Author Topic: impedance measurement with VNA using series, shunt/series through methods, graph  (Read 8924 times)

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Offline coppercone2

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https://www.mwrf.com/technologies/test-measurement/article/21849791/copper-mountain-technologies-make-accurate-impedance-measurements-using-a-vna

So I have finally put together a full decent set of equipment for my 300MHz VNA, including a resistive splitter and a directional bridge. I see the different ways of setting the system up offer a good amount of measurement range, but I am a little short on details.



It looks like the only other piece of equipment I need is a big choke. Any ideas for a J2102B-N common-mode transformer replacement?

has anyone been using these methods? The price of a impedance analyzer is deadly. I never saw a thread specifically about this article but these discussions popped up once or twice. I don't think you need a transformer if the ESR is high.

I was surprised by the accuracy graph. 2.5 percent error is pretty fucking small. I just wonder what it looks like without the transformer, but its still leagues cheaper then a IA.

I am wondering if it can characterize a piezo

I never saw this graph in keysight literature for some reason, they only show the middle section of the graph. did I just miss it? I kind of feel like if you measure it with a few methods and compare them, you should be able to determine if the measurement is off the rocker?

What devices does this measurement method have the most trouble with (practically speaking?, like real world components and systems that fall into impedance ranges that might be confusing without a bridge)?

If you overlay all 3 measurement graphs for a component, it may be interesting. I want to try it on something because I just got a floppy drive so I can download stuff off the VNA.

The main point of the thread is to show the error graph, I thought it was ALOT worse for some reason. Maybe this will increase peoples interest, because I thought it was a seriously dodgy solution, but it looks practical, like I thought it was something like 20x the error. The author does a measurement at 90Mhz
« Last Edit: April 10, 2021, 06:48:21 am by coppercone2 »
 

Offline joeqsmith

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...
has anyone been using these methods? The price of a impedance analyzer is deadly. I never saw a thread specifically about this article but these discussions popped up once or twice. I don't think you need a transformer if the ESR is high.

I was surprised by the accuracy graph. 2.5 percent error is pretty fucking small. I just wonder what it looks like without the transformer, but its still leagues cheaper then a IA.

...

What devices does this measurement method have the most trouble with (practically speaking?, like real world components and systems that fall into impedance ranges that might be confusing without a bridge)?

...

The main point of the thread is to show the error graph, I thought it was ALOT worse for some reason. Maybe this will increase peoples interest, because I thought it was a seriously dodgy solution, but it looks practical, like I thought it was something like 20x the error. The author does a measurement at 90Mhz

Yes, I use it.  It's helpful for measuring low impedances.   The common mode choke is used to break the ground loop of the two cables.  You may find that at the frequency you want to run that the cable's loss will dominate and the choke isn't needed.    You may also find you need to add some amount of attenuation to help with the match which can improve the measurement. 

Funny you bring this up now as I added support to measure ESR to my software and planned to demo it using a few different parts.  I haven't started recording yet, so if there is something specific you want to see outside of the basics let me know and I will see if I can add it. 

The industry standard way used a resonant coaxial-line made by Boonton (or custom).   



How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
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Offline coppercone2

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what transformer do you use? not sure if I even need it because the low esr thing is not really useful to me, so I doubt I will need a coaxial resonator either, I thought maybe its more useful for ferrites if you can figure out which method to use. I wanted to overall the graphs on some component to see if its possible to determine where its bogus
« Last Edit: April 10, 2021, 08:48:50 am by coppercone2 »
 

Offline joeqsmith

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what transformer do you use? not sure if I even need it because the low esr thing is not really useful to me, so I doubt I will need a coaxial resonator either, I thought maybe its more useful for ferrites if you can figure out which method to use. I wanted to overall the graphs on some component to see if its possible to determine where its bogus
what transformer do you use? not sure if I even need it because the low esr thing is not really useful to me, so I doubt I will need a coaxial resonator either, I thought maybe its more useful for ferrites if you can figure out which method to use. I wanted to overall the graphs on some component to see if its possible to determine where its bogus
The common mode choke is used for the shunt through method.   I have a cable with a fair bit of ferrite added to it on the shelf.   For the demo, it will not be needed.   For demonstrating series and shunt methods,  everything I have shown has been direct. 

ESR would be important ....  well.... errr .. when isn't ESR important.   A friend of mine had purchased two of the NanoVNAs to learn more about them.  They were going to start off spending some cash.   I had seen an article about the Nano one of the RF sites I use.   They sent me one with the idea I would show them how to use it (reason for first video).   Their interest came about when working on a new high speed digital design and sorting the power distribution network.  I gave them a hands on demo using a VNA, which must have sparked an interest.   I've designed a few small switching supplies over the years, and again ESR has played a big part of that.    The reason I added and plan to demo the ESR measurement on the NanoVNA was actually for them.   

We have gotten a lot of millage on these low cost VNAs.  Working within their limits (knowing what their limits are),  they throw up some decent data.   Just a side note, they never purchased anything besides the first NanoVNA.  For learning the basics, that thing makes for a perfect starter VNA!  They are waiting to see if we can find something better.
 
Around 24 minutes in, I run the same test circuits that I had used to demo the Nano.  These are all shunt and series, direct connections.   
     
« Last Edit: April 10, 2021, 10:51:59 am by joeqsmith »
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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thats why I am not that interested in it, it seems most useful for high speed digital systems to have low ESR, or things that are just way too fast for the equipment that you probobly wont get lucky with cheap parts (microwave).. it makes me think of coin miners lol

at least the super low range of ESR that is in the threads like 'how do i get capacitors for my mysterious 10000 amp 3.15vdc asic mystery (*miner*) board that is at some mysteriously cheap cost limit despite the asic (usually no expense spared), because its meant to generate a profit based on cryptoequations

the other side of the curve can be used for chokes, it seems unexplored

maybe you can use this new low esr measuring technology for improving TDR
« Last Edit: April 10, 2021, 07:45:36 pm by coppercone2 »
 

Offline joeqsmith

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Forgetting the microwaves, ESR would be important even with a 20A 40KHz switcher.   You are correct that the lower ESR, we can pump more current through them.  If you use AlumElec, temperature is a big factor in their life.  Lower ESR, lower temps, maybe less parts, lower costs.   It's just one aspect of the design. 

For power inductors, it seems normally I am looking at where they saturate.  I pulse them and look for the knee where they go non-linear and you are left with the E^2R losses.   Then back it down.   For RF, Q.

For TDR, I just use S11 like every other software.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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I mean that trying to catalog low esr capacitors might have a use in a TDR if you are not interested in high power switching supplies
 

Offline coppercone2

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you almost overlayed those three graphs on a single component, I see you have the fixture in the video

would you mind showing me what the inaccuracy looks like when you put the 3 curves right on top of each other with the same axis on the same part measured 3 ways across the range? i don't need any error bars or anything, just if you can show them unbounded in the same place, I figure its gonna be some kinda W shapes, I don't have the sma edge launch connectors, only waveguide coaxial couplers that I probobly should not use a edge launch connectors, so I can't make a a nice fixture


I kinda wanna know what happens when you get it completely wrong, and how it looks like, so i am not on a wild goose chase when I do this

but I have a interesting idea now, I want to see if this medical superglue sensitizer works on gluing teflon, to see if i can cut strips of teflon on a milling machine to make my own PCB on copper plate with super glue. I can glue polyethylene like its bonded, and that is close to the hardest thing to actually glue. I think if I measure the thickness of an end mill and blah blah maybe i can cut some very good PCB up
« Last Edit: April 12, 2021, 05:28:19 pm by coppercone2 »
 

Offline joeqsmith

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From reading your post a few times, I am not sure I understand what you are asking.   That videos a half hour long.   

Start with defining what type of component you are asking about and then what property of that component are you wanting measured.
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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don't care, one of the graphs should be accurate and two should be inaccurate
 

Offline Bud

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I never saw this graph in keysight literature for some reason, they only show the middle section of the graph. did I just miss it? I kind of feel like if you measure it with a few methods and compare them, you should be able to determine if the measurement is off the rocker?

There are several HP/Agilent publications on different impedance measurement methods. Search by " RF I-V " keyword.
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Offline coppercone2

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they said 10%, its more like.. around 2 percent MAX.

I glanced at that and other agilent diagrams before and it was basically neglected because 10% makes you feel like your in 1940, it sure makes me wanna buy a impedance analyzer. They all over state the error hard core, to the point of making it sound like a useless technique... go figure

On the high end your going at 1% error according to the calculation. Thats ten times, an order of magnitude, from the serious hand waving numbers thrown around. but then again why would I trust the manufacturer of $ $ $ $ impedance analyzers with a competitive technique

I bet you other stuff is going to get in the way of that nice error calc but I am thinking they are probably over stating it to benefit their sales.

process: significant accountable error at some small range within the whole thing going from zero to infinity, better with some other equipment
business: its infinitely bad , damn the whole range 
new graph : looks like you can get by with some style, your not in E6 carbon composition land like you were lead to believe
« Last Edit: April 13, 2021, 06:19:07 am by coppercone2 »
 

Offline joeqsmith

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don't care, one of the graphs should be accurate and two should be inaccurate

Not correct.  I could make all three methods inaccurate  by much higher than 10% error with ease.  Similar, you could make any of the three methods look better than the others depending what you measure.   It shouldn't be a surprise that there are cases where a free HF meter will out perform a high dollar VNA.  You would want to pick the method based on what you are trying to measure that would produce the least error.  Same for the tools you use.       

https://coppermountaintech.com/measurement-of-electronic-component-impedance-using-a-vector-network-analyzer/
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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Right, I am getting at the point that I feel like its under marketed, without seeing the possibilities backed by the analysis I thought its basically a useless endeavor / student demo thing to use a VNA for anything other then 50 ohms. it looks like you can actually do useful engineering work. It seems alot of literature just demonizes the VNA for these goals like it will give you total bullshit, it seems possible that in alot of situations it might provide an adequate measurement.

I am wondering if there was alot of other people that had this mis conception, I was basically scared to measure anything other then a known 50 ohm filter network or part, making me lean to things like test oscillators in situations where a VNA might have been good enough, faster, more convenient, etc.

The literature is really marketed at people with DEEP pockets.
« Last Edit: April 13, 2021, 10:38:37 pm by coppercone2 »
 

Offline joeqsmith

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I think I showed a 10K on the high side and 25mohms on the low.   Never thought that was a big deal, well, maybe for these low cost VNAs.   I'm pushing the limits of dirt cheap.   You can imagine if they could only measure 50ohm loads,  they wouldn't be very useful but it would sure make the math for them much simpler.   

Quote
Any ideas for a J2102B-N common-mode transformer replacement?
I did attempt to make a transformer with parts I had on-hand.    After 5 attempts with cores I had on-hand, I'm not even close.   That J2102B is showing almost -50dB at 1KHz.  -110 at 1MHz.  I assume this is the common mode attenuation.   Interesting problem.  If you find any papers that talk about it, please post them here.    If I come up with something that looks remotely decent,  I'll publish it.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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i see china has the same idea as you to make a long ferrite, they are all over ebay

this probobly has to do with expensive not off the shelf

I read on this post by teslacoil that he says to use multiple ferrites for different ranges, but thats only with induction impulse for loop testing

that thing is HUGE for the power level it does

so probobly the idea is to first separate magnetic and RF, work on RF because there is alot of materials available for chokes that are cheap, then the magnetic range has to do with $$$ industrial secrets for switching supplies, so ignore it completely, i almost expect people to ask whos asking and why and report on it to sales partners on the low frequency range when probed, greedy grant scientists, etc

This is probobly what the forum knows what to do with, so a 43 seems most linear


this seems interesting, because its how you make a home made binocular coreish thing


https://dokumen.tips/documents/common-mode-chokes.html


This is the same type of product, yes?
https://palomar-engineers.com/antenna-products/Coax-Common-Mode-Noise-Filter-c21444130

« Last Edit: April 14, 2021, 02:58:22 am by coppercone2 »
 

Offline joeqsmith

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I was attempting to make something that would fit in roughly the same size box as the part you were looking at rather than loading up the long cables.   

The best one I was able to come up with from the junk box used two different core types.   

On the plus side, for your application its all small signal stuff.   Seems like a fun and potentially useful project for the RF experimenter.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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did you see the chinese ones? they put like 30 ferrites in a row on a PCB, over a strip line I imagine, for like $40
 

Offline joeqsmith

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This is the same type of product, yes?
https://palomar-engineers.com/antenna-products/Coax-Common-Mode-Noise-Filter-c21444130

Depends how you define "same type".   We like cars here, if we consider a Volkswagen Beetle is the same type as a McLAREN F1, sure.   But when your first  post mentioned the J2102B-N, I assumed we weren't talking about the Beetle.   

From the data I attached for the J2102B, I assume that the common mode attenuation is 110dB at 1MHz.  At 100Hz, its better than 20dB.   Again, it's two different applications and will depend what you are trying to do.   The ones you posted are not characterized and 1MHz is about the lower limit (the upper limit of the J2102B),

did you see the chinese ones? they put like 30 ferrites in a row on a PCB, over a strip line I imagine, for like $40

Yes, and again it's fairly common practice.  You could buy a few different ones and measure their common mode attenuation.   If you like, I could post data for what I have tried so far as well.  If you collect the S-paramter data, we could overlay everything with one of the Touchstone viewers.   So far,  mine have VERY poor performance compared with the Picotest J2102B-N.   It would be interesting to see how they compare.   

From Picotest's site:
Quote
The J2113A is a better choice if you need to measure below 3kHz and below a maximum frequency of 500kHz. Due to the limited CMRR, the J2113A is not recommended below 1mOhm. If other cables are used with either solution, there may be significant degradation in measurement accuracy below 1MHz due to poor cable shield resistance and isolation.

I also assumed you were interested in going below that 1mOhm limit.   This is basically what I am going after and want to know if a transformer could be made on a modest budget that allow these measurements.   So far, I am saying no but I'll have some new parts in a few days and take another stab at it.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline joeqsmith

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Looking at Picotest'd data, you have to ask how the J2102A has gain below 20Hz.   And how do they pull off 6dB or so of attenuation with the J2102B at 10Hz, let alone 50dB with the J2113A.  Really makes you want to see the S21 data for these in this area.   

I think I need to ask around and see if anyone I know has one that can make a few measurements for us.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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there might be something to learn from the construction of one of those more industrial common mode chokes, perhaps with ferrite arrangement, construction methods or something, which is why I asked. They might show up on ebay cheap because it seems like some generic industrial dumpster crap. Sometimes the industry gets something really right and then uses really cheap parts and it can be improved, and maybe the mechanical design is good for putting different components in it. I was thinking about getting a common mode noise filter then taking it apart to see if it gives clues, maybe its some how segmented, etc, I have a feeling if someone asked that company 'hey can you extend this range' they might know exactly how to do it, and its capped off because of a specification, we know that cheap corporate will often kill anything non essential to the specification, but it might be possible to add back in. For all I know the original design could have been 50Hz to 500MHz and the boss just said 'we need to make this cheaper'.

I think getting the low frequency range with ferrite might be more difficult then the high frequency range.

I have something like this for a telephone line, its a little addon you put on it as a noise filter, inside is a rather complex network of inductors and stuff, maybe its common mode, I never exactly figured out what it is, but maybe the topology is adaptable. 

the palmer design likely has something to do with DSL.

This one looks like it can do everything but the LF
https://palomar-engineers.com/ferrite-products/ferrite-beads/FT240-3I-ID=1-4-AC-DC-Coax-Noise-Filter-RFI-Range-1-300-MHz-Bulk-Pack-of-10-p90491324

I know when you use multiple EMI filters they have impedance problems that degrade their performance but usually it does not cause a resonance IIRC, it just makes the proceeding one work much less, in some cases and they do not 'stack' their attenuation, but that is for CM/DM LC filters, I never thought about pure CM filters, for me the common mode behavior was always more confusing ,especially since torroidal chokes typically  are atually DM/CM filters, not pure CM, but thats with wire, coax + chokes is unstudied by me.


Active is completely different, I did not see it in the original article
« Last Edit: April 14, 2021, 07:56:58 pm by coppercone2 »
 


Offline joeqsmith

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there might be something to learn from the construction of one of those more industrial common mode chokes, perhaps with ferrite arrangement, construction methods or something, which is why I asked. They might show up on ebay cheap because it seems like some generic industrial dumpster crap. Sometimes the industry gets something really right and then uses really cheap parts and it can be improved, and maybe the mechanical design is good for putting different components in it. I was thinking about getting a common mode noise filter then taking it apart to see if it gives clues, maybe its some how segmented, etc,

If you buy something, measure it first and then take some pictures for us.   

https://community.flexradio.com/discussion/7354715/multi-band-common-mode-choke

The materials I am using are much better than the 43 for this application but again, I am not even close to hitting the numbers shown from Picotech.  Digikey does not carry the materials I would like to buy.   

I asked around to see if anyone I know has one but no luck.  I would really like to know how that data Picotech presents was taken.  Maybe there is a video floating around the internet.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 

Offline coppercone2

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did you look at mag-inc ferrites? they have ALOT
 

Offline joeqsmith

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did you look at mag-inc ferrites? they have ALOT

This is who made the old cores that I mentioned.   I downloaded their catalog and it looks like they still offer similar parts but I have not tried to find a distributor.  It may be a problem to buy them as a hobbyist in <5pc qty.  Let me know if you find anything out.       
 
I'll write Picotest and Coppermountain and see if we can sort out these graphs for starts.   I have not actually tried to measure anything with what I have yet.  Once the new parts arrive, we can try a few experiments.

If you read their application note, notice the graph on page 9.  I suspect No Coaxial means just a straight connection to Port 2 with coax.    Notice how they converge at 1MHz.   In my demo, I was testing at much higher frequencies, except for the resistors which are were not stable enough.   This demo, while fun, shows some of the limitations of these low cost VNAs.  Still, they were good enough to show why the shunt through is pretty much the standard when looking at low impedances.   
 
https://www.picotest.com/measurements/images/download/the%202-port-shunt-through-measurement-app-note_REV1_073118.pdf
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA & V2 Plus 4 may be found here:
https://github.com/joeqsmith
 


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