviation, which then eats up the real S11 curve and results in kind a linear curve as the noise level rises all over the spectrum.

author: gf
author_rank: Super Contributor
timestamp: September 15, 2024, 07:00:58 am
content: S11 = (ZL-Z0)/(ZL+Z0), where Z0=50 ohms.The expected value of S11 depends on ZLalone. Consequently, if your load's ZLwould be (say) 51 ohms (independent of frequency), then its S11 would be (51-50)/(51+50)= 0.009901 or about -40dB (also independent of frequency). Only the deviation from the expected value (ZL-Z0)/(ZL+Z0) is caused by noise and residual errors which cannot be calibrated out.This was just an example for an purely ohmic ZL. Eventually, ZL(f) can be frequency-dependent and complex-valued and is determined by the model parameters you enter for the load.EDIT: When the calibration is applied, it attempts to adjust the raw readings so that the VNA displays S11(f) = (ZL(f)-Z0)/(ZL(f)+Z0) when you measure the load, where ZL(f) is calculated from the model parameters you entered. You cannot figure out if the true value of your load deviates from ZL. The calibration procedure relies on the model parameters you enter being consistent with the calibration standards you are using.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 15, 2024, 02:21:27 pm
content: Quote from: Pinörkel on August 31, 2024, 10:43:27 pmQuote from: Pinörkel on September 01, 2024, 01:49:00 amShowing FR-4 insertable T-Check next to the last coaxial one I made. You could try and put something together from OTS parts like shown but I don't think you would find adapters to get the phase matched. A non-instertable setup like I had demonstrated with the LiteVNA and transfer relay would avoid that problem. The coaxial one was tested on the PNA using a set of Agilent standards for a reference.For the math side of things, I suggest downloading the old HP application notes and start reading. There is a treasure trove of information out there and it is all free. Just be aware that these are not the gospel and I have caught errors in them. I would have thought that book you purchased would have dove into the math as well.***Showing homemade T-Check with transfer relay and original NanoVNA. This would have been FR4 standards with ideal model.https://youtu.be/GJNMnq8eD0E?t=2433The V2Plus4 with a mechanical transfer relay and FR4 T-Check. Both cases, it looks like I ran it to 50MHz was all and results were not great. When shown with the Agilent, we are using the proper coefficients. I would imagine if I derived the coefficients for the V2Plus4 standards, then measured the T-Check, it would have greatly improved.https://youtu.be/XaYBpPCo1qk?t=2173

author: joeqsmith
author_rank: Super Contributor
timestamp: September 17, 2024, 03:18:00 pm
content: Just to add to GF's post, or further confuse you... Ignoring the math, if you tell the VNA an imperfect load is perfect, the VNA will show it as perfect. If you tell the VNA it is not perfect, it will show it as imperfect. Even is the load is perfect and you tell the VNA it is imperfect, the VNA will show it as imperfect. You can really see this with high reflection standards (short and open) as rather than a dot, you will see the rotation.If you cal a perfect standard, and tell the VNA it is perfect, then measure an unknown, you will get the actual value.If you cal an imperfect standard and you tell the VNA it is perfect, now if you measure a part that is perfect, it will show it has errors.If you were to cal using say a 1Mohm resistor for your 50 ohm load cal standard but you know the coefficients for it and enter them into the VNA, what will happen when you try and measure a 50 ohm load??***I think if I wanted to try and improve the T-Check on the LiteVNA 64, we would need to:Start with a non-insertable setup so we can use a standard T an load for the T-Check.Use a known good setup to characterize the LiteVNA's standards. This would mean using a high quality set of adapters to connect our low quality standards to.We would also want to validate the T-Check's performance with that same setup.Then it's just a matter of using the calculated coefficients for the low quality standards, running a full 2-port calibration (with the transfer relay...) after locking down the cables (tape). Then measuring the T-Check. Looks like it was pretty poor even at 500MHz in that one video.As far as transferring the coefficients to others, hard to say. The ones I received are not marked like the ones you show in that paper, making me suspicious. Even if they did look the same, how tight are they controlled?***40&42 showing the standards that were supplied with the latest LiteVNA 64 revision 64-0.3.1.44 showing the standards supplied with the latest and previous standards supplied with the LiteVNA 64 and also the standards supplied with the the V2Plus4. The two sets of standards were modified with end caps that allow me to lock the center conductors in place with a wrench.The DCR for the most current load is 51.057 ohms. The previous measures 50.890. I think to try the T-Check, I would want to select something better to use with the T.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 17, 2024, 11:18:09 pm
content: I finally got my first delivery of experimentation material, which is some Belden H155 PE cable and fitting BNC connectors from Telegärtner. The Telegärtner connectors were the only non-no-name connectors I could get and I had used this German brand before on my RG58 cables. To get the connectors on, I first had to design a simple 3d-printed coax stripper matching H155 cable and the datasheet of the Telegärtner connectors. It worked out very well on the first try. Why H155? I wanted to try out a low loss cable stiffer than RG58 with non-foam dielectric for better impedance stability. In addition to that, there are some return loss measurements for my specific application case and H155 cable available that I could compare my measurements against.So, I expected a cable with a much better return loss than my previous RG58 and a better impedance stability with an average impedance value somewhere close above but not necessarily at 50Ω. The following shows, what I got in terms of reflection coefficient and time domain step response, in comparison to my previous RG58 cable:RG58, H155 and H155 (cable reversed) time domain step response:From this I get that the impedance is indeed more stable, but still about 2Ω along the cable. Consequently, I would expect effects of the reflections at the cable end (BNC to SMA Adapter + SMA 50Ω load) to be more dominant than on the RG58 cable. Unfortunately, the impedance of my DIY H155 cable is further away from 50Ω than my RG58 cable.RG58 reflection coefficient:H155 reflection coefficient:The main thing, I get from this is that there seems to be a strong resonance phenomenon on the H155 cable. Maybe this is a result from the less pronounced local impedance changes causing less distributed reflections, the lower theoretical cable loss and the cable acting as kind of a resonance cavity at certain frequencies. My H155 cable has a length of 36", so it is electrically short and the reflections caused by the connectors and adapters at both ends could bounce around and resonate. On the RG58 cable this effect(which is also kind of visible) may be much less prominent due to many additional reflections along the cable and the worse theoretical return loss. To my surprise the general shape of the frequency dependent reflection coefficient is very similar on the two cables, except the H155 performing worse at frequencies below 500 MHz. I am still searching for the much better return loss to show up somehow.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 20, 2024, 01:32:49 pm
content: HP used to make a similar BNC connector to this Amphenol part.https://www.digikey.com/en/products/detail/amphenol-rf/112332/3088368Left Amphenol 74868 UG-88B/U (assembled for photo, not actual install)https://www.amphenolrf.com/031-2.htmlRight is old HP 1250-0256 with section of RG400.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 21, 2024, 05:31:17 pm
content: LiteVNA 64 was calibrated using the modified standards from the V2Plus4 (sorted ANNE terminator) from 300k-9GHz, 1kHz IFBW, ideal model. The two Amphenol BNC SMA adapters (C & D) from:https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5602515/#msg5602515were attached. Note the Port1 cable was locked in the vice during calibration and other measurements.Next I measured the insertion loss of a few cables. Their lengths were also noted along with the connector brand (if known).***Link for ProbeMaster cable:https://probemaster.com/bnc-cables-2/Also attached photo of the Pasternak RG-400/U. Datasheet may be found:https://www.pasternack.com/images/ProductPDF/RG400-U.pdfAt 1GHz its 14.7dB for 100' or 0.91dB for our 74inches. Which is very close to what we measured with the added loss of our adapters and connectors.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 22, 2024, 01:12:14 pm
content: Quote from: joeqsmith on September 21, 2024, 05:31:17 pmSo it seems like the liteVNA can produce some usable quantitative results.I tried to reproduce your insertion loss measurement with my test setup. The liteVNA was warmed up one hour, and then calibrated with the supplied cables and standards from 100k-6.3G, 1kHz IFBW, ideal model. I did not go all the way up to 9G, because I stumbled upon a strange calibration artifact, which I will try to show later. Then, I connected one of my cheap SMA-M to BNC-F adapters to the double SMA-F adapter at the cable on port 1 and a SMA-F to BNC-F adapter to the cable at port 2. The DUT cables then went in-between the BNC-F adapters.As cables, I had:RG58 C/U MIL C17F, 107 cmDatasheet: 1GHz 51.8 dB Attenuation dB/100m -> for 107 cm : 0.55426 dBBelden H155, 91.7 cmDatasheet: 1GHz 29.6 dB Attenuation dB/100m -> for 91.7 cm : 0.271432 dBThis is what I got:The image shows two measurements for each cable in the 100k to 1G range. One measured with 801 points from 100k to 6.3G and one with 801 points from 100k to 1G. The one with the denser sampling showed a large peak at the beginning going up to about 5.2, a strange small disturbance at 100MHz (which stays there, even when varying the sweep parameters) and some periodic superimposed oscillation. Unfortunately, the measured insertion loss values are not even close to the theoretical values I listed above. Maybe I messed up something with my measurement setup or my SMA to BNC adapters are too bad. At the moment, I do not have sufficient materials to test the insertion loss of only the adapters.I also got myself some PCB mount BNC-M connectors to experiment with building BNC calibration standards, like describedhere. It seems those are only available from china manufacturers and no known quality manufacturer makes those. The first try was a complete and perfect failure.The springy outer conductor of the connectors is too small in diameter by about .3mm and does not make good contact to any mating BNC-F connector. So, the connectors themselves are really bad. Nonetheless, I created an open and short by filing down the center conductor and then soldering a small tinned copper disk for the short, just for the fun of it. The result was smith chart that looks like Indiana Jones whip after calibrating. For open, short and load (even below 1GHz) the smith chart does not even get close to where it is supposed to be and jumps all over the place when just looking at the connector. I really wonder how the guy from the website linked above managed to get anything out of these connectors, but it may also well be my soldering skills.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 22, 2024, 02:30:21 pm
content: I'm guessing much of that loss is the adapters and connectors. I suspect the switch points have changed over time. Because I don't run the VNA standalone and have no use for any features beyond getting the data into the PC, its rare I would change the firmware. Shown with the VNA uncalibrated and a thru attached at 801 and 1601 points. Note the higher spikes at the start.If I increase the number of data points to 2001, calibrate and measure the thru, it is within 0.04dB. Now when I insert a 48" section of RG58U and BNC connectors/adapters, my LiteVNA64 does not exhibit the large transients you show.You could try taping the cables to the table to prevent them from moving during the calibrations then measurement. You could also try running the same firmware just to remove that variable.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 22, 2024, 02:52:23 pm
content: You do not specifically state that you recal the VNA when you are changing the number of points and frequency range.While the software has a limited interpolation if you work inside the frequency range that the VNA was calibrated, changing the number of data points would lead to some pretty strange results. Shown with the same setup but I have changed the number of data points. Of course the software would tell you there is an problem (CalRange error is active) but by design, it is not going to prevent you from doing such things.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 22, 2024, 04:17:30 pm
content: Assuming you are calibrating the VNA for each setup, here is the LiteVNA with the two ports open after calibration using 2001 points, ideal model as before but with 100 averages. Noise floor is about -100dB with mine. I expect yours would be the same.I then attached two Coline 20dB attenuators in series and again averaged 100 times. Note the min/max is enabled in both plots to show the peaks.The spec +/-0.2dB frp, DC to 1GHz with these where I measure about 1dB with the two. Again though, the VNA was calibrated with the SMAs and we have added these adapters which are another error source.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 22, 2024, 08:33:56 pm
content: Quote from: joeqsmith on September 22, 2024, 02:30:21 pmLooks like I get some better connectors then. Unfortunately, I can not really compare the ones from Telegärtner to another renowned brand, because Telegärtner seems to be kind of unknown outside of Germany. However, I have heard people say that their adapters might be similar in quality to same-priced Radiall, H&S and Pasternack devices.Quote from: joeqsmith on September 22, 2024, 02:52:23 pmRegarding the artifacts, I mentioned earlier, I already had a suspicion, that they might be interpolation errors. Though, I thought that I had recalibrated for the 100k to 1G range, I redid the measurement and the large spike at the start, the hiccup at 100M and the oscillations disappeared. The rest of the curve was very close to the two earlier measurements.Quote from: joeqsmith on September 22, 2024, 04:17:30 pmThis is what I get for the same parameters:Did you enable averaging before or after calibrating? Since calibrating takes a lot longer with averaging on, it seems to make a difference.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 22, 2024, 09:56:57 pm
content: Quote from: Pinörkel on September 22, 2024, 08:33:56 pmAfter.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 23, 2024, 12:43:42 pm
content: There are a few things about the LiteVNA you should be aware of that may cause you some problems. When I showed the uncalibrated through measurement, you could see the switch points. Normally these wouldn't cause a problem but if you are looking to measure losses in the 20dB or lower, these can be problematic. The easiest way to see the problem is to cal the VNA for say 50k to 1M with say 800 points, then connect a step attenuator between the two ports. As you add more loss, you will see a step in the response. These happen at discrete frequencies. The steps can be more than 2dB. As you adjust the attenuator, the steps polarity can change.It seems member Dislord had explained this in my large thread. I think they provided the switch points and went into details of what was happening. I added support to change some of the advanced settings of the LiteVNA to try and mitigate these problems. There is no fix that I am aware of. This was long after I had stopped updating the manual so the only place it is documented is in the forums.So when you see these spikes and shifts in your data, just be aware that the VNA can also attribute to them.There are a few things that the original NanoVNA did much better than the LiteVNA and V2Plus. I was surprised when I first bought the V2Plus4 how poor it was by comparison. The designer of the V2Plus claimed that the LiteVNA was a ripoff of their design. It certainly suffers from the same problems. In some cases, you can work around it by using both the tools. Of course, you still have that square wave drive. I think when you get down to that level, it's time to think about getting a used high end system.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 23, 2024, 03:54:15 pm
content: Going back a few years, here you can find the discussion switch points.https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg4136797/#msg4136797After normalization, attached showing the 400kHz switch point with and without AGC when measuring a 30dB attenuator.Also shown is how the step can change location depending on attenuation. The leading glitch at the low frequency starts showing up below 16dB. You can toggle the attenuator between 16 and 17dB and watch the leading glitch come and go.I'm sure most of the people using the LiteVNA are aware of the shortcomings like this but there isn't a good source for all the little traps for people just getting started.So if you see something really odd that you can't explain, it may very well be inherent to the VNA.

author: DiSlord
author_rank: Regular Contributor
timestamp: September 24, 2024, 04:50:20 pm
content: LiteVNA use 2 generators MS5351 (from min to 100M) and MAX2871 to 6.3 - 6.4GHz (and t ~8G use harmonic mode)On low frequency range also switch IF800Hz - 20k use 6k IF20kHz - 400k use 12k IF400kHz - 100MHz - use 60k IF and MS5351> 100MHz use 60k IF and MAX2871Exist 2 problems:Different IF on OpAMP give little different gain and phase rotation (compensated on last firmwares but not ideal) also OpAmp gain not ideal linear (calibration allow fix only linear errors)As result:- difficult fix problems on switch 20k, 400k and 100MHz (errors small, but exist)

author: joeqsmith
author_rank: Super Contributor
timestamp: September 24, 2024, 05:11:27 pm
content: Thanks for chiming in. While its pretty easy to detect the switch points and I agree that for the most part, they really don't cause a problem. I have always wondered about what appears to be a change in the switch points based on signal level. Where the low frequency switch points seem to be very predictable, the 100MHz one appears to move. Eventually locks in. See the zoomed in area of the previous data set for a better view.You can see I am running very old firmware and maybe some of this has been improved. That version I am running I think was when you opened up the harmonic mode for me to make it unlimited.

author: virtualparticles
author_rank: Regular Contributor
timestamp: September 25, 2024, 03:06:12 pm
content: Quote from: pdenisowski on July 07, 2024, 07:42:41 pmThis is how I would do the measurement as well. TDR measurements made with VNAs are helpful, but frankly, no VNA manufacturer provides metrology for this mode. The TDR method involves Time Domain inversion and requires aggregating a thousand or so frequency measurements. Determining uncertainties for this is impossible, as far as I know. VNA TDR has the advantage of making a series of measurements in a narrow bandwidth instead of a single measurement in a GHz or higher bandwidth, resulting in a 1000x better signal-to-noise performance. This virtual TDR will work over hundreds of feet of relatively low-loss cable.Seehttps://coppermountaintech.com/time-domain-processing/

author: joeqsmith
author_rank: Super Contributor
timestamp: September 25, 2024, 03:22:08 pm
content: Quote from: virtualparticles on September 25, 2024, 03:06:12 pmAs I understand it, the OP is not asking about a section of coax. For their cable, they are considering the entire system. We know Tektronix had made a custom connector to improve the return loss. I don't see how you would use the method you linked to evaluate the effects of the connectors, adapters, workmanship... Using the LiteVNA with TDR is not good enough to see a lot of detail but you can get a relative feel for which combinations are better.With that in mind, how would you recommend they measure these effects?

author: virtualparticles
author_rank: Regular Contributor
timestamp: September 26, 2024, 04:35:03 pm
content: Quote from: gf on September 02, 2024, 08:39:19 pmIn practice, we "Zoom in" on time-domain ranges using the Inverse Chirp Z transform. Zero padding is feasible but increases computation time. See my write-up athttps://coppermountaintech.com/inverse-chirp-z-transform-for-vna-time-domain-processing/for more information.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 26, 2024, 04:49:38 pm
content: So you are suggesting TDR?***While GF seemed to have missed my comment about zero padding, I have used that method for some time. This is what the OP is currently using.

author: tggzzz
author_rank: Super Contributor
timestamp: September 26, 2024, 04:53:27 pm
content: Quote from: joeqsmith on September 26, 2024, 04:49:38 pmif you want to see how impedance varies as a function of distance, then TDR would be my first choice.Examples: "wideband" digital signal integrity on PCBs, connector "suboptimal characteristics", faultfinding in inaccessible locations, e.g. inside aircraft fuselages.OTOH, if you are interested in "narrowband" frequency domain behaviour, then a VNA would be my first choice.As is usually the case, "there is more than one way to skin a cat".

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 28, 2024, 12:36:23 am
content: I am not sure if i will stumble on effects related to the switching points, but I will try to keep their existence in mind. The Inverse Chirp Z-transformation sounds interesting. I will try to read the paper in the next weeks and see if I can understand it.In the meantime I invested roughly a liteVNA64 value in better adapters. I am glad I did so, because that lead to a bunch of new insights (part of which had already been predicted by the more experienced members in this thread). Basically, I got several types of SMA to BNC, SMA to SMA and BNC to BNC adapters from Telegärtner, including what looked like a decent quality 50Ω Terminator and some port savers.As far as I can see, this BNC-M terminator seems to perform even better than the Fairview Microwave ST3B-F I originally had in mind for terminating the cable. The following picture shows an S11 measurement taken with the liteVNA, calibrated with the supplied cal set. The black curve is the S11 of the liteVNA SMA calibration load, the red curve shows my old 50Ω terminator, i had lying around and the other three curves show the new TG terminator for three different connection states (e.g. extra pushing or pulling at the BNC connection before measurement). When comparing with the curves from the respective datasheets it performs better that the ST3B-F and the Mini-Circuits BTRM-50+.My next (expected) finding was that my no-name SMA to BNC adapters are crap. On my new set of adapters all connections give an incredibly better haptic feedback when establishing an loosening the connections. Everything feels way more precise. Also they perform much better in a direct comparison. For that, I calibrated the liteVNA and then connected my H155 BNC cable via an SMA-M to BNC-F adapter to the liteVNA cable with the SMA through adapter. On the other side of the cable I connected a 50Ω termination and measured the TDR with Solver64. Then I swapped out the adapters and terminations at both sides of the cable between the no-name and Telegärtner ones and attached the liteVNA SMA load, the old 50Ω BNC termination and the new Telegärtner 50Ω BNC termination. The test setup and variations are shown here (the golden SMA adapters are the no-name ones):One could immediately see that at least the SMA part of all the no-name adapters is unusable and produces a downward spike in the TDR graph that is not there with the quality adapters. The upwards spikes in the graph are caused by the BNC connections. I verified that via another measurement with an improvised bad quality BNC cal kit which made the curve start at the top of the first spike and go downward from there. As expected there are two spikes at the end, when connecting a BNC terminator via a double BNC-F adapter. My old BNC terminator produced a spike at the end that went up to over 80Ω, and some oscillations. As expected, the cable part measured the same on all measurements, except for temperature drift variations and the offset caused by the SMA to BNC adapter between the cable and the liteVNA through adapter. Regarding the observed offset, my brain is still not sure why the hiccup caused by an adapter should influence the absolute value of the cable measured after the adapter. Is that because of an insufficiently dense sampled input signal resulting in integration errors on the way to calculating the TDR, which then of course sum up over the integration distance?I hope I will find the time to redo the recent S21 measurements with the new adapters this weekend.@joeqsmith: While playing with exporting Touchstone s1p and s2p files from Solver64, I noticed that Solver64 respects the locale of the PC it is used on pretty much everywhere in the UI, including said export. However, the Touchstone file format seems to be only defined for using the English decimal separators. Consequently, Solver64 outputs unreadable Touchstone files on all PCs with a locale using other decimal separators. Of course this also affects the SnP load function. Solver64 will not load correct Touchstone files on PCs with other locales. I fixed this by post-processing the files and replacing all commas with a point, so METAS VNA Data Explorer can open them, but maybe there is an easy fix for this in future releases.

author: charliedelta
author_rank: Regular Contributor
timestamp: September 28, 2024, 05:01:27 am
content: Anyone tried to use a VNA to measure longitudinal balance?I want to build a LISN for CAT5 or CAT6 cable and need to verify the longitudinal balance?Any ideas welcome?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2024, 04:00:48 pm
content: Quote from: Pinörkel on September 28, 2024, 12:36:23 amGrowing up in the USA, we were taught the metric system at a young age, and were told we would switch soon. Now that I am reaching the end of my life, I have seen little progress. I have worked for industries that should have pushed to correct it. As dumb as switching between imperial and metric is, I can't imagine having to use commas in some cases and decimal points in others. Growing up with our hybrid "impertric" system, I am guessing you also just accept the stupidity of it all.If you select a country code that uses the comma for a decimal place, my software uses the comma for everything being displayed or exported. I assume that these countries have tools that support your choice. It sounds like that may not be the case. Today, I force the exported Touchstone files to use the comma if this is what you have chosen. It would actually make the code cleaner to export data using only the decimal. I had added this after someone had asked about it.The Touchstone import was something I added for my own debugging. It only supports files that use a period for the decimal point.If I force the Touchstone export to always use a period for a decimal point, does it cause problems for people in other parts of the world?***If you do decide you want me to change it, let me know what OS you are using, how you have it setup (screen shots). First step would be to replicate what you are seeing.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 28, 2024, 06:17:11 pm
content: Quote from: joeqsmith on September 28, 2024, 04:00:48 pmKind of... if I ever think "It cannot get worse", I have a look atthis. Soon after, hope and humor are back on track.Quote from: joeqsmith on September 28, 2024, 04:00:48 pmI noticed there was an issue, when METAS VNA Tools and Copper Mountain S2VNA could not load the files, because they are apparently hard-coded to use a period. In S2VNA the UI even becomes non-functional (buttons do not update any more) and after some time it spams the user with hundreds of error dialogs. The thing is that the Touchstone file format specification does not define, but only imply that a period has to be used as a decimal separator. The fault here is that the file format neither defines the separator, nor offers a way to specify it in the file. Thus it becomes a file format that would require to look at the data and make a guess, which is inherently bad. So your choice of handling it is not wrong (in fact, I think, it is a better interpretation of the format specification), but will not work with many programs and devices that use Touchstone files. Since there is no standard compatible way to specify the separator in the Touchstone file, a configurable option in Solver64 for just the Touchstone file decimal separator would be a way to handle the faulty standard. However, most Solver64 users may live in the US and it may very well not be worth the effort. The only drawback of an external correction is, that the compatibility of corrected files is inverted. After correction all progams but Solver64 can read them.As it is maybe of interest: I had a short look at S2VNA Software to see, if it does anything different when extracting time domain data from an S11 measurement. Apparently ist is very close to what Solver64 does, but I could not find a way to display the Y axis as impedance:

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2024, 06:37:40 pm
content: QuoteAgain, if you want it changed, I am not asking what my software looks like but what OS you are using and how you have if configured. I use 10 and noticed all the BS they have added to support these formats that we can't agree on. MS can't even give you simple area as I assume it is more screwed up than that. Imagine the time that was wasted coding the OS, testing it.... Just stupid IMO.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 28, 2024, 08:48:42 pm
content: Quote from: joeqsmith on September 28, 2024, 04:00:48 pmIf I use the import, it only works with files that use the same decimal separators as defined in the current system number format settings, which is what I would expect.Quote from: joeqsmith on September 28, 2024, 06:37:40 pmThere are even countries that have mixed use of number formats (Map).I have attached my number format settings for testing. I use English Win10 and German Win7, but the number format settings are the same on both. However, please do not invest time into this if there is a chance that the result is worse. I already regret my last improvement suggestion, since it lead to the time domain graph defaulting the X-autoscaling to being off, resulting in several extra clicks, every time I use it.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2024, 09:50:49 pm
content: My ignorance is unlimited! So you use the common for a decimal and a period for a separator. Crazy stuff. I think this is a few minute change. Let me test it first and I'll upload it for you to try.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 28, 2024, 10:09:55 pm
content: Quote from: joeqsmith on September 28, 2024, 09:50:49 pmThat is exactly what would make it difficult to detect the number formatting. At work, we write all our scientific papers in English, since most literature and papers are in English. Consequently we also use English number formatting. However, many PCs although set to use English UI language, are set to use German date and number formatting and of course metric units.In the meantime I redid the S21 measurements with my RG58 C/U and H155 cables. The new adapters improve the insertion loss at 1GHz by approximately 0.1 dB per cable. The H155 cable is now missing approximately 0.2 dB to spec and several RG58 C/U cables are missing 0.2 to 0.3. This difference might be from to the two BNC connectors at each cable. If that is so, I wonder why the insertion loss of the RG58 connectors is higher that the one of the H155 connectors.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2024, 10:26:31 pm
content: I've made the changes and tested both the export and import on both the 32 & 64-bit systems using both decimal formats. It does what I expect but that by no means suggests it solves your problem. Try it and let me know.

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2024, 10:39:13 pm
content: Quote from: joeqsmith on September 28, 2024, 09:50:49 pmThat's why I18N libraries are used. (I18N is internationalisation).Wait until you see how numbers are written in the world's most populous country: search for "crore" and "lakh", and see how 3*106is expressed.

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2024, 10:42:20 pm
content: Quote from: Pinörkel on September 28, 2024, 10:09:55 pmThe nice thing about date (and times) is that there are so many standard forms to choose from. See ISO8601

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 28, 2024, 11:28:36 pm
content: Quote from: joeqsmith on September 28, 2024, 10:26:31 pmI just tested export and import and if there is anything different than in the previous version, I am not able to spot it. Exported files, as one would expect, still contain the decimal format set in my system settings and imported files have to be in that format to be read properly. I also tested swapping my systems decimal format and the input/output in Solver64 changed as expected. As a result, all "," in the Touchstone files still have to be replaced with "." after export, so other programs like METAS VNA Data Explorer would read the files, which always expect the English number format.Apart from that I noticed that the time domain x-autoscaling is on by default again, which is nice. Thanks for that.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2024, 12:20:19 am
content: Quote from: Pinörkel on September 28, 2024, 11:28:36 pmQuote from: Pinörkel on September 28, 2024, 08:48:42 pmOdd, as that change was made on the 9/2/24 in version 5.07.https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5627253/#msg5627253Interesting is I downloaded the latest version and tried it with German settings and it does not work. Odd as the version number is correct. Let me have a look.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2024, 12:58:04 am
content: Something went wrong when I copied the files to the second PC. You were running the same version. I went through the rebuild again and uploaded. I just tried to download the new files back to the other PC and it seems to work. Should display 5.09.

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 29, 2024, 08:59:34 am
content: Quote from: joeqsmith on September 29, 2024, 12:58:04 amNow, it is working. So much easier to get files into METAs. Thank you very much.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2024, 02:55:47 pm
content: Quote from: Pinörkel on September 29, 2024, 08:59:34 amMost of the feedback I receive is from people who struggle with the basics. It's rare to discuss problems like this that actually could be addressed. So thank you for bringing it to my attention, and taking the time to test it.Does working suggest you have tested both the export as well as the import and it now works as you expect? Both import and export of S1P and S2P should now work the same.

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2024, 03:53:45 pm
content: Quote from: virtualparticles on September 26, 2024, 04:35:03 pmVery powerful algorithm that few folks know about, good to see the CZT and ICZT hasn't been totally forgotten and displaced by the FFT!! Back in 1980 we developed a handheld special real time SA (3~4watts) based upon the CZT, utilized custom very long CCD devices as the Complex Convolvers. This was developed for finding and "Zooming In" on certain signals of interest.Going the conventional DSP and FFT route would have taken the equivalent of 1/3 of a Cray One Computational Power at the time, which wasn't going to yield a handheld!!Nice articleBest,

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 29, 2024, 09:09:43 pm
content: Quote from: joeqsmith on September 29, 2024, 02:55:47 pmBy working I mean that exporting s1p and s2p files now produces files that can be read by third party software out of the box and imported in Solver64 again. I also tested this by disabling sweeping and manually clearing all graphs in between. Loading s2p files jumps me to the 2-port-plot display, but that has never displayed any data for me (maybe because the liteVNA is not a 2-port-VNA?).By the way, I have a related question regarding the "Export Data to Clipboard" function in the graphs. Is there any special reason (besides it being the display format of the graphs) that you chose to export numbers in the compact number format rather than the scientific number format?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2024, 10:11:40 pm
content: Quote from: Pinörkel on September 29, 2024, 09:09:43 pmIf you load the FHX13X_FHX14X.s2p that is located in the touchstone directory I would expect you to see the attached image. If not, something is wrong.While the LiveVNA64 and V2Plus4 are both 2-port, 1-path devices, my software supports use of a transfer relay with these along with a 12-term error model. This is how that T-Check you mentioned is measured.If you tell the software to create a 2-port file but you are not using a transfer relay, I think I save S11 to S22 and S21 to S12.Quote from: Pinörkel on September 29, 2024, 09:09:43 pmExporting data like you mention is built into LabView. See the following article:https://www.ni.com/docs/en-US/bundle/labview/page/exporting-data-and-images.html

author: Pinörkel
author_rank: Regular Contributor
timestamp: September 30, 2024, 11:45:20 pm
content: Quote from: joeqsmith on September 29, 2024, 10:11:40 pmAh, yes that works. This file is quite interesting, since it is also the only file METAS can display a TDR graph for. It does not do that for any s1p or s2p file I produced with Solver64. I tried to figure out the reason for the s2p files, but both files have nine columns and a correctly specified option line:Code:# Hz S DB R 50(Solver64)Code:# GHZ S MA R 50(FHX13X_FHX14X.s2p)Nothing I tried, like modifying the option line, swapping the line endings, removing the S12 and S22 columns or defining additional SNP file parameters is able to make the Solver64 file work in Time Domain Mode in METAS and nothing is able to break the FHX13X_FHX14X.s2p file so it no longer works. Since METAS can display all S parameter data in a Solver64 exported file it should be no read error. Also, it does not seem like METAS can not display time domain information for files with DB format. If I define the option line in FHX13X_FHX14X.s2p to be DB, METAS can still display time domain graphs. Very confusing.Quote from: joeqsmith on September 29, 2024, 10:11:40 pmYes, that is how my saved files look like.Quote from: joeqsmith on September 29, 2024, 10:11:40 pmAh, that explains it, thank you. The thing with this format is, although it is nice to read for a human, almost no other software supports it. Especially no Excel clone. I fiddled together some formula to convert it, but that involves creating an extra column for each data series and applying the formula, before the values can be processed further.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2024, 12:28:51 am
content: Quote from: Pinörkel on September 30, 2024, 11:45:20 pmSee attached file for waveguide filter using dB format created with my software with LiteVNA and home made X-band extender.Quote from: Pinörkel on September 30, 2024, 11:45:20 pmI would just post process the Touchstone files.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2024, 12:42:19 am
content: S1P files, Open and Short***Load METASE and select a file like 70mmBettyAireline.s1p in the Touchstone directory. Select TDR. You should see the blank screen. Now select Control F11. You should see the errors why this particular file can not be processed.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 01, 2024, 02:26:57 pm
content: Quote from: joeqsmith on October 01, 2024, 12:42:19 amHere, all your files open and display TDR graphs for "Band Pass Impulse" mode, but not for the "Low Pass Step" and "Low Pass Impulse" modes. Ctrl+F11 does nothing for me. However, your command line screenshot was very enlightening. To me it seems like METAS at some point divides the maximum frequency by the number of sampling intervals in the and cannot process further if that does not match the actual largest sampling interval size in the file.If I change the start frequency to 0Hz before exporting in Solver64, METAS can display all time domain graphs in any mode. The scale on the X and Y axis does not match what I see in Solver64 (even when I leave the velocity factor at 1). Impedance values and lengths are off. Since the length on the X axis in Solver64 matches with the length of my cables when the velocity factor is set right, I would trust Solver64 more at this point.If I change the maximum frequency in my files by subtracting the start frequency from the last points frequency, METAS can display all time domain graphs in any mode. However in this case, the "Impulse" modes get an additional bump at the beginning and the "Step" mode a frequency dependent drift.Looks like METAS might have some bugs, but at least the vnatools group does not seem to be very responsive to error reports. Interesting is, however, that it displays stuff before the zero point in TDR mode.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2024, 11:24:50 pm
content: Quote from: Pinörkel on October 01, 2024, 02:26:57 pmSelect the Options pulldown and select Console (same as ctrl F11).I am currently running 2.8.8875.19141. Maybe your German settings cause it problems. You could ask them, or just try using the USA settings and see it it behaves correctly.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 02, 2024, 03:11:05 am
content: Quote from: joeqsmith on October 01, 2024, 11:24:50 pmSeems like the reduced version called "VNA Data Explorer" that I am using does not have that. I will try to get the full version.Also, I have added my three experimental Touchstone files to my last post for comparison.

author: gf
author_rank: Super Contributor
timestamp: October 02, 2024, 06:15:33 am
content: Quote from: Pinörkel on October 01, 2024, 02:26:57 pm1) Note that the non-ideal, non-zero impulse width (and possibly Gibbs artifacts) introduced due to the VNA's upper frequency limit and windowing in the frequency domain extends from the center of the impulse to both sides. If the impulse is centered at or near t=0, then the impulse shape also extend to negative time.2) Keep in mind that IFFT is circular, therefore the time axis wraps around, too. Anything you see at negative time could also be wrapped positive time.EDIT: Also think about the implications when you try to integrate the impulse response on the (circular) time axis in order to get the corresponding step response. The starting point of the integral on the time axis definitively makes a difference.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 02, 2024, 12:55:31 pm
content: Quote from: Pinörkel on October 02, 2024, 03:11:05 amCorrect. This is supported with the VNA Tools. This version of their tools offers some other features as well that may be of interest. Eventually, I plan to use it to attempt to characterize the included standards for that T-Check experiment.The do support a polar plot but sadly, no Smith chart. The one thing I really dislike is the inability to change the trace colors. Yellow on white, really difficult to see.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 02, 2024, 09:43:04 pm
content: I slowly get the impression that, even if I had a highly precise characterized calibration set, it would be quite difficult to get precise impedance values from a measurement, as long as the data is not recorded and evaluated by an expensive professional VNA. Every time a transformation to time domain is performed, there are apparently possible differences in the calculation, regarding the data interpolation, integration, different window functions, which all lead to differences of multiple ohms, as can be seen in the differences between Solver64 and the METAS VNA Tools when processing the same data.Apart from that, I played again with my initial idea of using a BNC calibration set to minimize the number of measured devices around the cable. For this, I modified the Chinese PCB mount connectors, I had bought for playing around. As already mentioned the ring of springy contacts in the male connectors had a too small diameter to make contact with the inner side of the outer contact in the female connectors, which is the reason I had classified the connectors as unusable. Just for verifying if that connection issue was the cause of the bad calibration results, I bent all of the contacts slightly to the outside, so the would be able to make contact. Although the result was the opposite of good mechanical quality (and will still go to the bin), my modified open and short and the good quality BNC terminator from Telegärtner were able to produce a pretty good calibration. It was not even half as susceptible to reconnecting the BNC connectors than I thought.When comparing my H155 cable using this BNC calibration and another calibration done with the liteVNA SMA set and putting an SMA-M to BNC-F adapter before the cable, I noticed something. Maybe not starting the measurement right at the reference plane of the BNC connector of the cable but at an earlier SMA reference plane is not a bad thing at all. As can be seen in the attached image the green curve taken with the BNC calibration starts right at the top of the peak produced by the BNC connection and then goes down. Since this peak is not symmetrical, there is no way of knowing the correct impedance offset of the values after the peak, especially, since the peak is most likely the place with the highest probability of integration errors. The SMA calibrated white curve includes both sides of the BNC connection and we know from this measurement that the BNC reference place of the BNC connection deviates significantly from 50 Ohms. So in this case the curve is nailed to 50 Ohms at a point where we know for sure it is not 50 Ohms. I also made a copy of the green BNC calibrated curve, changed its color to pink and moved it over the white SMA calibrated curve for better comparison. It seems to match the curve quite closely, except for the peak at the beginning, which is apparently about one Ohm higher. I think, that only one curve can be correct about the height of the peak. But I do not know whether the BNC calibrated curve has a starting artifact or the SMA calibrated one is too flat because e.g. it does not sample the highest position(which the BNC calibrated curve should by design) or there is a sampling/integration error.

author: gf
author_rank: Super Contributor
timestamp: October 03, 2024, 10:43:35 am
content: As I said, it is important where you start to integrate the impulse response in order to get the step response. The integral should include the whole extent of the first impulse. If you have a discotinuity at (near) the calibration plane (t=0), then the first impulse is centered at (near) t=0 and the impulse width extends to negative time. Therefore you should start integrating at a negative point in time in order to include the whole impulse. If the software can only start integrating at t=0, then you can try to shift the first impulse into the positive time region by applying some electrical delay. This works at least with the internal TDR of the NanoVNA firmware. I can't comment how that applies to Joe's Solver as I've never used it.Furthermore, I'm also not sure if it is a good idea to use a rectangular window function in conjunction with a wideband DUT. The abrupt truncation at the upper frequency limit of the sweep turns an ideal impulse response of the DUT into an impulse with Dirichlet-kernel shape (showing lots of ringing which is not present in reality). So I'd rather use a non-rectangular window function even if it results in a softer (wider) impulse shape (but you'll get less unreal overshoot/ringing in the step response). To show the difference, I've attached the IFFT impulse responses of rectangular (blue), van Hann (brown) and Blackman (yellow) window function. Which one would you consider the best approximation with finite support? Certainly not the blue one. IOW, where would you start to integrate the blue impulse so that the integral includes thewholeimpulse?

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 03, 2024, 12:56:21 pm
content: @gf: Thank you for this very useful explanation. So, the filter windows function is basically one of those trade-off choices like it is in computational image processing. Luckily, I bought two SMA port saver in my last adapter purchase, which, as I now realize, can also be used as signal delays. So I made some quick measurements, of my H155 cable with none, one and two port savers inserted between the calibration plane and my SMA to BNC adapter (I will repeat these measurements later with better warmed up equipment). I also did some comparisons between rectangular and Blackman window functions in Solver64 and METAS VNA Tools. The results were quite interesting.I found that the measurements with one and two port savers in between generally tended to be more alike than the one without additional electrical delays in terms of absolute values for the cable impedance in the non-connectors region of the cable. As I see it, this suggests that working with an additional delay before going to BNC could be favorable in this case. However, this has to be taken with a grain of salt, because I noticed that I am finally in a region where repeatably tightening the SMA connectors properly makes a visible difference (thanks to joeqsmith for hammering that into my brain). For this, I used my improvised 3D-printed torque wrench and ensured that I used a method, that let me get the same results over and over again, when loosening and re-tigthening the connector. So, I am kind of sure that I am at least sufficiently repeatable there without tightening to a specific calibrated torque value.The second interesting finding was that using different windows (e.g. Rectangular vs. Blackman) made a large difference in terms of absolute impedance values in Solver64, but nearly no difference in the VNA Tools. In the VNA Tools I only observed less pronounced peaks, kind of like a low pass filter effect. In Solver 64 on the other hand, the mid section of the curve jumps by nearly 1.5 Ohms, which may hint at large integration differences ath the peak caused by the first BNC connection.Solver64 electrical delay and filter function comparison with three electrical delays (no, one and two port savers inserted behind the calibration plane). The upper three curves are with with Rectangular window function, the lower three ones with Blackman:The Blackman curves, and this is also true for the Hanning filter, show more pronounced ringing in the step response, which is kind of the opposite of what gf predicted.One comparison of different filters in VNA Tools. I still have not found an easy way to get time domain loadable s1p files from Solver64 into VNA Tools, so only one curve here. The first image shows Rectangular and the second one Blackman:Here, the curve with the Rectangular window shows, as predicted by gf, less ringing. Also the low pass components of the curve do not jump up and down, when switching filter windows, like they do in Solver64.

author: gf
author_rank: Super Contributor
timestamp: October 03, 2024, 04:39:38 pm
content: Quote from: Pinörkel on October 03, 2024, 12:56:21 pmWith "electrical delay" I actually meant a virtual delay, applied mathematically by the VNA software/firmware. I'd be surprised if Solver would not offer "electrical delay" or "port extension" settings, or both. (When using NanoVNA standalone, you can set EDELAY in the GUI.)QuoteYou mean in conjunction with low pass TDR, or in general? Lowpass mode requires evenly spaced frequency points from 0Hz to fmax. The S11readings for frequencies below the VNA's lower limit down to DC are usually estimated by extrapolation. Just a guess, but I could imagine that the VNA tools might expect these extrapolated values to be already present in the loaded s1p files if you want to calculate a lowpass TDR?

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 03, 2024, 06:41:56 pm
content: I just repeated my measurements multiple times and got the same results, except for some really minor deviations.Quote from: gf on October 03, 2024, 04:39:38 pmAh ok, my bad. Solver64 indeed has a port extension parameter. If I set it to e.g. -10000 ps, I can actually see the same curve parts as in VNA Tools. If I choose the delay large enough that the curve starts in that flat region at the left, then the resulting curves are very close in amplitude to each other. If I hit that bathtub at the beginning or the subsequent peak, the rest of the curve shifts vertically, which might be an integration issue. That aside, switching filter window functions still causes the curve to hop around in Solver64, while I cannot observer this in VNA Tools.Quote from: gf on October 03, 2024, 04:39:38 pmYes, the issue VNA Tools has with the Solver64 touchstone files is, that the maximum frequency is not a multiple of the largest frequency step. Lets say I measured from 100 kHz to 6.3 GHz with 801 points. Then I have to manually edit the frequency of the last point to 6.2999 GHz, to enable VNA Tools to compute the time domain graphs. I am pretty sure that this messes something up, and produces bad measurement values, but I have not found a better workaround yet.

author: gf
author_rank: Super Contributor
timestamp: October 03, 2024, 07:24:44 pm
content: Quote from: Pinörkel on October 03, 2024, 06:41:56 pmWhat if you choose a start and stop frequency which are both an integer multiple of the step size, for example 6300 points from 1MHz to 6300MHz?

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 03, 2024, 09:20:57 pm
content: Quote from: gf on October 03, 2024, 07:24:44 pmThat would of course work, but leaves me with no resolution for looking at short cables or discards a lot of the lower bandwidth. The liteVNA only supports about 1024 sampling points. So, I could maybe go with 6.3M to 6.3G and 1000 points (or 9M to 9G and 1000 points for trying to squeeze more resolution out of the liteVNA) to get loadable files that contain the required harmonic grid. I am still not sure what I am loosing by not being able to utilize the frequencies below 6.3M. Maybe, it is best to use multiple calibration ranges here: One for TDR that satisfies harmonic constraints and another one for return loss measurements that includes lower frequencies.

author: gf
author_rank: Super Contributor
timestamp: October 03, 2024, 11:19:26 pm
content: Time domain resolution is a matter of the highest frequency, not a matter of the number of points.For a given resolution, the length of the time axis (before it wraps around) is proportional to the number of points.For a long cable you either need a large number of points, or you need to reduce resolution by choosing a lower stop frequency.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2024, 02:27:56 pm
content: Quote from: Pinörkel on October 03, 2024, 09:20:57 pmI think the only way to get this to work is add a new option for the METAS software. When selected, rather than step = (stop-start)/points I would use step = stop/(points-1), then prepend the first frequency with the start. This obviously would cause a step in the first data point but it would always be less than the max step size. So the stop frequency would always be a multiple of the step size.For example you want to sweep from 50k to 6.3G using 401 data points. The software would sweep 50k, 15.750M, 31.5M, 47.250M ... 6.3GBut again, I want to be very clear. The LiteVNA can be used to maybe 8.5GHz. It's just not going to provide you with enough range to provide the resolution needed to look at such short discontinuities like a connector would have. At best you could maybe tell which connector combinations offer the least reflection.

author: gf
author_rank: Super Contributor
timestamp: October 04, 2024, 09:36:04 pm
content: Quote from: joeqsmith on October 04, 2024, 02:27:56 pm@Pinörkel: Let me add: Assuming a velocity factor of (say) 2/3, an 8.5 GHz lowpass mode TDR with (say) Blackman window has an intrinsic step response rise time of about 135ps or 27mm, and a sampling interval of about 59ps or 12mm (w/o interpolation). If you relate this to the size of the connectors, I think it becomes evident, why you cannot see many details happening inside the connector.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 05, 2024, 01:36:04 am
content: Hmmm, why is everyone suddenly assuming that I want to have a detailed look at the connectors?That would make very little sense given the fact, that BNC connections are pretty crappy in the world of VNAs. I am just trying to improve my measurement skills (which works well due to the tons of useful information everyone is generously dropping in this thread) and to optimize my measurement setup around the cables to be reasonably good for getting as much as possible out of the limited capabilities of the liteVNA.At the current point I am able to take fairly reproducible TDR measurements of my cables. The results, do not allow me to derive any reliable conclusions about the absolute cable impedance yet, partially, because I do not have sufficiently characterized calibrations standards. However, I can detect significant deviations from the target of 50Ω and I can see which cables have large local or global impedance variations. In addition to that I discovered that I can even detect quality issues of my BNC crimp connectors resulting from crimping quality variations by comparing the respective TDR peaks, which is also nice.On the non-TDR side, I can now measure the quality of my BNC terminations and do repeatable return loss and insertion loss measurements. At the moment, return loss and insertion loss measurements for my cables do not line up with what I would expect yet and I still have to determine if that is due to measurement errors on my side, insufficient equipment or simply my DUTs not being comparable with the reference measurements I am comparing them with.With respect to getting the absolute cable impedance values as good as possible I am currently investigating the software side of things, since I have observed large differences between software solutions that transform my S parameter measurements into the TDR domain. I found that on the time/length scale of the TDR graphs, the Solver64 software seems to align very well with reality when using the correct velocity factors for my cables, one thing where the VNA Tools seem to be completely off. The impedance values on the other hand appear to be more consistent in the VNA Tools than in Solver64 which I would base on the previously described behavior when switching the filters. Since I am not sure what is causing the observed differences between the software solutions it is possible that one software has an issue, both have an issue or none of both have an issue and the issue is only hiding in my brain. Maybe I should try to implement a minimal version of the time domain conversion on my own to get an impression what is happening there and to have some code I can play with and get direct feedback what changes to the code have which effects.Once I have sorted this out, I will try to optimize my calibration gear, by getting a better kit, or finding an opportunity to characterize one using well known standards.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 05, 2024, 02:02:12 am
content: QuoteWell, you have started double posting in two threads making it more difficult to track.Read my response:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5666095/#msg5666095

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 05, 2024, 11:46:10 am
content: Quote from: joeqsmith on October 05, 2024, 02:02:12 amSorry, i missed that. So, as I understand it, you are saying that the amplitude variations I observed are most likely a result of the lost signal amplitude that cannot be recovered by the IFFT because of the low-pass applied prior to multiplying the signal with the filter function in frequency space. If so, would it not be possible to compute the actual signal amplitude lost by low-pass filtering and to compute an amplitude loss correction factor from that? Or would that be nonsense and introduce a systematic error?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 05, 2024, 01:19:34 pm
content: Easiest thing would be to move up the cutoff.As mentioned already, this filter is after iFFT where the smoothing filter is prior to it and part of the data collection.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 05, 2024, 04:53:39 pm
content: A simple demonstration of my current proposed changes to Solver64Update readouts when loading S2P filesCombine stability trigger to other cursor handler, all cursors now trackChange 2-Port Adv. Plots to SI unitsChange TDR/TDT impulse curves to use the amplitude scaleSwap TDR/TDT samples/seconds graph axis selectionExpose all TDR/TDT filter settings to userAdd support for METAS time domain modes by using uneven step sizeUse zero phase filter for TDRShowing effects of increasing the low pass filter's cutoff frequency. Windows shown are rectangle, Kaiser, gaussian, Blackman, flat top, Hanning, Hamming, Welch.Shown also it the same data loaded into METAS. Note that the time scale is double on METAS. Like length, I am looking at the end to end, NOT the total travel time.Of course, if the goal was to replicate METAS, I could change this and ditch the zero padding..... Then again, we already have METAS so that's a pointless exorcise.None of these new settings are sticky but I could certainly add them to the defaults.***The phase error between the two programs was due to my use of a FIR filter. Updated to show a zero phase IIR.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 06, 2024, 12:47:08 am
content: Quote from: joeqsmith on October 05, 2024, 04:53:39 pmThat is indeed looking very nice.Quote from: joeqsmith on October 05, 2024, 04:53:39 pmYes, having both options is most likely better. Adding the new settings to defaults could be advantageous for some workflows, but also raise the probability to forget that those settings have been changed.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2024, 01:09:26 am
content: Let's try looking down into the muck. Cal the VNA, 401 pts, 4kHz IFBW, 50k-8.5G. Now let's reinsert the load. You can see there are obvious differences between how Solver and METAS process the data. I have included the Touchstone file if you would like to try other viewers and compare results.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2024, 01:37:09 am
content: Staying within 50k-6.3G, lower the IFBW to 1k, enable the smoothing filter, and only run the SOL (leaving the load attached), it should measure something very close to 50ohms at the the reference plane (time=0). Smith chart shows both smoothed data (red) and unfiltered (yellow) with a 100X magnification. Lucky I had added that for our R&S/Tektronix experiment....

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 06, 2024, 01:37:25 am
content: Quote from: joeqsmith on October 06, 2024, 01:09:26 amFascinating... the funny thing is, I do not even get results similar to yours when I open the file in METAS and adapt the settings accordingly. I found amath document(page 73), where they describe what they do and for the TDR low pass, it looks very basic without even using a cut-off.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2024, 01:46:06 am
content: That's strange. I would expect the exact same results when you import my Touchstone files. I just tried it with their Data Explorer and get the exact same results as with the VNA Tools. Make sure you are using the exact same settings.***... and the same revision of the tools.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 09, 2024, 07:24:16 pm
content: Quote from: joeqsmith on October 06, 2024, 01:46:06 amThis is what I get, when viewing the files and trying to match your settings. For the METASTDR_load.s1p file it looks like totally different data. For the METASTDR_load_smoot.s1p file, unlike in your screenshot, Solver64 here seems to agree with the display in METAS.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2024, 07:43:23 pm
content: I don't think Solver will allow you to import a Touchstone file and process the T-Check. But, maybe it can.... That import processing is very limited and something I had added for debugging. So, I am not surprised your screen shots with Solver look different.Your last graph in METAS using the smoothed data appears correct. I have no idea what is going on with the unsmoothed data.***Can you tell I am not thinking at all about what you are posting and have T-Checks on my mind.***Ok, the smoothed graph imported into solver also looks the same as what you show. Odd it would have been so different from my previous post. No idea why METAS has the offset.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2024, 08:02:07 pm
content: I just downloaded it back to my PC and ran a diff.. No problem.Processed it with METAS, and now we have a three different results.Forgot to set the start time. I get the same results as your graph. I am using a different version of METAS on that other PC. I'll try it again later and report back.At least things are making sense.***There was no difference between versions of METAS. I then repeated the same test with the smooth filter active and got the same results with Solver. Saved the file and loaded into METAS, matched Solver. So, I am not sure what I did but it appears I overwrote the files. Oh well, looks like they basically all match until we look at the tiny errors.I did post about the offset seen with METAS in their group to see if they could explain it. I would expect that T0 to be basically dead on at 50 ohms.***Looks like I had turned off the smoothing filter prior to exporting the Touchstone file. The problem is still the same (offset) but I have attached the data with the smooth active. I did not take the time to calibrate and torque it this time.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 10, 2024, 10:18:00 pm
content: Quote from: joeqsmith on October 09, 2024, 08:02:07 pmSomething similar already happened to me multiple times: Suddenly, the TDR graphs in Solver64 just make no sense. Later it turns out that I just forgot to turn off averaging in the main panel.I am still trying to figure out the best way to get reproducible impedance values. For that, I experimented with various differential measurements, where I added or removed things in the setup. One of those experiments was measuring two BNC cables in series, connected by a double BNC-F adapter. What I expected was that the TDR graphs of both cables would show the same shape and impedance, just their order was swapped. Instead, I consistently got higher impedance values on the cable further away front the VNA, no matter what cables I tried and how many times I redid the calibration or reconnected the BNC connections. Also, the H155 cables curves are not ascending as much, when the cable is the second in line. May that be an effect of accumulating integration errors, an effect of cable loss or something completely different?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2024, 04:19:18 pm
content: Quote from: Pinörkel on October 10, 2024, 10:18:00 pmWouldn't you expect some upward slope with longer cables due to the series resistance of the cable? Easiest way to determine if something is up with the cable's impedance is measure it in both directions. If you see that same upward slope, then you are dealing with the series resistance.I am not aware of any VNA software that attempts to compensate for the series resistance of the cable.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2024, 04:22:36 pm
content: Showing all 17 windows with the lowpass cutoff set to a high value. As expected there is basically no effect.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 11, 2024, 06:48:22 pm
content: Quote from: joeqsmith on October 11, 2024, 04:19:18 pmBased on the attached screenshot of two different H155 cables, also measured in reverse, I would say that seems to be the case. With "series resistance" you mean the DC resistance, or the frequency dependent cable attenuation? Since most cables have specs with attenuation given for different frequencies, I always wondered why I could not find a setting in Solver64 or METAs to compensate for that. So, I installed S2VNA, one of the VNA software packages from Copper Mountain Technologies and that software indeed has some cable correction settings to specify the velocity factor, return loss at a specific frequency(at least that is how I understood the last two settings). However, putting the attenuation for H155 cable in that setting, I got no visible effects on the TDR lowpass step trace. The TDR bandpass trace, on the other hand, was affected by an entered attenuation.Quote from: joeqsmith on October 11, 2024, 04:22:36 pmHmmm, I would have expected a quite noticeable lowpass effect on the curve, especially with flattop.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2024, 07:46:33 pm
content: Our friend Joel to the rescue.. (thanks to the power of Google and the internet):https://community.keysight.com/s/question/0D55a00008NHdPpCAL/tdr-impedance-measurement-theoryYou could try using various resistors rather than the cable. Measure their values with your multimeter then with the VNA. They should track somewhat close. This should give you some idea on the accuracy.Quote from: Pinörkel on October 11, 2024, 06:48:22 pmWhy? The whole point was to show that as we increase the cutoff frequency to where the low pass has little effect, the window type also has little effect. Think of a 1Hz signal that we put a 100Hz low pass on. Does it have much of an effect?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2024, 05:35:17 pm
content: Looks like the Keysight link is now dead, but thanks to Google, here are a few more articles written by Eric Bogatin that may help:https://www.signalintegrityjournal.com/blogs/8-for-good-measure/post/1212-instant-cable-testing-and-the-surprising-resultshttps://www.signalintegrityjournal.com/blogs/8-for-good-measure/post/1282-test-your-tdr-with-a-dmm

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2024, 09:41:55 pm
content: In my main software thread, I was attempting to measure my T-checks using the LiteVNA.https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5656063/#msg5656063While the overall results were poor, one of the things I tried was adding support of characterized standards (databased standards).I thought I would show the difference between the three different error models when measuring a section of air-line and my Beatty standard. The calibration standards are the same set I have been using from the V2Plus4 with the second best Mini0-Circuits ANNE terminator.The ideal model is what I have always used with these low cost VNAs, except when working with the waveguides.I had calibrated my old Agilent with an ECal that I had setup using a set of know Agilent standards I borrowed from 300K-9G. This was then used to measure the four low cost standards. These S-parameters are read by Solver for the databased model.METAS was used to fit the S-parameters to create the Agilent coefficients which were then fed into Solver.The differences between all three models was very subtle.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 12, 2024, 10:27:46 pm
content: Quote from: joeqsmith on October 11, 2024, 07:46:33 pmThe explanations on that website are very educative. Of course the reflection at some point is affected by the series resistance up to that point and the seen TDR impedance at the end of the cable must be bigger by the resistance of the cable. Had a look at the datasheet of my H155 cable and also verified, that the center conductor resistance is lower than 0.015Ω for my three cables. However, even if I take that value and add the outer shield resistance, I still have a very small value that is totally dominated by the cable impedance changes of my cables.Quote from: joeqsmith on October 11, 2024, 04:22:36 pmThat is because my brain is still not used to how the cuttoff frequency is tied to the half amplitude point of the lowpass filter function an thereby determines its width. I did not realize that you raised the cutoff frequency that much.Quote from: joeqsmith on October 11, 2024, 07:46:33 pmThose are very good links, thank you. I just tried that before you posted the additional excellent links and of course messed up the measurement method. Since I do not have any SMA connectors lying around, I soldered some female gold plated pin header contacts to a BNC stub, so I could insert some 0.1% metal film resistors there. I measured the resistors on my Keithley 6500 and checked with the "Impd Rectangular" function of Solver64 in the range of 100k to 200k (as close to DC as possible). As I learned from your third link, this was not how to do it, so I also measured in the convergence region of the TDR graph, with rectangular and Hanning filter. This is what I got:ResistorDMMImpd RectangularTDR convergence rectTDR convergence Hann20 Ω19.94 Ω20.2 Ω27.9 Ω16.3 Ω47 Ω47.01 Ω46.5 Ω46.45 Ω44.3 Ω100 Ω99.98 Ω111.1 Ω73.3 Ω103.75 ΩSo, my first measurement idea got close, but messed up the 100 Ω resistor and the two TDR based measurements were jumping all over the place depending on the selected filter function. At least, my measurements were quite reproducible.When looking at the cables that were measured in the linked articles I have the impression that those were orders of magnitude better with respect to impedance stability than my cables.Also, If I understand correctly, the series resistance effect should also be visible when viewing non-VNA-based TDR measurements, like with a Tektronix 7S12 TDR plugin. However, the TDR traces in the article that did TDR measurements on some Tektronix cables (link), I cannot see this effect at all.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 12, 2024, 11:37:49 pm
content: Quote from: joeqsmith on October 12, 2024, 09:41:55 pmSo, according to your experiments it would be sufficient for low cost VNAs to get a a good quality load and not bother getting a characterized calibration set because the difference is hardly noticeable on these devices?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2024, 11:46:06 pm
content: Quote from: Pinörkel on October 12, 2024, 10:27:46 pmNot sure why but all I see now is a white page. I should have converted that to PDF as it was a good post from Joel.Quote from: Pinörkel on October 12, 2024, 10:27:46 pmYes, it was a pretty big change.Quote from: Pinörkel on October 12, 2024, 10:27:46 pmAs you have found out, the heavy lowpass filter is causing some errors with this test. Solver 5.10 will make all of these settings available to the user. Most likely, I will pick some values that are a bit more friendly. Shown using the TDR to measure from 10 - 500 ohms.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 13, 2024, 12:46:51 am
content: Quote from: joeqsmith on October 12, 2024, 11:46:06 pmYou most likely have some sort of script blocker active(like NoScript), either in the browser or a network based one. Anyways, I have converted it to PDF with (expanded posts) and attached it.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2024, 02:20:22 am
content: Thanks for the conversion. I have Ublock and Badger, but they were running when I first viewed it. Odd.I have seen a few posts about the need to have the standards characterized for accurate measurements. I've seen posts like the one you linked where people have measured them. I haven't seen a lot of data showing what, if any improvements they saw or where it mattered.

author: gf
author_rank: Super Contributor
timestamp: October 13, 2024, 06:30:56 am
content: Joe, to be able to understand what is going on: Where in the processing chain is this lowpass filter applied? Is it after IFFT, applied to the time domain impulse response?

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 13, 2024, 11:45:30 am
content: Quote from: gf on October 13, 2024, 06:30:56 amThe process should be that the filter function is applied before transforming from the frequency domain to the time domain using the IFFT. At that point the frequency domain should have the shape of a harmonic grid, extended by a conjugate complex version(mirrored copy into the negative domain) and an added DC point at zero. The filter function is then multiplied onto the frequency data with its center at zero (which equals a convolution in the time domain) and its width determined by the cutoff frequency. While scaling the filter function horizontally, I would also expect some vertical scaling to be applied to the filter function, maybe to make the area below the function stay the same, but I am not sure about that. My original theory was that this last part done wrong could have been responsible for the observable amplitude jumps when switching the filter functions.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2024, 05:21:20 pm
content: Quote from: gf on October 13, 2024, 06:30:56 amRead the followinghttps://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5666745/#msg5666745https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5627253/#msg5627253https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5666095/#msg5666095https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5626535/#msg5626535

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2024, 05:30:56 pm
content: Quote from: Pinörkel on October 13, 2024, 11:45:30 amSolver does offer the ability to export Touchstone files which can then be post processed (by METAS or your own custom software).

author: gf
author_rank: Super Contributor
timestamp: October 13, 2024, 08:42:33 pm
content: Quote from: Pinörkel on October 13, 2024, 11:45:30 amThe quantity that affects the height of a step (after the step has fully settled) is the DC gain of the lowpass filter, which must be 1. In the frequency domain, only the DC bin determines the DC gain, and for a convolution kernel in the time domain, the sum of taps must be 1.

author: gf
author_rank: Super Contributor
timestamp: October 13, 2024, 08:44:45 pm
content: Quote from: joeqsmith on October 13, 2024, 05:21:20 pmQuoteThanks, I had missed that. Is it just a FIR filter? Or something "magic" (nonlinear / adaptive)?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2024, 09:44:30 pm
content: Quote from: gf on October 13, 2024, 08:44:45 pmRead the following:https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5666975/#msg5666975

author: joeqsmith
author_rank: Super Contributor
timestamp: October 17, 2024, 06:41:17 pm
content: I have not made any progress with understanding the offset seen with METAS. I downloaded CMT's software and it appears to work very similar to METAS. IMO, this is a problem with how they both process the data. Maybe Brian at CMT can chime in.@virtualparticleshttps://groups.google.com/g/vnatools/c/GhsrPlfaDVA?pli=1Quote****I have provided one last example on the METAS group showing a fixed RL of 10dB @ 0 deg. I would expect both programs to show a stable impedance at T0.Or maybe not... Seems there is still going to be a step.. May not be a good example.

author: gf
author_rank: Super Contributor
timestamp: October 18, 2024, 12:13:44 am
content: Quote from: joeqsmith on October 17, 2024, 06:41:17 pmThere is a reflection due to an impedance discontinuity at t=0 (from 50 Ohms to 96 Ohm), therefore you get a step at t=0. The step represents the impedance transition. You see the new impedance not before the step has settled. Due to practical limitations the step can only have limited risetime, smearing the step symmetrically. I.e. the edge starts to rise before the discontinuity, and ends after the discontinuity.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 18, 2024, 01:13:00 am
content: Quote from: gf on October 18, 2024, 12:13:44 amI agree, that is what I would expect. Sadly I screwed the pooch when I posted that data and had TDR only select, but the CMT plot was correct. I have included the METAS plot here. The point was that rather than seeing T0 at 50, then stepping to 96, it actually shows T0 at 68. 50 is prior to T0.***Note also that the CMT software also was very close to 68 at T0. I would think in the real world, T0 would be 50.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 18, 2024, 01:25:29 am
content: Flipping the phase has no effect. Shown here with a fixed return loss of -160dB at 180 degrees. Both CMT and METAS will use 50 ohms, not at T0 but at the flipped negative start.

author: gf
author_rank: Super Contributor
timestamp: October 18, 2024, 06:41:38 am
content: Quote from: joeqsmith on October 18, 2024, 01:13:00 amIdeally (if your VNA could measure up to infinite frequency), you are supposed to get 50 at t<0, 96 at t>0, and adiscontinuityat t=0, so that you cannot tell what the the value at exactly t=0 really is.Due to the limited upper frequency of the VNA and the windowing, the discontinuity gets "smeared" and turns into a transition region, spanning approx. -110ps to +110ps in your last example. You are not suposed to see 50 at t=0. That's inside the transition region. You are supposed to see 50 prior to the transion region, and 96 after the transition region, as you would see 50 before an ideal step, and 96 after an ideal step.What is the expected value of the smeared step at t=0? S11 at f=0 is -10dB, or 0.31623. Since the impulse response of the window function is symmetric, 50% of 0.31623 are supposed to be integrated before t=0, and the other 50% are supposed to be integrated after t=0. So the smeared impedance step is expected to cross t=0 at an impedance value of 50*(1+0.15811)./(1-0.15811) = 68.781 Ohm. [ But you won't care about the value in the transition region anyway. ]EDIT: Also note that the conversion from S11_step(t) to impedance(t) is nonlinear, so we don't get (50+96)/2 Ohms at t=0, even though the S11(t) step is point-symmetric. The corresponding impedance step not point-symmetric. Only for small S11, the conversion becomes approximately linear.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 18, 2024, 01:04:47 pm
content: Quote from: gf on October 18, 2024, 06:41:38 amThe problem I am having is the algorithm they use doesn't agree with what I "feel" is happening in the real world. Assuming we calibrate to the reference plane, that's the only location we actually know what is going on. As we move away from the reference plane the errors increase. The idea that the actual cal'ed value doesn't occur at the reference plane but before, and the transition region isn't after the reference plane makes zero sense to me.Your comment about "... you won't care about the value in the transition region anyway. " is spot on. This whole idea of looking at the TDR at this level of scrutiny isn't something I had considered, until the OP started this thread. I blame them.

author: gf
author_rank: Super Contributor
timestamp: October 18, 2024, 05:54:00 pm
content: Quote from: joeqsmith on October 18, 2024, 01:04:47 pmIf you look at the impulse response instead, then I think your feeling won't complain about the horizontal position of the impulse, as it peaks exactly at t=0, at the reference plane. See attached image. You would rather complain if the peak would not be at t=0.But if you integrate this impulse response in order to get the step, then of course the edge starts to rise before t=0, because the impulse width is > 0, and the impulse is centered at t=0.If you want to know the horizontal position of an impedance discontinuity, then look at the impulse response, not at the step.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 19, 2024, 03:40:30 pm
content: In the case of our perfect load being attached to the VNA after cal, similar to how I expect the step to be 50 at T0, I would expect the impulse to be zero, then slowly increase as we move away from the reference plane.I understand what CMT and METAS are doing math wise but again, I am having problems visualizing it mechanically.While playing with CMT and METAS, I noticed there are differences in how they process the data. It's noticeable as we move further away from the reference plane. As far as changing Solver's TDR, I would want to understand these differences before deciding how to proceed.I was looking at Eric Bogatin's presentation on LeCroy's TDR (linked below). I thought I would see how their software handles the reference plane, but it appears that they do not allow access to their software.I like how the CMT software allows you to select the time scale for the round trip. I may add that to Solver.

author: Pinörkel
author_rank: Regular Contributor
timestamp: October 19, 2024, 04:10:05 pm
content: Quote from: joeqsmith on October 18, 2024, 01:04:47 pmFor the amount of interesting and useful information this thread already unearthed, I gladly accept that blame.

author: gf
author_rank: Super Contributor
timestamp: October 19, 2024, 08:08:55 pm
content: Quote from: joeqsmith on October 19, 2024, 03:40:30 pmWell, just mandate your feeling to accept that you are not getting a causal or minimum phase response, but a zero-phase response. That's what a windowed IDFT is supposed to do. The implementation has not really room for interpretation in this regard. Sure, to make your feeling happy, you could add an artificial delay, but I don't see how this is really useful at the end for other purposes.QuoteCan you post a screenshot with the difference and the corresponding s1p file?[ Btw, except for vector averaging of the complex-valued S11(f) of multiple traces (in order to reduce noise), don't pre-process the acquired S11 data you save to the s1p file. No smoothing or similar "cheating". ]

author: joeqsmith
author_rank: Super Contributor
timestamp: October 19, 2024, 08:23:00 pm
content: It's not the delay, but rather the impedance offset at T0 that I question.Quote from: gf on October 19, 2024, 08:08:55 pmSorry, you lost me. If you referring to Solver, it is up to the user to decide the number of samples, IFBW, averaging, smoothing and calibration they want. The program will use these settings to preprocess the data prior to saving the Touchstone file.For synthesized Touchstone files, there is no need for any preprocessing. I just create the data I want.

author: gf
author_rank: Super Contributor
timestamp: October 20, 2024, 12:39:52 am
content: Quote from: joeqsmith on October 19, 2024, 08:23:00 pmQuoteAssume that you feed a 100 Mhz CW signal into a spectrum analyzer. At which frequency would you like to see the peak of the bell-shaped curve? I guess you would like to see the peak at 100 Mhz and not at say 101 MHz.Now you do a TDR and there is a discontinuity at t=0? Why would you like to see the impulse response peak somewhere at t > 0, and not at t=0 where the discontinuity actually is? Doesn't it make sense to place the peak eaxactly at the position of the point of interest?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2024, 02:42:06 am
content: With a perfect load at T0, I expect there to be no discontinuity and it to display an even 50 ohms.

author: gf
author_rank: Super Contributor
timestamp: October 20, 2024, 02:16:26 pm
content: Quote from: joeqsmith on October 20, 2024, 02:42:06 amIdeally, yes. In the presence of noise, no. It also depends on how you extrapolate frequency points which have a frequency below the VNA's lower limit, which includes in particular 0Hz.To get a feeling for the uncertainty, let's do a quick and dirty monte carlo analysis, create 1000 synthetic readings with S11(f) = 0 plus -60dB of Gaussian noise, and look at the TDR traces.Code:S11 = 0.001 / sqrt(2) * (randn(401,1000) + 1i*randn(401,1000));S11(1,:) = 0.001 * randn(1,1000); % DC, must be realS11 = [ S11; flipud(conj(S11(2:end,:))) ];w = repmat(ifftshift(kaiser(801,13)),1,1000);imp = fftshift(real(ifft(S11.*w)));figure 1plot([-400:400]/18,50*(1+cumsum(imp))./(1-cumsum(imp))); grid onEach of the traces in figure1 should be horizontal at 50, if the readings were noise-free. So obviously there is quite some uncertainty due to the noise, and it increases with time/distance. Already from the left boundary up to t=0, quite some uncertainty has been accumulated by the integration. To improve the uncertainty near t=0, it may help to rotate the time axis so that it starts not at -22ns, but at say -1ns. [ Note that the time axis resulting from the IFFT is circular and wraps around. ]

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2024, 06:25:55 pm
content: In the attached image, the reference plane is shown as T0. Anything to the left is <T0. Anything to the right, >T0. The SOL were measured at the reference plane. The shown system has some disturbance before the reference plane. We have attempted to remove these errors with the SOL.We attach a DUT to the reference plane, but look at the area <T0. Are we able to accurately measure the disturbances?

author: gf
author_rank: Super Contributor
timestamp: October 20, 2024, 07:27:25 pm
content: What is the length of your time axis? And what is the expected time/distance to the disturbance?Is it possible that the time of the disturbance is beyond the right end of the time axis and did wrap around into the negative region (maybe even multiple times if the DUT is very long)?In order to get a longer time axis, you need to reduce the spacing between the frequency points (i.e. either increase the number of points, or reduce the stop frequency of the sweep).

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2024, 12:29:18 pm
content: Quote from: gf on October 20, 2024, 07:27:25 pmPer the drawing, the length of >T0 is longer than the disturbances at <T0.***Not that it matters as the simple question is, can we measure anything meaningful to the left of the reference plane using TDR? Because you want some details, I have changed the drawing to show some rough lengths. A 21" section of coax attached to the VNA, followed by 12" of air-line which has a disturbance. Our DUT will be a 10" section of ridged coax with a shorted end. The reference plane is defined where this ridged attaches to the air-line. I have chosen these values as they are parts I have on-hand and if there is a question as to what happens, it will be easy to measure it in the real world.The VNA is setup to locate the shorted end of our ridged cable. The disturbance in the airline is closer the the reference than the short.When we look at the data in the region of <T0 with the CMT or METAS, do we see this disturbance?

author: gf
author_rank: Super Contributor
timestamp: October 21, 2024, 08:02:43 pm
content: My understanding is that disturbances between VNA port and calibration plane are supposed to be compensated by the calibration (provided that they are within reasonable limits). They don't show up in the reported S-parameters anymore. So basically you measure the DUT as if it were connected to an ideal 50 ohm VNA port located at the calibration plane.Quote from: joeqsmith on October 21, 2024, 12:29:18 pmIMO not. But if a disturbanceshows upat a negative time, it can be located at a positive time in the real world.As I already mentioned earlier, the IFFT time axis is circular and has a finite length.Suppose your time domain points after the IFFT are [ -5,-4,-3,-2,-1,0,1,2,3,4,5 ] ns.Then everything you see at (say) -3ns could habe happened at 8ns, or 19ns, or 30ns, or 41ns, etc. in the real world.All times -3ns+N*11ns, where N is an integer and 11ns is the length of the IFFT time axis, are aliases of -3ns.If you decide that you want to display the circular time axis starting at -2ns, you can rotate it and get [ -2,-1,0,1,2,3,4,5,-5,-4,-3 ] ns.But then you could also label it [ -2,-1,0,1,2,3,4,5,6,7,8 ] ns, because 6ns is an alias of -5ns, 7ns is an alias of -4ns, and 8ns is an alias of -3ns.EDIT: The DUT is not limited to a single reflection, but can produce multiple reflections, possibly bouncing back and forth. Then the last one can be much farther in the future than 2x the one-way travel time of the DUT. So a time axis length of 2x the one-way travel time of the DUT is not necessarily enough to avoid ambiguities.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2024, 09:08:12 pm
content: Quote from: gf on October 21, 2024, 08:02:43 pmWe agree.Quote from: gf on October 21, 2024, 08:02:43 pmSmall steps but it's good we can agree on such basics.I would also state that as we move from T0 to >T0 we have an increase in error as you have shown in your modeled demonstration.https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5683481/#msg5683481Now if we agree that the region of <T0 is meaningless, what possible purpose does displaying it have in the real world? CMT and METAS must feel it has a use.Looking at your previous data, like CMT and METAS, you show the perfect convergence happening in the region of <T0, where it it meaningless. I would expect a perfect 50 only at the reference plane. It then diverges as we move toward >T0. To date with Solver, I do not allow the user to look at meaningless data before T0. I also converge to 50 at the reference plane. If you have a discontinuity, it still converges to 50 at the reference plane as that is the only location we actually know is 50.https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5667377/#msg5667377Anyway, interesting discussion. I could be totally wrong with my thinking and would welcome any published text from the main suppliers on this subject.***I should be clear that I was thinking that as we move toward <T0 the error in the meaningless region also increases, and the error band would resemble a butterfly.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2024, 09:32:15 pm
content: For others that may not agree that once we have calibrated to the reference plane that using S11 TDR can not be used to looking at what is physically happening prior to the reference plane.Showing the describe setup with VNA+coax+beatty+coax+short. First with no attempt to calibrate the VNA. Then with the calibration applied between the Beatty standard and our shorted coax. Note after calibration, there is no sign of the discontinuity caused by the Beatty standard. We have corrected for it.Good when things do what you think they should....

author: gf
author_rank: Super Contributor
timestamp: October 21, 2024, 11:14:39 pm
content: Quote from: joeqsmith on October 21, 2024, 09:32:15 pmCould you post the .s1p (with calibration applied)?I'll reply to your other message tomorrow.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2024, 11:35:53 pm
content: Quote from: gf on October 21, 2024, 11:14:39 pmNo problem. See attached.***Just an FYI, this is using that METAS mode I had mentioned earlier. METAS will read this file directly. Oddly, when I tried CMT, it could import and plot the TDR data without these changes. Just another difference between the two programs.

author: gf
author_rank: Super Contributor
timestamp: October 22, 2024, 09:28:07 am
content: Quote from: joeqsmith on October 21, 2024, 11:35:53 pmI just want to extract the numbers and don't care so much about format compatibility with a particular software.There are still disturbances visible in the -2...0ns region. Sure, they are small, but compared to the noise in the flat -12...-2ns region, they are statistically significant. I wonder, is it due to imperfections of the calibration?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2024, 01:23:47 pm
content: Quote from: gf on October 22, 2024, 09:28:07 amConnections were finger tight, and not secured. That said, my attempts to sort out a good calibration for the LiteVNA64 have not yielded good results when using the T-Check method to verify them. You can read about that starting here:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5656063/#msg5656063If you're not much of a reader, I strongly suspect I am limited by the VNA itself. I now have the latest revision (LiteVNA64 64-0 3.2). It appears to have lower noise than the previous two revisions. I ran a T-Check with finger tight connections and the results appear much better. My plan is to check it again after allowing the VNA to warmup and using proper torque.