[Solved] NanoVNA says impedance of my 3200R resistor is 542R-1250X at 50 MHz - Page 1
Forum section: electronics / projects

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 15, 2019, 05:38:26 pm
content: I recently purchased a NanoVNA for playing with radio electronics, mainly HF radio for now, and I've already measured some filters and the result looks good. Now I'm trying to understand the behavior of toroid impedance transformers and baluns - it seems every amateur radio operator can wind them, but I never get it, so I decided to do some experiments.I etched a single-layer FR4 test fixture with some holes and two SMA connectors, so I can solder different components and see how it measures. My methodology is basically: (1) solder a transformer on it, (2) solder a resistor dummy load, (3) do a one-port S11 measurement to find the return loss (or SWR), and keep tweaking it and repeats the steps. If the results look reasonable, I assume the transformer is doing impedance transformation correctly, then I can wind another identical transformer, connect them back-to-back, remove the resistor, and do a two-port S21 measurement to find out the insertion loss.It worked fine at HF. But I started to get strange results when I move into VHF (output transformer needed for an upconverter, and it's only 30-60 MHz). As a sanity check, I removed everything from the test fixture, leaving only the SMA connector, and a 3200R metal film resistor and did a single-port measurement. NanoVNA reports the impedance 3200R-411X at 50 kHz as expected, but at 50 MHz, astonishingly, it becomes 542R-1250X (Z=1362R). I think it's why I was getting strange S11 measurement for my transformer - even a resistor doesn't behave like a resistor for me. See the Smith Chart attached.I've never used a VNA before and I don't know understand the RF black magic here (I thought 100 MHz would be black magic, but apparently even 30 MHz is black magic). What could be the reason of these measurements? Is it a real result, or it's just a bogus artifact of the VNA (or, should a do a SOLT calibration on my test fixture)? Is it the parasitic elements in my test fixture? Parasitic elements of the resistor (but I heard ordinary metal film resistor works up to 400 MHz without seeing significant change of impedance)? Transmission-line effect? I suppose there can be some transmission-line effects, but I didn't expect to see an effect this great.Update: Case closed. It doesn't work because I shouldn't attempt to do it. A VNA is not designed to work under significant impedance mismatch. If you measure S11 for a mismatched load, the results are bogus.

author: jmelson
author_rank: Super Contributor
timestamp: October 15, 2019, 07:49:51 pm
content: If it is a wire-wound resistor, I can totally believe this reading. The R is low due to interwinding capacitance, and the X is there due to inductance.See if you can find a real carbon composition (bulk carbon element) resistor and check it. Even carbon-film and metal-film resistors often have helical grooves in the element that create inductance.Jon

author: exe
author_rank: Unknown
timestamp: October 15, 2019, 08:00:04 pm
content: photo of resistor?

author: ogden
author_rank: Super Contributor
timestamp: October 15, 2019, 08:21:12 pm
content: Note that best accuracy of 50-ohm matched RF instruments is measuring impedances which are as close to 50Ohms as possible. The rest of the questions could be answered by following video:https://youtu.be/pXjFS2MhuqI?t=1

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 15, 2019, 08:32:44 pm
content: Here is the resistor.I think it is a metal film resistor, is it? I purchased them from a random online vendor, the vendor claims it is "precision metal film resistor", 5% or 1%, don't remember. I hope they don't sell wirewound resistors masqueraded as fake metal film resistors in Shenzhen, but please double-check....Also, the board has been reworked for nearly 30 times with a lot solder and nasty paint residue from the magnetic wires. Perhaps these substances are somewhat conductive or capacitive?

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 15, 2019, 08:38:53 pm
content: And yes, the coax has been calibrated with a set of open, short, and 50R load standard before all measurements. Otherwise the capacitance of the coax distorts the measurement so much and creates bogus results. After calibration, a 50R RF dummy load measures as a clear dot on the Smith Chart.I'll try a SMD resistor later.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 15, 2019, 08:50:50 pm
content: Quote from: jmelson on October 15, 2019, 07:49:51 pmOops, I wrote the wrong sign. The measurement is actually 542R MINUS 1250X. If it's inductive, I would've accepted it. But you see, on the Smith Chart it's clearly CAPACITIVE. It's why I suspect the results are completely bogus. Perhaps it's out of calibration due to the effects of the fixture, or perhaps this VNA is not even designed to measure anything two order-of-magnitude greater load than 50R... Or there is a hidden capacitor somewhere I cannot find.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 15, 2019, 09:49:18 pm
content: I did not have 50R SMD resistors in the partbin for now, so I tried more THT resistors in my collections. First I tried a 47R resistor, the VNA shows pretty accurate and flat result throughout the spectrum. I later tried a 910R resistor, at 50 kHz it's accurate, but at 50 MHz, the real part becomes 693R and capacitive. Previous strange results during transformer testing was probably an isolated issue due to forgetting to calibrate the coax, not related to this.Anyway, I think... Case closed. I learned it the hard way: One should not attempt to measure S11 using a VNA if there's a significant impedance mismatch. It's a stupid thing to do, because the result will be completely unreliable. Trap for young players!

author: Bud
author_rank: Super Contributor
timestamp: October 15, 2019, 09:54:28 pm
content: You got it correct, none of the VNAs would accurately measure such large impedance.

author: virtualparticles
author_rank: Regular Contributor
timestamp: October 17, 2019, 12:55:04 am
content: au contraire!! One can make extremely acccurate impedance measurements with a VNA. See "Make Accurate Impedance Measurements Using a VNA", Microwaves and RF, June 21st, 2019 written by yours truly.

author: hendorog
author_rank: Super Contributor
timestamp: October 17, 2019, 12:59:21 am
content: Quote from: virtualparticles on October 17, 2019, 12:55:04 amThank you sir, that is what the crowd has come to seeLink here:http://www.clarke.com.au/pdf/CMT_Accurate_Measurements_VNA.pdf

author: ogden
author_rank: Super Contributor
timestamp: October 17, 2019, 04:53:45 am
content: Quote from: hendorog on October 17, 2019, 12:59:21 amNote that said accuracy can be achieved with CMT VNA that has whooping 140dB dynamic range and very good reflection measurement accuracy. NanoVNA is far from that. Actually it would be good to have similar impedance error chart for nanoVNA. Keysight impedance measurement handbook is another good paper to read for VNA beginners:https://literature.cdn.keysight.com/litweb/pdf/5950-3000.pdf

author: hendorog
author_rank: Super Contributor
timestamp: October 17, 2019, 09:41:11 pm
content: Quote from: ogden on October 17, 2019, 04:53:45 amI think the point of the document is that using the correct method makes an enormous difference to the accuracy which can be obtained - whatever VNA used.I agree it would be good to get a similar chart for the nano, but I expect it is going to be very frequency dependent in the nano. So picking a limited frequency range will likely be needed.No doubt a CMT VNA is way better than the Nano, but the paper is based on the CMT datasheet spec, which I assume is worst case over the entire span - all the way to 6.5GHz.The Nano even in its fundamental range below 300MHz will still be worse, but maybe at or below 10 MHz it might not be terrible in comparison - as we are comparing it at 10MHz to the worst cast CMT VNA spec at 6.5GHz. No maths was harmed while writing in this sentence, so feel free...There is still fixture removal maths and a common mode transformer required for series measurement. Also I assume the lack of 2 port calibration will be an issue. Maybe 2 port 1 path?

author: ogden
author_rank: Super Contributor
timestamp: October 18, 2019, 05:18:48 am
content: Quote from: hendorog on October 17, 2019, 09:41:11 pmIndeed. NanoVNA have many ranges. The higher the range the worse performance: 1~100MHz, 100~300MHz, 300~450MHz, 450~900MHz.QuoteEverything is there already. NanoVNA have both, fixture removal (as delay setting) and full SOLT calibration for S11 and S21. Note that NanoVNA do not have hardware for S22 and S12, accordingly no calibration for those.

author: hendorog
author_rank: Super Contributor
timestamp: October 18, 2019, 08:32:53 pm
content: Quote from: ogden on October 18, 2019, 05:18:48 amI think there is a bit more to it:Fixture removal is more than just fixing the delay - as described in the paper. Maybe that is good enough, but it will add to the error.The need for the common mode transformer is also mentioned in the paper. I don't know how much error that will add either.The SOLT calibration on the nano, and in the software I've seen for it, is just a response cal on the through. It doesn't correct for load match or source match. That will increase the error for series measurements. There is an improved cal type called enhanced response which improves this, but still doesn't correct for the load match of the CH1 port. You can use an attenuator to improve that match, and that will degrade the dynamic range.http://literature.cdn.keysight.com/litweb/pdf/5965-7709E.pdfThere is another type of calibration which was implemented on the 8753 + T/R test set called 1 path 2 port, which requires the DUT be manually reversed. I think that cal type can be almost as good as a full 2 port cal if the switching hardware existed in the nano.I'm not sure if I am up to working out the maths on these errors...By pulling the data from the nano and using the calibrations available in scikit-rf or equivalent, the maths issues here could be resolved. Still need to build or buy a balun though.

author: ogden
author_rank: Super Contributor
timestamp: October 18, 2019, 08:46:43 pm
content: Quote from: hendorog on October 18, 2019, 08:32:53 pmNano does full phase/magnitude SOLT calibration for both reflected S11 and thru S21 and not as you say "just a response cal on the through". Please explain what you mean by saying so and why you think it is so.

author: hendorog
author_rank: Super Contributor
timestamp: October 18, 2019, 09:36:03 pm
content: Quote from: ogden on October 18, 2019, 08:46:43 pmOk you have prompted me to have a closer look at the code and the nano is a bit better than I thought. It isn't what I would call 'Full calibration' though:Where I'm coming from is, in order of accuracy:1. Response calibration is the lowest form of through calibration. This is just a normalisation and corrects for Transmission tracking.2. * Enhanced Response calibration. Uses the OSL information to correct for source match as well. It doesn't correct for load match. You can use an attenuator to improve load match.3. 1 path 2 port calibration. I _think this should_ correct for everything and be almost as good as a Full 2 port cal, but requires a manual DUT reversal.4. Full 2 port calibration, the ultimate, but it is not possible on the nano.* I now think the Nano implements Enhanced Response cal. Whereas previously I thought it was just a Response cal. So one step better than I thought. Which is good.I'm not sure if this is widely known. I haven't seen this laid out anywhere.Here is a page on ER and Response cal:Response:http://ena.support.keysight.com/e5071c/manuals/webhelp/eng/measurement/calibration/basic_calibrations/thru_response_calibration_transmission_test.htmEnhanced Response:http://ena.support.keysight.com/e5071c/manuals/webhelp/eng/measurement/calibration/basic_calibrations/enhanced_response_calibration.htmEdit: all of the above is related only to the _through_ measurement - i.e. S21. Perhaps that wasn't clear either.S11 measurements are based on the OSL cal. That is fine as long as you disconnect two port devices and terminate them with a good load. Otherwise the measurement will be influenced by the nano not correcting for Load match.

author: ogden
author_rank: Super Contributor
timestamp: October 19, 2019, 07:04:44 am
content: Quote from: hendorog on October 18, 2019, 09:36:03 pmNo. You are drifting away from topic. Accurate impedance measurements mentioned in thepaper you linkedusesonly S11 and S21. As you already find out, Nano does both.QuoteObviously. "T" letter in the SOLT abbreviation means "through". And S11 do not have "through" because it is single portQuoteTerminating open ports of DUT is impedance measurement basics, but thank you for reminding - perhaps someone will benefit from this info.

author: hendorog
author_rank: Super Contributor
timestamp: October 19, 2019, 11:29:50 am
content: Quote from: ogden on October 19, 2019, 07:04:44 amNope. The point is valid. The link I referenced doesn't mention that cal type, which is why I pointed that out as a separate item.It is a cal type implemented on the 8753. The 8753 supported an 85044 T/R test set.Actually being able to measure S11 and S21 isn't the point. The point is how accurately you can measure them. That accuracy is dependant on the type of cal you do. This entire discussion is about how to measure various impedances accurately is it not?

author: ogden
author_rank: Super Contributor
timestamp: October 19, 2019, 12:00:51 pm
content: Quote from: hendorog on October 19, 2019, 11:29:50 amI did mean that NanoVNA S21 and S11 measurementand calibrationcapabilities are enough to perform measurements described in the article. I suspect that you think that S21 thru calibration do not measure/calibrate Port2 impedance. Well, it does. BTW components such as resistor & inductor (some other components as well) are "single port devices" that do not need to be rotated during impedance measurements.

author: hendorog
author_rank: Super Contributor
timestamp: October 20, 2019, 07:57:55 pm
content: Quote from: ogden on October 19, 2019, 12:00:51 pmI give up. We are miles apart in understanding here.

author: ogden
author_rank: Super Contributor
timestamp: October 20, 2019, 09:22:24 pm
content: Quote from: hendorog on October 20, 2019, 07:57:55 pmDon't give up. Explain. In more details thanlike:Quote from: hendorog on October 19, 2019, 11:29:50 am

author: hendorog
author_rank: Super Contributor
timestamp: October 20, 2019, 09:27:12 pm
content: Quote from: ogden on October 20, 2019, 09:22:24 pmIt is entirely explained in the previous post.If you don't understand, then ask specific questions and I'll answer them - within my own capabilities to do so.

author: ogden
author_rank: Super Contributor
timestamp: October 20, 2019, 09:38:29 pm
content: Quote from: hendorog on October 20, 2019, 09:27:12 pmYour previous post was something like "I give up because I am way smarter".Besides your arrogant attitude, that does not explain much. So please be so kind and either provide link to post or even better: full quote. Then I will continue with questions.

author: hendorog
author_rank: Super Contributor
timestamp: October 20, 2019, 10:20:02 pm
content: Quote from: ogden on October 20, 2019, 09:38:29 pmLook, you are making comments like: "I suspect that you think that S21 thru calibration do not measure/calibrate Port2 impedance. Well, it does."I have repeatedly explained the calibration options available for the port 2 calibration, but you say something like that.Then:"BTW components such as resistor & inductor (some other components as well) are "single port devices" that do not need to be rotated during impedance measurements."Sarcastic much? They are only single port if measured in S11 shunt. The pdf which Brian (virtualparticles) wrote explains in detail that there are different methods. There are even pictures on the first page. Then there is a chart which helps us decide which method to use. It's gold.You just don't appear to be interested in learning, all I am seeing from you is a desire to argue.

author: ogden
author_rank: Super Contributor
timestamp: October 21, 2019, 05:10:17 am
content: Quote from: hendorog on October 20, 2019, 10:20:02 pmYes. You made such impression. Especially when you said following: "3. 1 path 2 port calibration. I _think this should_ correct for everything and be almost as good as a Full 2 port cal, but requires a manual DUT reversal" when we talk aboutimpedancemeasurements ofsymmetriccomponents like resistor, capacitor, inductor. Also knowing that NanoVNAdoes SOLTI calibration, you believed that it is just scalar analyzer. Whatever. As you promised to spread your wizdom, please stay tuned. Questions will follow. I will try to ask them one by one.QuoteHere's question: how and why impedance measurements can be improved by rotating symmetric component (DUT), let's say resistor, on S11+S21 instruments like NanoVNA and/or 8753 + 85044 T/R test set? I am listening. Please no obscure, better explain it well. Yes, I am interested in learning and others may benefit as well.

author: bson
author_rank: Unknown
timestamp: October 21, 2019, 04:25:13 pm
content: Any time delay calibration error will show up as a phase deviation in S11 measurements, which will then become reactance when deriving the impedance. For this reason, time delay must be measured using a reference load and derived from the phase measurement of the reference load/open to null it out. For RF you need reference data for the load/open (the calibration load, not input load), but for 10-50MHz this shouldn't be necessary - but it's definitely necessary to properly null it. (Possibly the short also.)

author: hendorog
author_rank: Super Contributor
timestamp: October 21, 2019, 08:03:36 pm
content: Quote from: ogden on October 21, 2019, 05:10:17 amYour ownisn't helping, and insulting me isn't helping either. You have only made general comments which are something like: 'nanovna can't do it because it has poor dynamic range', and then 'nanovna has everything which is needed' and then 'you think the nano can't measure phase'.You are just as confusing to communicate with as I don't understand how you are jumping to these conclusions.Firstly, let me say this: I agree that the nano can do these S11 and S21 measurements. I never said it couldn't. <-- Read this again.The point I am making is that there are different levels of accuracy which can be achieved.The measurement can be more accurate - or at least as accurate as possible - if we:* Use the right fixture for the measurement <-- see Brians paper* Do better fixture removal than just port delay only* Use a better calibration algorithm.* Limit the frequency range of the measurementOn the calibration algorithm, as I indicated I am not sure if the 1 path 2 port will turn out to be better than Enhanced Response.But lets find out...I will try to find a document describing it and will come back with a reference. Its implemented in scikit-rf I believe.In the meantime here is something I found where the 8753 designer talks about it briefly:https://community.keysight.com/thread/5003Quote

author: ogden
author_rank: Super Contributor
timestamp: October 21, 2019, 08:36:51 pm
content: Put analysis of my conclusions away and let me learn. You did not answer my question about DUT such as resistor, manual reversal and how it improves impedance measurements. Note that I am asking you specific question, aboutreversal of DUT which is resistor. Hopefully I do not need to explain what is resistor and how it differs from fixture with resistor in it? Quote from: hendorog on October 21, 2019, 08:03:36 pmThey talk about network analysis of two port devices such as filters amplifiers and so on. We talk about impedance analysis of symmetric components.

author: hendorog
author_rank: Super Contributor
timestamp: October 21, 2019, 09:32:31 pm
content: Quote from: ogden on October 21, 2019, 08:36:51 pmNo problem, same applies for me.The DUT reversal is _only_ related to the one path two port calibration. This is not a common thing anymore as everyone has 2 port analysers.A symmetric component is a special case of a two port component. In the case of a perfectly symmetric component and fixture then I agree with you - you don't need to reverse the DUT. That is an optimisation of the general case of one path two port.I'm trying to find some documentation which is very clear and unambiguous about one path two port, but I haven't succeeded yet.I don't know enough myself to answer the question about how it works. If I could then I would be off building VNA's for a livingBut anyway, here is a page from scikit-rf about it:https://scikit-rf.readthedocs.io/en/latest/examples/metrology/TwoPortOnePath,%20EnhancedResponse,%20and%20FakeFlip.htmlThe FakeFlip covers the case of a perfectly symmetric component.I think that page might have been what sowed the seed in my head in the past that this type of cal could correct for all errors. This comment is what I remember:QuoteI'm at work, so I can't spend much time on it.

author: ogden
author_rank: Super Contributor
timestamp: October 21, 2019, 10:26:22 pm
content: Quote from: hendorog on October 21, 2019, 09:32:31 pmGood. If we talk about reversal, then SOLT-calibration of the *fixture* both ways (on S11+S21 VNA) could possibly increase de-embedding precision (of the fixture). This potentially will increase precision of DUT impedance measurements which as we (at last) agreed, do not need to be reversed.

author: hendorog
author_rank: Super Contributor
timestamp: October 22, 2019, 12:01:51 am
content: Quote from: ogden on October 21, 2019, 10:26:22 pmGood.Now that we have got past that.The issue I have been talking about here is how to correct for the load match error in the nano. And of course how much it matters.

author: ogden
author_rank: Super Contributor
timestamp: October 23, 2019, 12:03:26 pm
content: Quote from: hendorog on October 22, 2019, 12:01:51 amYes, you are talking about that - I noticed. We can continue. As you already learned, reversal of the DUT won't help in our case of symmetric component impedance measurements. What remains - nothing much actually. Psuedo-Full Correction (FakeFlip) you linked seems like possible solution to improve something here, yet article does not cover details. You are welcome to explain how it works and what could be actual improvements using FakeFlip calibration.One thing for sure that can be improved - return loss of NanoVNA ports. Currently RL of Port2 is kinda poor (to say it politely): @299MHz it improves from -28dB (48.1+j3.1 Ohm) to -48dB (49.9+j0.4 Ohm) by adding quality 50Ohm 10dB matching pad. Port1 matching can be inferior as well. Disclaimer: I talk about NanoVNA I have which is black *clone* with no shields.

author: hendorog
author_rank: Super Contributor
timestamp: October 23, 2019, 08:27:24 pm
content: Quote from: ogden on October 23, 2019, 12:03:26 pmExcellent, there is no need to proceed any further in that case.

author: ogden
author_rank: Super Contributor
timestamp: October 23, 2019, 09:37:42 pm
content: Quote from: hendorog on October 23, 2019, 08:27:24 pmWhy don't you explain how FakeFlip works.

author: hendorog
author_rank: Super Contributor
timestamp: October 25, 2019, 04:27:33 am
content: Quote from: ogden on October 23, 2019, 09:37:42 pmI can't help you there.But I did keep looking for a good doc, and then I found this video which covers it perfectly I think.

author: ogden
author_rank: Super Contributor
timestamp: October 25, 2019, 07:59:50 am
content: Quote from: hendorog on October 25, 2019, 04:27:33 amThanx! Apparently we both looked. I foundthis wiki pagewhich covers your case perfectly I think.

author: virtualparticles
author_rank: Regular Contributor
timestamp: June 08, 2020, 01:31:58 pm
content: This is a real can of worms. One-Path, 1-port calibration is also called "Enhanced thru" calibration. The two main contributors to S21 uncertainty are "Transmission Tracking" and "Isolation". The Isolation error, or the leakage from port 1 to port 2 within the VNA is usually below thermal noise in the measurement bandwidth of most VNAs. Unfortunately, the isolation of the Nano is quite awful, so this calibration must be performed. After calibration, the isolation cal should hold for an hour or so. After that one will begin to see the noise creep higher at the high end of the frequency band.Transmission tracking is approximately equal to the sum of the products of the Raw Port 1 match and the Residual Port 2 match and the Raw Port 2 match and the Residual Port 1 match. To clarify the terms here, Residual means the "apparent" value after mathematical correction has been applied. A One-Path, 2 port VNA does not have a Residual Port 2 match calculation so the Transmission Tracking becomes a function of the sum of the Raw source and load matches alone. This results in ripples in the S21 measurement.ThanksBW