impedance measurement with VNA using series, shunt/series through methods, graph

[coppercone2]

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

[joeqsmith]

...
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).   

[coppercone2]

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

[joeqsmith]

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.   
     

[coppercone2]

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

[joeqsmith]

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.   

[coppercone2]

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

[coppercone2]

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

[joeqsmith]

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.

[coppercone2]

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

[Bud]

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.

[coppercone2]

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

[joeqsmith]

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/

[coppercone2]

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.

[joeqsmith]

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.   

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.   

[coppercone2]

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

[joeqsmith]

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.   

[coppercone2]

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

[joeqsmith]

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:

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.   

[joeqsmith]

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.   

[coppercone2]

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

[coppercone2]

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

[joeqsmith]

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.   

[coppercone2]

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

[joeqsmith]

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

[virtualparticles]

Right, I am getting at the point that I feel like its under marketed

You're right! I wrote the article quoted earlier. It also appeared in Microwaves & RF. It blew me away when I was able to accurately measure a 1 milli-ohm resistor without some 4-lead ohm-meter. Please note that the deep "V" in that chart is only because I was lazy and didn't care to do the math to round it off. I was only concerned about the cross-over points so one could determine which method was best for what impedance. The ability to make this measurement is pretty awesome for anyone doing a big FPGA board with lots of power supplies. One can use two probes on a power supply and you'll see the high-pass response created by the internal control loop of the supply chip. When you go beyond the "natural frequency" of the control loop, it gives up and allows the output voltage to wiggle. A poorly designed LDO or switcher might exhibit a large overshoot in that high-pass response indicating instability usually due to a poor choice of filter capacitor with ESR which is too low.

The common mode inductor is probably not needed unless you are looking below 10 kHz. If you need to obtain one, PicoTest has what you need. They also have a lot of educational information on the site. Steve Sandler is brilliant.

Cheers!

Brian

[joeqsmith]

I wrote the article quoted earlier.
....

The common mode inductor is probably not needed unless you are looking below 10 kHz. If you need to obtain one, PicoTest has what you need. They also have a lot of educational information on the site. Steve Sandler is brilliant.

Brian, do you still have the transformer?  If so I would be very interested in seeing if you can replicated their graph I linked.   

I did write them today but no response.

[joeqsmith]

I have collected a fair number of cores over the years made from various materials.  From attached, I have some fairly large ones.  If I understand their graph, 30dB of common mode attenuation at 100Hz, seems unobtainable.    Nothing in my stash will come close.   Not to mention, you still have that 6GHz upper end to deal with.     

The transformer to the right was my last attempt. 

[virtualparticles]

Hey Joe!

My lowest frequency is 9 KHz on an S5065. Is that low enough? Were you just interested in the S-Parameter measurement through the thing? I do still have it. Steve gave me one since we're collaborating on some things and cross-marketing. If you want to get through to Picotest you have to go through Emma, his admin lady, emma@picotest.com. Steve is awful about answering his email.

Best,

Brian

[joeqsmith]

Brian,

Thanks for offering to help.  If you could look at the common mode attenuation from 9KHz to 1MHz, that would be great.  Basically, I want to see if this is what their graph is showing:

https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3548742/#msg3548742

Just a simple log sweep and and attaching the S21 data here would be very helpful.   At 9KHz, I think they are showing 65dB of CM attenuation.   

***
Also, could you include as many details about your setup as possible if you decide to measure your transformer.  Drive level, picture of cabling and anything else that you feel may be important to replicate their graph.   

[virtualparticles]

I'll see if I can get that done next week. You can reach me at brian.w if you like as well.

Best,

Brian

[coppercone2]

the nice thing that this low limit makes the bridge useful, you can measure that with a cheaper bridge then use the VNA  for HF, just slow, but probobly more useful and accurate to have in the lab

rf bridge = expensive, but this takes care of that to some regard, then you still have a bridge for ultra precision and this range that is not covered with complex equipment, I think this will be my route, its more interesting for me anyway.. others are likely too impatient, since using a bridge is actually work

[joeqsmith]

4a, my attempt at making a toroid.  What did I learn?  Not to attempt to make your own toroids. 

The new cores arrived.  You can see that the box of cores is overflowing.  The new cores show some improvement but still not even close to the graphs presented by Picotest. 

I had seen another article that was more detailed on PDM from the same person.  Here he adds a little more detail about his transformer.
https://www.signalintegrityjournal.com/articles/1544-why-2-port-low-impedance-measurements-still-matter

Of course, I had started out using coax as I plan to use the transformer at much higher frequencies..     

My mom drowned her dumb kids.

 
The following snips were taken from a 2018 webinar put on by Copper Mountain Technologies and Steve Sandler of Picotest.  Here Mr Sandler offers us a few small bread crumbs.   

   

[coppercone2]

I want some kind of flexible former you can put on a torroid with silicone like those cell phone things that stick without glue then put glued up wire on precisely then remove the formers, so you can get the winding perfect, like set them down and measure each of them with a caliper before you glue the wire in then pry them out after it sets a little, it would be a bunch of little triangles or rombus with miters that you make in a silicone mold

[joeqsmith]

With the 3D printers, have you considered making a custom form for each core? 

I saw someone using wood with holes drilled in it. 

[coppercone2]

Maybe a water soluble one? I think the problem I was imagining is the former interacting with it. Or castable wax.

I have a DLP printer but the problem is 1) wax is expensive (not really)
2) making a bunch of conic sections in CAD is going to kill me (seriously I hate this shit)

If someone made a configurable 3d printer file to print formers based on parameters that can be measured (i.e. specify a torroid with thickness, diameter, inner diameter, radius 1 and 2 (iif they are different) and coil form, then I would do it

They have parts generators like this, the scripting is probobly simple if you know the language and program, but I don't like dealing with arcs.

The resolution I have is great and I don't mind printing at 12 seconds a layer as per the datasheet to burn my printer. I just don't know how you would get the bottom layer, you would need a scafolding to hold it togehter as you glue it down with wax, then cut away everything (so its held from the top by 'rods'. That way you can have true seperation between the parts, but this only makes sense for fairly large diameter wire, so the wire can actually make contact with the part.

[joeqsmith]

I wasn't able to find the article with the wooden forms.  There was another showing a plastic guide for aligning the wire but I couldn't find it as well.  May be all for the better.  They were both fairly crude and I am not sure they offer any benefit. 

So far, I have not come up with anything good enough that I would be concerned with how the wire lays. 

Did you end up ordering any of the low cost loaded cables or the ready made transformers?    I would still like to see what you come up with.   That can't be worse than the ones I have tried.   

I received my 1mOhm 1% 50ppm resistors along with a 100uOhm. 

[joeqsmith]

The new test loads next to the parts used during the the demo.   

[coppercone2]

I am saving money on this thing to try to make a little vise/fixture for clamping parts and maybe springs and silver plate (kool amp), I have been meaning to get something for a while.. don't expect me to gamble on ferrites and I hate buying those Chinese boards but I know everyone else loves em

What I want is a non solder tester, which I feel is very important. I really don't like resoldering SMD parts, it makes me nervous, mainly the reason why I don't already own one.

I got a bunch of pins but they are all too long and the travel is too long, but I see now, it might be an option to use those press fit PCB connectors that someone made a thread about, they come with their own spring, they just need to be fixed to a adjustable mechanism of some kind.


this, but sideways (read post 3) might make a easy to make RF launcher and receiver
https://www.eevblog.com/forum/rf-microwave/vertical-mount-connector-to-stripline/

[joeqsmith]

I had made a couple of cheap clamps for both shunt and thru measurements. 

[coppercone2]

I can't tell, how did you get spring? Do you just wedge?

those things I linked have a spring, so you can probobly adjust them on a threaded thing to make really good direct contacts, maybe you neeed to undercut the component a little so it does not short it or put some insulation but it should be close to perfect, but since its supposed to go into a strip line, it should be as good as you can get touching a component RF wise

I am also thinking you can solder a hard line to connector, then drill it out, then cut it open after inserting the center pin and splice in a new piece of coax to have the entire spring assembly on a conventional plunger terminal inside of the dielectric. Might work best with high gauge cable but you would still change the diameter (LM400) by drilling.

[joeqsmith]

I can't tell, how did you get spring? Do you just wedge?

The two screws allow the coax to be adjusted.  The part is placed into the channel, I then press the coax against it and finger tighten the screw.  There is a small bit of Telflon on the end of each screw.    The shunt jig is similar.   

I had thought about adding a spring that would apply pressure to the coax but so far, this seems to work fine.  I also thought about using a jeweler's file to cut a V in the coax to provide a 2-pint contact but again, it did not seem like it was a problem. 

[joeqsmith]

I'll see if I can get that done next week. You can reach me at brian.w if you like as well.

Best,

Brian

Brian, could you do me one other favor when you have time to look at that transformer.   Could you please just measure the DC resistance between the two shields?   I have a sinking feeling...

Thanks

[coppercone2]

oh, yeah, I would call that a press fit. its not as nice as constant force spring but thats almost a moot point

[coppercone2]

this is what I am thinking,

then you can use plungers with a texture

it would probobly be better if you made your own 'hard line', which I thought about also, to house the plunger with better impedance match, but then you need to bond it to regular coax anyway , not sure how much difference is, but it should give lower DC resistance I think with plungers

Hmm, what I wanna find now, is the LMC400 hard line

maybe one of the other LM cables has a center conductor that will snug fit the pin. i need to look at the specs, if the center pin can be replaced with a plunger then you can just cut away the outside of the connector to get a real nice thing going without any BS

people with high speed VNA might have to do more work, but I only go to 300MHz, so I can use alot of different cables

[joeqsmith]

If I were to add a spring, the section of coax would be the plunger.    I was thinking to just make a small clamp that went around the coax and connecting it to the screw with a tension spring.   Very simple. 

Shown with a 0.050 ohm resistor inserted.  It's not setup for this large of a package.  The meter has not been zero'ed or warmed up and is reading a bit low.  Not to mention, the lack the 4W.  Good enough for what I use it for.     
 

[coppercone2]

I think you can just solder pogo pins on top of that PCB so they are across the resistor, expose the silkscreen and solder them down

I just bought a bunch of brass tubes and other stuff and I just figured this out, so the part is basically seated where its soldered in but held together by pogo pins

is there a problem with this? 

it makes every SMT fixture ridiclous.
well at least very expensive in comparison

[joeqsmith]

At 300MHz, you can get decent performance using techniques that may not work so well at 3GHz. 

Look at this attenuator I made using about as bad as techniques as possible. 
https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2924286/#msg2924286

An addition of a bit of metal and we are within a half dB up to 300MHz.   
https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2924796/#msg2924796
Of course, things get pretty bad after 300MHz.    I would say for a 300MHz fixture, some sort of pogo pin may be fine. 

You reminded me of this article using solder braid for a fixture. 
https://www.edn.com/solder-wick-trick-characterizes-bypass-caps/

Note his transformer...
https://www.edn.com/pcb-fixtures-improve-component-measurements/

[coppercone2]

But you can make the pogo pin protrude only like 1mm,

I mean on the PCB you have, the traces leading to the resistor, in the series config, you scrape the solder resist off of them, lay the pogo pins down on the PCB so they are touching across the SMD pad facing each other, like a bridge over a valley, so its naturally short circuited. Put the part in there to unshort circuit it. I don't think this will effect things very much, you might need to put a tiny copper finger under the moving part of the pogo pin, right up a mm from the head. I think it will be good to quite a high frequency. If its gliding on top of a bed of copper fingers it should still be considered part of the 50 ohm stripline right?

[virtualparticles]

Hi Joe,

Didn't have time to do a chart but you can pull what you need from the S-Parameters that one of our Apps guys created for me. I hope this helps!

BW

[joeqsmith]

But you can make the pogo pin protrude only like 1mm,

I mean on the PCB you have, the traces leading to the resistor, in the series config, you scrape the solder resist off of them, lay the pogo pins down on the PCB so they are touching across the SMD pad facing each other, like a bridge over a valley, so its naturally short circuited. Put the part in there to unshort circuit it. I don't think this will effect things very much, you might need to put a tiny copper finger under the moving part of the pogo pin, right up a mm from the head. I think it will be good to quite a high frequency. If its gliding on top of a bed of copper fingers it should still be considered part of the 50 ohm stripline right?

Pins I have are longer than the boards (think Evert Charles bed of nails).   For me, it's not a big deal to solder the parts down.   If I added pogos, I would need to come up with a way to cal out their effects. 

[joeqsmith]

Hi Joe,

Didn't have time to do a chart but you can pull what you need from the S-Parameters that one of our Apps guys created for me. I hope this helps!

BW

It does and answers the reason I had asked that last question.    I would still like to see if you could replicate their data if you get a chance.   

[coppercone2]

you can get pogo pins that are about 1 inch long and very thin,longer if you use them with the housing to make it replaceable.

you can't do the same cal procedure? I assume you solder in a solid SMD jumper (high current solid metal)

[joeqsmith]

you can get pogo pins that are about 1 inch long and very thin,longer if you use them with the housing to make it replaceable.

you can't do the same cal procedure? I assume you solder in a solid SMD jumper (high current solid metal)

The boards are about and inch long total.  For a thru jig, you have about 1/4" for the pogo pins.   

When I use the boards you mention, I have a set with standards mounted to them.   The parts I test are mounted the same board and mounted just like the load.  I use the same connectors as well.  Basically, I want to remove much of the errors.   Adding pogos would toss that out the window and I wouldn't gain anything.   I'm not suggesting you shouldn't do this.   As you said, you don't want to solder SMD parts.       

[coppercone2]

well its mainly for MLCC, I am not sure about inductors, I assume they are pretty robust, but I know ceramic caps break ALOT with fracture. i figure resoldering makes this worse potentially.

I do wonder if press fit is better though the way you do it because is really beefy, you have piezo and magnetostriction, i wonder if it can do something with the springs to form some kinda pendalum. it might need another tiny clamp, it is possible it might be causing the pogopins to vibrate back and forth kinda like a shaker, your method is truely massive in comparison to the transducer, this might effect anything with L&C,.

though, that giant fixture might make it easy to over pressure a capacitor also.

[joeqsmith]

I am not aware that I have ever damaged a capacitor with these jigs, or any part for that matter.  No doubt, it could happen.   The bigger problem is getting the parts inserted and aligned.  Tweezers.   Now that I finally broke down and bought a small set of Pace tweezers,  I'm about as fast soldering and removing them from the test boards.  Those were one of my better investments. 

[coppercone2]

i have a pair of those, still the problem is the capacitors can fail down the line. i dunno why i really care but if I wanted to sift through say an entire box of them to test everything, the solder would get old, even with pace tweezers, since you are supposed to clean all that stuff with braid when you are done, and testing a 100 strip is difficult to solder that much

all the proper work always shows after desoldering for resoldering you are supposed to start with a clean part that had all old solder stripped off as much as possible, presumably because its contaminated.

maybe you can use a solder pot after pace tweezers if that works good enough to remove anything excess but its too much for me, i would feel like im in china

[joeqsmith]

Wow, you are actually testing all your capacitors?   I can't think of a time I have sorted through a reel of ceramic capacitors.   I normally pitch any parts after testing them.  I have some of these boards with parts permanently mounted to them for demo'ing the Nano.  I also have some parts mounted that I keep for my other VNAs to as a sanity check.   

Sounds like you are reading about mixing solders.  That's nothing I am concerned with at home.   You may be working more in production environment where I am just playing around with one of my hobbies for the fun of it.   

[coppercone2]

i wanted to test things like the old IBM experiments where you put like a whole bunch of decoupling capacitors on a board for ground plane things, generally I dont measure anything, and I am interested in things like vibration, shock resistance, etc. just experiments for myself to get to use/make cool gear like shaker tables

I see alot of new rules of thumb being made after a few VNA  experiments and people shouting all about them but the sample size is small, its new and popularity is very high, potentially dangerous (1uF ceramic  madness spam) , I feel like someone needs to look at those new theories with a bit of practical things in mind to see if its good advise to be shoving down peoples throat all the time) , there has been a bit of a 'decoupling revolution' on electronics forums in the last 5 years, some might call it new testament, that too many old timer engineers were doing things by rules of thumb and that its all wrong or useless etc (often suspiciously pitched with parts count reduction or miniaturization, in line with what the fab would want you to do for the sake of their packaging costs). when i showed some stuff i found to very skilled / long time people they were often a little suspicious, but where I found them in their point in life they had little financial support, personal drive, etc (understandable that you are not amped up about some place you left in the 90's and that you wanna do something else with your life without having a impedance laboratory in your basement)

generally at a company they only investigate this if sales fails to bullshit out of it so its non commercial

i feel like using a few classic approaches can recently get your circuit 'reamed' in the comments because of a few pictures and graphs on a few threads here and there (feels a little extreme, feels like saying that you pray to roman gods or something)

i have too much interest in grounding/electrical bonding. im not saying its not true but there is alot of stuff to look at, and alot of it is related to manufacturers providing more data and cheaper equipment, but with more advanced datasheets there is more confidence and less testing and more complex theories formed , which is something that i don't always like

[joeqsmith]

EEVBLOG is the only social forum I follow.   I must have missed out on this 1uF ceramic madness spam and decoupling revolution you mention.    The super low cost VNAs has certainly caused a bit of a trend for the RF hobbyist but more so for the antenna and 3D printer groups.  They need their 3 places beyond the decimal SWR readings and plastic cases.   Outside of all the noise, there are a few that have actually started to experiment with them.  IMO, that's a good thing. 

No doubt that as we continue to increase pushing for faster edge rates,  more details become important.  That doesn't mean that the beginner need place these same constraints on their first 555 timer circuit. 

[virtualparticles]

Which data am I trying to replicate? It's hard to doubt the Steve. Just look at this testimonial.   
"He's not just a man, he's a Universe."

[joeqsmith]

Which data am I trying to replicate? It's hard to doubt the Steve. Just look at this testimonial.   
"He's not just a man, he's a Universe."

Originally I was asking if you could replicate what I believe is the common mode attenuation of the transformer.   See their data here:
https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3548742/#msg3548742

That may be asking too much as you would need to set up the test to run it.   The easiest thing that would pretty much clear everything up now is just measure the inductance with an RLC meter between the two coax shields.   Then, measure the DC resistance with a basic ohm meter.    I would expect that the DC resistance will be sub ohm and the inductance will be VERY high.   Just let me know what RLC meter you used and what the settings were. 

[joeqsmith]

This was the 8th attempt, again coaxial design using multiple core materials.     

By far, it's the highest performance (most common mode attenuation from 10Hz to 1MHz) of what I have been able to come up with but it pales compared to the numbers Picotest has published.   Assuming that virtualparticles is able to measure the inductance of the shield, and we are able to validate their plots then I am going to have to give some more thought as how to pull this off.     

Because I am out of ideas for now, I went ahead and mounted it in a box and will try making a few measurements with it.   No, not with the Nano....     Although....

[virtualparticles]

Hi JoeQ

Measured from connector ground to connector ground. I used a pair of pigtails and a copper plate as a low inductance common ground as you can see in the picture. I calibrated to the end of the test cables, attached the cheezy pigtails and then used port extensions to get to the end of them. The attached S-Parameter file should be what you want.

Best,

BW

[virtualparticles]

Here is zoomed in to 1MHz so you'll have enough points to compare the charts

[joeqsmith]

Hi JoeQ

Measured from connector ground to connector ground. I used a pair of pigtails and a copper plate as a low inductance common ground as you can see in the picture. I calibrated to the end of the test cables, attached the cheezy pigtails and then used port extensions to get to the end of them. The attached S-Parameter file should be what you want.

Best,

BW

Thank you very much for running this test.   This data is very helpful.   

[coppercone2]

how big are your assemblies joe?

the biggest clues might be the weight and size of that thing.

Given how cheap companies are with product packaging, I have a feeling for some reason there is something moderately heavy and large in there. I figure the magnetic element is at least 1/3 the size of the assembly.

page 53 has the electrical parameters
https://www.picotest.com/downloads/INJECTORS/Injector%20Manual%201.5.pdf

unfortunately its weight is not given.

https://www.dacpol.eu/en/j2102a-common-mode-transformer-66605/product/j2102a-common-mode-transformer

Weight    0.210kg

the chassis is probobly light.

[joeqsmith]

I believe with the data virtualparticles has provided, combined with clips from the webinar,  we have everything required to replicate it if that was a goal.   During the webinar, Steve Sandler talks about the trade off when designing the transformers.  You may want to design a custom one for your particular application. 

Attached is some data I took tonight using my four test resistors (100mOhm 1%, 25mOhm 2%, 1mOhm 1% and 100uOhm 1%).  The data was collected both with and without the transformer.   We can clearly see the effects of adding the transformer at these lower frequencies. 

[joeqsmith]

I was curious about the 1mOhm part reading 30% high.   I tried changing the cable from RG400 to one of my good HP cables, no effect.   I moved the DUT right to Port 1 with a single adapter, no effect.   I increased the source 10dB, no effect.  I raised the frequency, no effect.   The noise floor is about 109dB where I am testing now which seems like plenty of margin.  It just likes that 85.6dB figure. 

So I tossed the old HP VNA aside and switched to an old RF generator and my SA.  No surprise, I got very similar results.  The noise floor for the SA is much better so I tossed in the 100uOhm.   

While it would be interesting to try and sort it out, I'm beyond the limit of what I can measure with the equipment I own.      It's possible my test boards are the source of the problem.  I doubt that the 1mOhm 1% 50ppm part is out 30%.   

[coppercone2]

how do you calibrate these things? I imagine the problem might be when you move the coaxial cable around or unscrew it after calibration.

if you have a DC or close microohm meter you might want to put your test fixture to that by making a kelvin that terminates in the SMA connector and fiddle with it

[joeqsmith]

For the old HP and the SA I get transmission only so I inserted a thru.  Everything is torqued.  It's very reproducible, even with two completely different setups.  The thru is the same PCB so we should be removing much of the error.

With these big power resistors, I can insert two banana to BNC and BNC to SMA on the two sides of the PCB.  Drive the one side with a current source and read the voltage on the other.     

1 mOhm
With 1.6716 Amps I measure 1.987mV or 1.189 mOhms. 
With 3.9334 Amps, I measure 4.666mV or 1.186 mOhms.   

100 uOhm
With 5.078 Amps, I measure 1.083mV or 213.3 uOhms. 

[coppercone2]

I think you will need something like a 4 terminal resistor to measure that accurately , 100uohm is just not happening normally with any kind of high precision that you want with a normal part regardless of the setup I think

https://www.globalspec.com/FeaturedProducts/Detail/OhmiteManufacturing/FKGK_Series_4Terminal_Resistor/312325/0

Maybe you need to make another VNA pcb with sense traces that are there for verification with a microohm meter or discrete microohm meter setup and not used with the VNA, they will effect it at some frequency but if you stay low you can maintain the correlation between both measurements. that part has a special pad btw, I suppose you can put a shorting jumper there to short the little trace to the big trace when you do a VNA measurement but you dont wanna solder near that thing

[joeqsmith]

Agree, as I said it's possible the test boards are the problem.    For the 1mOhm, I may be able to improve what I have.  Agree, the 100uOhm would be a more difficult problem.   If we ignore that for now and say the DC measurements are reasonable,  the VNA readings, while closer, are still off a fair amount.   There may be something else limiting the measurement. 

[coppercone2]

Maybe ultrasonic cleaning the contacts and deoxit gold?

Are you using high end VNA cables? I do wonder if they are the best type of cable for low resistance measurement, i.e. they have foam dielectric in them.. I only have 18GHz or more cables but they are thin and I don't have the parts for making the PCB yet unless I want to do it with waveguide launcher connectors (I found like 100 sma waveguide launchers in the trash a long time ago).

Now I am not sure what dielectric hard line solid sheath uses, but I have a feeling for that kind of stability you don't want foam in the circuit, and for some reason I think a higher wire gauge conductor is better.
I do have some hard line however, if I do this I could try to make BNC hardlines that go to the PCB if you feel like waiting a month. I think my rg141 is solid core and solid teflon dielectric

I need RG400 or whatever its called for my pogo pin fixture, the brass tubes should get here but when I think about it I think I need to redo the plan with a peice of hardline rather then a flex cable because mechanically some kind of bullshit will go down with anything but solid teflon insulation

Also, if you make your own cables, since it turns out bondable (not sure, still need to experiment), you can try to glue the pin into the connector after soldering or crimping to the center conductor by using a primer and loctite glue (medical) to bond the center pin  to the connector with more then press fit, i suspect as you are bending the cable the center pin is moving around inside of that plastic sleeve and causing a problem

of course what i recommend is going to go into full hardline for the fixture that is well bonded into place some how (maybe stainless zip ties on a metal frame) that goes to the VNA and then use all APC-7 connectors and retrofit the VNA with APC-7 so it can be put into exactly the same space every time and connected with genderless rotational sleeves so you can put it into a fixture bonded to the analyzer every time then tighten the sleeves without fiddling with anything, but that is like a 1000$ mod, because if you use bigender/genderless (what is PC now lol?) connectors you can put the fixture on machine pins and then do the same thing every single time. I am imagining a 1/4 inch plate or better bolted to the bottom of the VNA with pins on it to fit the fixtures*

*its been a while since I used APC-7 connectors so I am not sure they are truely machine pin compatible, because you might need to tilt them, don't do this unless you figure out a connector that will work, I don't feel like looking for one to see how flat the face is right now, or if there is any spring loaded stuff that you don't want rubbing on a slide fit in there. I think it can work if that is a problem if you put the pins orthaganol to the plate so the fixture slides on from the front rather then slides to sit on top of the plate, or you bend them, but I feel like hard line would be best left as strait as possible without bends introduced into it, ready made coupler/extensions might be the best to use

[joeqsmith]

I have modified both the 1mOhm and 100uOhm boards to try and improve the DC measurements. 

I'm sure the cables contribute some error, along with everything else.  The question is which ones are causing the majority of the error. 

Maybe ultrasonic cleaning the contacts and deoxit gold?

.....that goes to the VNA and then use all APC-7 connectors and retrofit the VNA with APC-7 so it can be put into exactly the same space every time ....

I'll leave you to do your thing. 

[JohnG]

For what it is worth, accurately measuring a 1 mohm resistance is challenging enough at DC, not to mention 0.1 mohm. I would be really surprised if the contact resistance was repeatable within 0.1 mohm.

Something else to consider is that the resistance an RF cable is probably several mohm, perhaps higher. Variation in temperature or connector pressure, etc. can give you substantial errors at this level. It may be that the initial accuracy of any calibration combined with the limited precision of the error correction model is insufficient.

Finally, most low-ohm resistors have a fairly large cross section, and skin effect can make the effective resistance higher. It doesn't really help if they are thin-film unless a lot of care is paid to the shape of the resistive region and the current return. A chip resistor will experience current crowding at it's edges, not just the faces. Even worse is that many low-ohm alloys are partially magnetic, which makes skin depth issues even worse.

Cheers,
John

[coppercone2]

for what its worth, if you use deoxit on a strip of copper, and test leads, it does increase repeatability when measured with real gold kelvin leads, for some reason, on a source meter

take 15 measurements and measure variance if you don't believe me. i used IET brand kelvin leads (not cheapo) and a keithley SM that was calibrated from a fluke multifunction calibrator at a modest current like 100mA, the source meter was powered up for 24 hrs before measurement

they are not lying

its also still not very stable

the usual i heard is "omfg its gold"

[joeqsmith]

I would never apply anything to an RF connector that was not what the manufacture recommends.  Now, if you found me an article published by Keysight, Copper Mountain Tech, Gore or manufactures of high quality RF connectors where they call it out as part of the maintenance and care, I would read it.   

[joeqsmith]

For what it is worth, accurately measuring a 1 mohm resistance is challenging enough at DC, not to mention 0.1 mohm. I would be really surprised if the contact resistance was repeatable within 0.1 mohm.

Something else to consider is that the resistance an RF cable is probably several mohm, perhaps higher. Variation in temperature or connector pressure, etc. can give you substantial errors at this level. It may be that the initial accuracy of any calibration combined with the limited precision of the error correction model is insufficient.

Finally, most low-ohm resistors have a fairly large cross section, and skin effect can make the effective resistance higher. It doesn't really help if they are thin-film unless a lot of care is paid to the shape of the resistive region and the current return. A chip resistor will experience current crowding at it's edges, not just the faces. Even worse is that many low-ohm alloys are partially magnetic, which makes skin depth issues even worse.

Cheers,
John

So far, at least for DC using the current source, it has been repeatable well within 1uOhm.   I can't answer for absolute accuracy.   Once I have the DCR sorted, I'll try and sort out the other main source of error.   Again, just an interesting experiment.   

************
Doing a search, I came up with the following thread:

https://www.eevblog.com/forum/testgear/measuring-low-impedances-with-a-vna/

Attached, showing the modification to the test boards.   At DC I measured the following:

1mOhm
Measured 0.9870A @ 1.0120mV or 1.025mOhms.

100uOhm
Measured 1.8929A @ 189.0uV or 99.85uhms.

Also shown is the last transformer.  The test setup was very similar to what virtualparticles shows.   Based on the data they provided, I suspect I am using the same coax, but more of it, more turns, and a mix of core materials.  The downside is I give up the higher frequency.   I plan to built two more to try and match the Picotest's transformers performance.  One using the hybrid, the other using a single type but I suspect more in-line with what they have based on the data we have been provided.     

[joeqsmith]

Showing results using the two smaller loads. 

[coppercone2]

I am guessing that using the 4 terminal SMD resistor in the same way would have the same results and its not worth doing for the price tag because its clear that the problem was resolved good enough, maybe there would be a very slight improvement, but it would restrict your PCB only for the weird resistors making it not very useful outside of a dedicated calibrator

what is the accuracy of the resistor? at 0.15, the error is very low, assuming you used a high accuracy resistor.

we probobly don't hear about splitting the trace under a normal resistor because its prone to manufacturing errors so no one bothered to write about a 'hack' like this despite the giant error reduction, since mass production would be upset, you probobly need alignment during reflow since that will no longer self align.

I am still waiting on some of the brass tubes and hardline to make a probe that has the plunger inside of the coaxial cable sticking out only a little. I have to solder the plunger into a thin brass tube that I got because its far too thin and slightly enlarge the hardline to fit that tube in the hardline, the calculations show I should have like slightly less then 50 ohms in that transmission segment (I get something like 43 ohms for around 1.5 inches). I will make the same fixture as you but with spring loaded pins rather then press fit for the center conductor and see how that works out despite the slight changing of the dielectric shell.

[joeqsmith]

All four resistors used are 1% parts.   From my previous post, measuring the 1.0 and 0.1 mOhm resistors with the two old HP bench meters with unknown accuracy I would guess we are well within 5%.   

Noise floor of the SA vs the VNA.

[joeqsmith]

Looking at S21 for various decoupling test boards.   Notice the three types of capacitors on this particular board.

With the V2 Plus working well into the GHz,  it may be useful to have some sort of power distribution graphs. 

[joeqsmith]

Basic example of PDN design talking about effects on phase noise.


Nice lab, but I don't see the Nano anywhere.. 

[coppercone2]

if you want to put a 3mm into a lmr400 cable with the center conductor removed, its a snug fit, and a non jacketed but still insulated cable makes a very light fit into a 7.2mm tube.

you need to widdle down the center conductor of the lmr400 cable to get it to solder into a brass tube however

[coppercone2]

So the problem is getting a joint between a 3mm brass tube filled with solder and a 2.7mm copper clad aluminum conductor cut, then gluing a peice of sleeve over it

I am going to try LF460 aluminum solder and flux first off. I think you need a lathe to make your own non open brass tube because I see nothing but problems here. If LMR400 was solid copper, there would be 0 problems. Now it looks like I need a miracle to solder this together or I need to add alot of series connectors

I am thinking to get CNT-400 cable, which has solid copper, unlike the modified LMR, supposedly. try making heads and tails out of this mess, looks like the wild west

edit: managed to tin the aluminum copper with the 460 aluminum solder and flux then bonded it with normal solder to the brass tube filled with solder, so its a 5 metal junction, its probobly pretty weak, but it seems strong enough for the application.

[joeqsmith]

Hi JoeQ

Measured from connector ground to connector ground. I used a pair of pigtails and a copper plate as a low inductance common ground as you can see in the picture. I calibrated to the end of the test cables, attached the cheezy pigtails and then used port extensions to get to the end of them. The attached S-Parameter file should be what you want.

Best,

BW

From Picotest:   

"We use a PCB FIXTURE to short the BODE 100 OUTPUT (center pin only) to both pins at the J2102B input.  Then we connect one receiver to the J2102 input and the other receiver to the J2102 output center pin."

Which makes sense but I wouldn't expect that to cause any difference compared with how you collected the data.   If I run a similar configuration on transformer #8 where the shield and center pin on the input side are shorted and then measure thru the center pin,  the results are the same as looking across the shield.  At 100kHz, it looks like your core measures around -24dB compared with transformer 8 at -37dB, and -90dB with the Picotest graph.   At 1MHz, your at -32dB compared with -47dB for #8 and  -110dB for Picotest's graph.    Again, this transformer would be very poor at higher frequencies but the plan is to construct three others that will attempt to get something closer to the data you have provided.   

Also, their response didn't address the gain shown in their plot.  I have written them back so hopefully we can get to the what is the cause of the discrepancy. 

Also shown is the transformer #9 which is also a hybrid.  It measures about 22dB dB at 100kHz and -32dB at 1MHz.   The old HP is limited to 150MHz.  It's obviously fairly flat in this region.   Transformer #8 also works well at these lower frequencies.   For fun, I stuck it across the NanoVNA.  I wouldn't put a lot of stock in the data above 300MHz.   Once I have the other transformers built, I will take the time to measure them and we can overlay the data with what you provided.   

[coppercone2]

I got a copper bar stock and silver plate so I can braze into a U shape and drill holes and add set screws and solder on spring couplers to make my test fixture

I am gonna make a U shape and then solder oversized silver plated berylium copper springs around the drill holes so when you insert the silver plated brass rod into the thing, the end of it contacts a the spring and makes a good coupling to the U shape body so its not going through set screws. I got tellurium copper for that so its easier to work with. I will cut it into 3 sections and silver braze it together.

it should beat the toolmakers vise I am using now.

I am just not sure how to get the drill holes right, the tubing measures differently in different points. I am not sure how much it will wedge so I might make sample segments and try to fit it in an aluminum block to see what snug is.. I wonder if I need to buy a rotary lap and a precision reamer.

[joeqsmith]

I was thinking something like this.   Copper clamps are soldered together at the spring end.  They would provide more than enough force to clamp an SMD in place. 

[JohnG]

First, a thanks to those pursuing this and showing the results. I did not think that it would make sense to do these measurements down to the single digit milliohms, but given the discussion in this thread, I am motivated to give it a shot.

Second, those are beautiful fixtures. I just got a used Sherline lathe with milling attachment, so they would be something to learn on. There is one gotcha, though. They can overestimate the inductance of a part that would normally be mounted over a ground plane. This will become significant as the width of the chip component becomes larger than the spacing between the surface layer of the PCB and the ground plane, at least for thin chip components.

Third, the solution to getting a wide frequency range CM-blocking inductor should be as simple as putting ones with smaller ranges in series. If the frequency range overlap is small, you should increase the total frequency range of CM rejection without affecting the differential mode of the 50 ohm line. The penalty will be an increase in total size and cable length. You could stack different cores if you wanted to lower the total winding length, but that will most likely hurt the high end rejection.

Cheers,
John

[coppercone2]

the pogo pins have replaceable contacts (they are a sleeve and a pin) so thats the only reason why i gravitate to them, plus it probably slightly hardened and gold plated.

If you do use pins, be aware my half finished one in the vise is a pain in the ass to load, since I do not have a external big spring, I need to position the part in tweezers then use another pair of tweezers to separate the pogo pin and slowly push it back into place. It is annoying and a few parts were shot away from me. I thought about making a little pedestal out of some material to rest the part on on top of a screw to adjust the height  but I am not sure what the screw should be made of, and it seems unnecessary unless i widen it alot to fit a big inductor or something.

A big external spring mechanism is superior so long the RF parameters are OK. I don't have the tools to make a nice joint (I do have a brass surface grinder sharpener fixture that I will likely never have a surface grinder for,which is almost perfect to modify because it has a smooth travel in a long piece of brass, but i also thought to just make an attachment to use that to dress my bench grinding wheels so its probobly better to leave that tool alone). Since I got some beryllium spring copper i will experiment with making springs, I don't have a knack for figuring out what kind of spring I need to get the mechanics right or the tools to make it operate smoothly. Maybe I will try to make a little one built like a watchmakers vise on my proxxon mini mill or modify a watchmakers vise (the one for holding the bowl part).


My current fixture has a discontinuity in the dielectric, I cut it as flush as I could with a razor to expose a piece of center conductor so I can solder the pin holder in the brass tube to it, then slid another piece of dielectric on and soldered the brass tube on top. However if you have the N connectors you can probobly solder the center conductor to the coaxial connector pin and the pogo extension and slide a single piece of dielectric back over that and solder the N connector over that single piece. I am not sure what that thin discontinuity does from razor cut, a TDR would tell me nicely, but I have a feeling it does not matter much at only 300MHz max. I have materials to try to make non interrupted ones if I get access to higher speeds but its already pretty interesting.

I got the aluminum solder to work alot better by using it alone for the joint instead of pretinning, to use my regular soldering iron with the special acid flux and clean it afterwards with mild abrasives and solder it like a normal solder joint, and to really scrub the aluminum solder down with stainless steel wool in soap water prior to soldering and cleaning with alcohol. It is actually more difficult to join the pretinned surfaces then just flowing a normal joint, I think the problems I was having was just really nasty solder, from my brazing drawer in the tool box, in the uncontrolled garage.. I had to throw out all my RG45 rods from corrosion and spent like 2 hours fixing the brass rods filler on the buffing wheel lol. I stopped trying to get the joint the right size and instead overfill it so there is a big bead and then use a dremel sanding drum to get it back into a cylinder shape then polish it from 120 to 2000 grit with the plastic radial disks along the entire splice before sliding the sleeve on top to reduce insertion force. The only problem my current fixture has is that one side needs to be rotated to get the pogo pins to make good contact (it is slightly off center because when I was soldering the pogo pin sleeve into the brass tube I did not have a nice centering fixture for it, I got lucky with one being dead center and the other one is sensitive to rotation or its misaligned. I also don't have whatever N connectors were used with the factory cable I modified, so one of my connectors is a right angle one, but it ends up working because I run the fixture ontop of the VNA when its inside of 19'' rack, next to the other rack, so I will probobly leave it alone for now. Anyway I am excited to see it worked and eventually I will do the refinements.

 I am also going to solder some of those micro alligator clips to the end of some pogo pins so I can put alligator clips inside of the fixture to test small leaded parts, those small copper alligator clips can be bent easily to conform around different diameter leads. Since they are replaceable its cool because you can solder stuff directly to the pogo pins and insert them into the fixture, and even seize the pins with solder to get rid of the spring action in such situations, so it makes a micro pin connector. You can even solder them to empty PCBs without coaxial connectors to make boards that slide into the gold sleeves, so long its acceptable the board is floating from ground, which still may be useful for testing series components chains.

[joeqsmith]

John,  the following is an article Picotest published on sub 100uOhm measurements: 
https://www.picotest.com/images/download/Ultra-low.pdf

A bit odd that while they have presented a picture of what is marked a 20uOhm test device, the picture they choose to include in the report for their DCR is showing 33uOhm.   Note how they used the J2102A rather than the J2102B.  The article was dated April 2018.  If you look at this undated paper, the graph from the VNA is from March of 2019.  So I assume they just didn't have the J2102B yet.

https://www.picotest.com/downloads/INJECTORS/Picotest%20Releases%20Update%20to%20Popular%20Power%20Integrity%20Test%20Tool%20J2102B.pdf


Picotest responded about the discrepancy but sadly I still can't make sense of it.  They talked about a possible reason for the gain:

It can have slight gain, because of the loading impedance of the probe (high Z) vs port (50 Ohms).

I could calibrate that out, but since this was only a relative measurement it wasn't important.

The point is that at low frequency, the common mode transformer can't help, the solid state isolator can. Above a kHz or so the transformer wins.

I won't pursue it further.  It seems like a lot of small, and what I would consider important details are left out of their documents. 

I've been pretty clear about my use of more than one core material.  From the S-parameter data, I suspect the Picotest design uses a single material.   It's possible that you could improve on their design using multiple materials with the same cable type and length.   If you decide to make your own, I am interested in seeing what you come up with.

[joeqsmith]

The P2130A DC Blocker lets through frequencies between 500 Hz and 8 GHz while simultaneously providing low insertion loss and excellent VSWR. Its maximum differential (input – output) voltage is 50V.

From the attached data, the typical insertion loss at 6GHz is around 0.8dB. 

I spent some time making a couple DC blocks to use with the transformer.  Still waiting on parts. 

[JohnG]

Thanks for the links. I have been taking a more careful look at the method.

I do intend to build a few CM transformers and see what I can do. If there are any interesting results, I will post them. It won't be for a few weeks, as I have other priorities right now. Unfortunately, I also only have a NanoVNA 2+4 at my locale. Might be fine to measure the inductance, but not sure how low an impedance can be measured. If travel starts again, I may be able to visit my company HQ and do some measurements on our Keysight VNA.

Cheers,
John

[joeqsmith]

I haven't looked to see if anyone has attempted to run the Plus 4 below 50KHz.  The software that our local member had modified for the original Nano supports down to 10KHz.   The one I have isn't too bad at 20KHz.   For your own transformer,  you may be better off waiting until you can get some time with some better equipment so you can see how they perform at lower frequencies. 

Look forward to seeing what you come up with. 

[joeqsmith]

The new cores arrived.  I tried making up a couple of binocular transformers.  The first with a single material which was a total bust.  The second using a mixture which worked much better.   

Shown are the -3dB and insertion loss at 6GHz, along with the common mode attenuation for the two best transformers I came up compared against transformer #8  and the Picotest.   
******

My last attempt used three different core materials which gains about 4dB in that 100kHz region.  Not too shabby.   

[coppercone2]

I tried my silver plate on tellurium copper, seems to work

working on the BeCu springs now

[joeqsmith]

You should post some pictures of this jig you are working on.   

From what I learned in that last round of transformer testing, I'm making one last attempt on material selection.  The goal is to get the common mode below what was achieved with #8, but get the insertion loss at the high end closer to the performance of the Picotest transformer.    I think it's doable and the cost would still be well below the price of the Picotest transformer. 

[coppercone2]

when its done, i have focus problems getting the small parts to focus and i might remake them so its a waste of time because i am an impatient worker, so it looks sloppy and half assed

The spring is a big unknown to me and I need to make the parts out of a piece of aluminum junk first to see if the tools I had in mind will work because I need to do the machining and then fixture/braze then solder

[JohnG]

If you are looking for best attenuation per turn at low frequency, it's tough to beat a toroidal tape-wound core, especially an amorphous or nanocrystalline one, e.g. Metglas or Finemet. For the most inductance per turn, avoid cut cores or "distributed gap" cores. Also avoid "square-loop" cores, and look for so-called flat-top annealed cores.

Here's an example: https://www.digikey.com/en/products/detail/T60006L2040W422/2258-T60006L2040W422-ND/12531985?itemSeq=363262270

It's not very tolerant of any DC, but CM DC should be negligible. The permeability starts rolling off below 10 kHz, but it's still pretty good at 100 kHz.

It will start to get quite lossy above that, but it's still CM loss so it helps. Combining with another ferrite core may help at the high end if needed.

Cheers,
John

[coppercone2]

Thats what I see in the highest power density dc switchers

[joeqsmith]

If you are looking for best attenuation per turn at low frequency, it's tough to beat a toroidal tape-wound core, especially an amorphous or nanocrystalline one, e.g. Metglas or Finemet. For the most inductance per turn, avoid cut cores or "distributed gap" cores. Also avoid "square-loop" cores, and look for so-called flat-top annealed cores.

Here's an example: https://www.digikey.com/en/products/detail/T60006L2040W422/2258-T60006L2040W422-ND/12531985?itemSeq=363262270

It's not very tolerant of any DC, but CM DC should be negligible. The permeability starts rolling off below 10 kHz, but it's still pretty good at 100 kHz.

It will start to get quite lossy above that, but it's still CM loss so it helps. Combining with another ferrite core may help at the high end if needed.

Cheers,
John

As a matter of fact,  all of the transformers I have shown used tape wound nanocrystalline cores.  And yes, I have combined these with other materials to improve the common mode attenuation at the higher end.   #8 for example used Mn-Zn.   

As I mentioned, that last attempt used three different materials.   I suspect the insertion loss for this design would be very good but I want to see if I can improve the common mode before building it up. 

[coppercone2]

Tried 11/32 and the letter drills around it, it looks like 11/32 is the best fit (hard to slide on though), so I bought a 11/32 reamer and a 11/32 adjustable lap and diamond lapping paste, so I can ream a 11/32 hole then try to lap it to a fit I like before I start messing with the springs. I think that might work.

[joeqsmith]

I have some short pogos but they use a ball at the end.  I wonder if they make anything short enough that you could have just mounted them to a microstrip.   At 300MHz, the performance may be decent. 

I ended up remounting transformer #8 using SMA connectors and removed some of the excess wire which slightly improved the insertion loss. 

Shown is the data for the #8 after these changes along with the 2pc hybrid and the last tricore transformers compared with the Picotest.  I collected the data the same way virtualparticles did to try and get a realistic comparison.    In the 10kHz region, all three have a better common mode attenuation.  I wonder if that is a limit of the VNA.    That tricore just misses at 150kHz to 300kHz.   The Picotest's insertion loss beats everything I have shown so far.

[coppercone2]

well there has got to be some some problem to miniaturizing a plus sized rack box to a deck of cards, that could be it.


i stopped playing with my sdr but people have all sorts of weird problems with SDRs.

i can try it on a e5100 in the future one day, i am sure some one here will beat me to it

i always run into problems making anything smaller, even the most simple shit, something always gets you.


i say this somewhat jokingly, if the designer called it 'pico' when its the size of a modem and you got something the size of a deck of cards called nano, and pico is smaller then nano, but likely made with higher design effort, I assume the person that thought it his design is nano and you could still foreseeably go to a pico missed something, when the original is 20lb.

[joeqsmith]

I was wondering if running the VNA near it's lower limit was causing a problem, but our friend Brian is no beginner.   I would put more stock in their data than the graphs.     

https://www.electronicdesign.com/home/contact/21132621/brian-walker

The rounded pogo pins I mention are about half the length of these.   

[coppercone2]

the harwin ones have sleeves you can put alot spring loaded things into

[joeqsmith]

I believe Harwin made the short ones I mentioned.   The smaller ones in the last picture were made by Everett Charles.  They may offer something you could use as well.   

https://ect-cpg.com/spring-probes

[coppercone2]

the high frequency probe makes this design obsolete

at least i get to lap something

[joeqsmith]

I don't know what they charge for them.  They look pretty fancy.   Still, in that lower frequency range,  I would just continue with the home made setup.

[coppercone2]

yes I do expect to call the police on the quote, for some reason I think I can buy a shiny new VNA and a HP test fixture for the price, after all the goal of all this is cost.

if you just factor in the cost of making it, and you go with all USA and pay extra for things like USA brand lapping paste and machine tools

~45$ for the reamer, lapping tool and lapping paste, plus you get to keep the tools, you can get by with just the drill and a file most likely because it probobly is not measurable, I just want it to feel smooth
~20$ for the TeCu block
~15$ for the BeCu spring
~15$ for a ready made LMR400 cable, so you can cut it in half and use those
~20$ for the brass tubes and you get to keep a bunch (I think putting the seek thermal on the andonstar microscope mount is a very very expenisve tool, with the help of a shiny brass rod, it feels like a real thermal microscope, not that i have seen one). yes brass hobby tube is excellent stuff, not just for railroad hobbyists
~30$ for silver plate (kool amp) and you get to keep 99%
~15$ for aluminum soldering stuff
~15$ for polishing stuff

realistically a ham already has 1/2 of this stuff

and whatever probes you get

so its like $130 and most of it is otherwise useful other then the cable and the copper block, giving you alot of capabilities, the reoccurring cost is the probe you might break after a long time, and that is just 1/2 of the probe you can replace.

for someone else though, it seems that getting the full bandwidth of the VNA is most important. I have a 300MHz limit.

[coppercone2]

here is a picture, I think it came out nice actually with the silver plating, I am still learning about applying it, it looks like the tip needs a bit more work on closer inspection. btw I left one side of the tubing normal after a pipe cutter cut so it kind of conforms around the end of the coaxial cable, and there is a layer of DP460 glue on the top also.

I also thought about making a heavy braid braid that shorts out the two tubes with clamps so you can use it to probe something weird and big like an air variable capacitor.

[joeqsmith]

Are these for two different fixtures, or do you plan to butt them up against one another and place the component you want to test between the two pogos?

I assume you will mount these to some sort of ground plate.  How do you attache them to the VNA?

[coppercone2]

like this, if the explanation is hard, maybe this crude picture will help. the spring is like a head band, or perhaps a turtle neck that you put on your head so when the tube slides through it presses against the spring slightly so the connection is made on the very end, i am not sure if it will help, i think its proper if the spring is dense and small. i will turn that copper block in the backdrop into the U shape by cutting it into 3 pieces and brazing it together, and ream/lap the holes so it feels good. the T's are set screw knobs. I just tested it by putting it in a vise. you draw the springs back with tweezers, then put the part in with another tweezers and let it go. once it operates on set screws a pedestal can be made. Imagine someone glued a big fat o-ring to the outlet of the shaft hole but the o-ring is just a tightly wound spring that compresses/deforms on the outside of whatever you put through it. I think since the spring i got is silver plated Beryllium copper I can just mill a ball nose ring around the hole to seat the spring in there, and heat it on a hot plate with a ring of solder in there to adhere the spring to the big block. when i do the other side, I would just flip it and lump a bunch of heat sink compound all over the top of the fixture so the top spring does not de-solder when I heat it up.

I think a turtle neck sweater is the best explanation of how it works. its a step above wedging steel wool into a EM hole cable pass through (bootleg RF gland of some kind). If you can make really good colets it would be better, but thats so far outside of what I can do its not funny

the drawing is realistic because thats probobly what it will measure as with the tools/skill I have available, it is unlikely to have many square surfaces

[coppercone2]

http://www.sinocableglands.com/sale-11945562-emc-emv-stainelss-steel-cable-glands-ss304-ss316-ss316l-for-shielded-emc-cables.html

this might be an option if I can find it in the right size, I did not think of EMC glands for some reason.

someone with money might want to try this, but its $50 a pop. getting kinda heavy.
https://canada.newark.com/lapp-usa/53112037/cable-gland-3-4-npt-brass-9-17mm/dp/61AC3758

the manufactured glands in the interests of sanity and durability probobly have multiple contact resistance points (i.e. spring wedged in something, rather then fused to the main structure, so making your own might have impedance advantage).

I might also make a shield around it with a little lid with hinges to put the parts in and tighten down the lid around EMI gaskets if I get that far.

[coppercone2]

Oh right, that is just for thru measurement.

For reflection (i.e. component directly to ground), I thought of of making the same fixture but having only one side with a hole, the other would be the wall of the fixture with a pogo pin inserted directly into it.

For shunt measurement, I am not sure. This is a difficult one requiring alot of mechanical bullshit. I would probobly put a watch makers vise with flats instead of horns with dielectric jaws into the fixture to clamp the part with a pogo pin under neath, so I can press all 3 pins into the part without disturbing it. It would be clamped into the vise securely and all 3 pins would be inserted in all 3 axis. I suppose I can do that for all of them so the part is not floating, but I am satisfied with how 2 pogo pins hold small parts, it would only be necessary for something large like a big SMD inductor or capacitor.


I would need to remake the watchmakers vise out of plastic I think. It might be a significant parasitic capacitance. Maybe 3d printed with a nylon screw.


If anyone knows what I am calling a watchmakers vise, because of a ebay mis-name
https://www.esslinger.com/watch-case-opener-for-screw-on-style-backs-pocket-wrench-tool-square-pins/?gclid=CjwKCAjwkN6EBhBNEiwADVfya8e3HYAwocSj_Q63cwjbZArCKY7UxaCrfh994lWfYb5NdS_8zF3xMBoCgNcQAvD_BwE

its actually a removal tool. So that made out of plastic to hold the part in place with a pogo pin coming out the bottom and pogo pins coming from the sides.

I think that would be a good mechanism to clamp weird parts, if someone can make one for a 3d printer that is compatible with a nylon screw..

[coppercone2]

I am wondering since you have a 6GHz VNA, with the whole deoxit thing, can you try to put some adapters on it and then put a coupling thats really greased up (even with something else like wd40) and see if it makes a difference on the graph ? like really dirty with the oil being sloppy applied so it goes over the dielectric and stuff.

 in the interest of reducing mating cycle damage, no change would be beneficial, and be a bit of a mythbusters. its not defined

[joeqsmith]

I am wondering since you have a 6GHz VNA, with the whole deoxit thing, can you try to put some adapters on it and then put a coupling thats really greased up (even with something else like wd40) and see if it makes a difference on the graph ? like really dirty with the oil being sloppy applied so it goes over the dielectric and stuff.

 in the interest of reducing mating cycle damage, no change would be beneficial, and be a bit of a mythbusters. its not defined

I would never apply anything to an RF connector that was not what the manufacture recommends.  Now, if you found me an article published by Keysight, Copper Mountain Tech, Gore or manufactures of high quality RF connectors where they call it out as part of the maintenance and care, I would read it.

You are certainly free to buy your own test equipment and treat them as you wish.   I would even follow along if you decided to post about it. 

[coppercone2]

guess its going to have to wait an ebay deal then, i don't think its cobalt thorium g liquid

[joeqsmith]

For starts, you could buy the V2Plus and a few bags of cheap connectors.   Buy enough and maybe you can draw some sort of conclusion. 

If you are looking for low wear and long life from your $1000 metrology grade adapters,  you may want to consider including some of this in your testing:
 

[joeqsmith]

Distributors are slowing down.  The order for the last set of parts was finally processed.   Maybe next week.   

In the meantime here are a few posts on RF connector care.   I don't think you are going to find anything about WD-40 or other chemicals being recommended but again, I would certainly be up for reading about it if you do happen to find anything (from a reputable source).       

https://dl.cdn-anritsu.com/en-us/test-measurement/files/Manuals/Instruction-Sheet/10100-00031C.pdf

https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1ma99/1MA99_2E_CoaxConnectorHandling.pdf

http://na.support.keysight.com/pna/connectorcare/Connector_Care.htm

If you read this article, you may find a few problems:
https://www.microwaves101.com/encyclopedias/connector-care

Still some very good information on this site.

Here's an old paper from HP
https://www.ece.ubc.ca/~robertor/Links_files/Files/ConnectorCare.pdf

[coppercone2]

https://blog.solidsignal.com/reviews/using-dielectric-grease-weatherproof-coaxial-connectors/


Most coaxial connectors will not contain much air space in the active cable core area when correctly assembled. Filling the connector with a dielectric grease with dielectric properties matching the cable core dielectric material (commonly foamed polyethylene) will not cause any significant measurable shift in connector impendence or VSWR and in some cases will improve it.

thats for packing too

I assume they might mean analog VSWR meter on a rooftop antenna. when I zero my VNA just redoing the connection makes it drift though. I put deoxit on it long ago to protect it from humidity. just thinking about it, it might be safer on 75 ohm stuff for testing because it does not have the foam. i think your concerns are about the foam absorbing oil?

I thought to get a few adapters that are 75 ohm for easy cleaning and experiment on that. I suspect an ultrasonic bath in hot alcohol will clean them up so I can do proper before and after testing and see if cleaning has an effect. I think its solid teflon and not PEfoam in the 75 ohm connectors. I think they might be more stable, especially when soldering, so I tend to use 75 ohm on stuff that has a weird impedance anyway, since its harder to destroy, you can get aggressive with soldering the body. For something like attaching it to a high thermal dissipation object with solder (brass tube), the 75 ohm connector survives unscathed but a 50 ohm connector is delicate and if you are not super careful (and its difficult to be so this careful because its a difficult solder joint), they tend to bulge/deform and even smell bad. Shields also are more forgiving.

[joeqsmith]

I wouldn't turn to the cable or IT guy to be an expert on RF connector use.  You may find the ham and CB groups have a lot of wives tales as well.  You wouldn't turn to Facebook and Twitter for medical questions would you?     

https://youtu.be/Y0sg9G5BBVU?t=112

I suggest following the real experts for both your RF and medical questions.   

[JohnG]

If you are leaving your VNA out in the weather, then use of a dielectric grease might make sense to help prevent or reduce moisture infiltration and subsequent corrosion. I'm not exposing mine to the elements, though.

It also is used in applications where there might be high voltage. In the RF world, this would be in the transmitter output path, with a reasonably high power transmitter.

I'd be pretty dubious about it for signal measurement. SMAs might tolerate it, but most fancier connectors seem to have substantial air space. It's supposed to be air, or some kind of gas. Also, greases seem to have a bad tendency to migrate to where they are not wanted. It's a lot safer to use some connector savers.

I would not make any assumptions about cable dielectric formulation and cable impedance. I have seen all kinds of dielectrics for 50 ohm coax.

Cheers,
John

[coppercone2]

I think there is also something to  be said about the dollar value of the product, paying off <1uM precision lathes (apc7 is centered to 500nm), etc. I do know that metal things want oil. it good, even on gauge blocks you rub it off before you use it

my hunch is that there is a frequency where this begins to be noticeable, there is probobly an operating region where it does not matter, where it matters a little and when it screws with you

also people often dislike lubricants because it leads to over tightening or difficult torque setting adjustment (how do you compensate for oil), dry makes this more stable for the wrench. This can lead to a boondogle over oil stability, how to not oven oil stuff, etc. Steel has a 'slightly oiled' specification.. and thats for a bolt. try to translate that to a SMA torque wrench. Like with plastic piping, they say screw the wear, the worst thing is over tightening the plastic causing it to burst. in these coaxial connectors, spring deformation from over tightening can be catastrophic. I can see nothing by problems for a company that advocates use of oil but then tries to force people to use a particular oil that they categorized because its just oil, no one will really follow that because too many people have their own opinions on lubricants, and it gets rid of easy cleaning, since the oil might trap crap in it and make the connectors on average for some customers fail alot faster, whereas the life from just blowing it out with duster might be better unless the upmost care is taken, when you know things just wont happen correctly when like, non mechanically inclined people are asked to fuck around with grease in really posh laboratories, and since we are mixing RF and light so much in laboratories now, its a menace to optical systems.

[joeqsmith]

The last set of cores arrived.  From 10KHz to 1MHz, the home made transformer has higher common mode attenuation than the PICO.   Adding one or two more turns would certainly help the low frequency performance but it hurts the insertion loss.    I suspect with the materials used, the home made transformer would out perform the PICO below 10KHz.   

The home made transformer's insertion loss is about 0.5-1dB worse at 6GHz.   Picotest's site claims "Maintains 50 Ohm transmission line integrity to at least 6GHz".   Shown with data plotted up to 8GHz.   
 

 

[joeqsmith]

I am not sure why the Picotest transformer has such a mismatch but we can use the frequency between the peaks to determine the length of the cable.  We can also take a guess at the cable type based on the loss.   The match appears to be better with the home made transformers.   Possibly due to higher quality cable and connectors.   

To give you some idea on the price,  the majority of the cost of these two transformers was the connectors.  The nanocrystalline cores came next, followed by the box, then the Mn-Zn cores, hardware and coax.    The transformer on the right was about $200, more than half of that was connectors.   

[JohnG]

These are interesting results, but I am a bit confused. How exactly is CM attenuation being measured? I looked back through this thread, but never found a very clear definition. In addition, it seems as if this would be dependent on the measurement instrument, since this would determine the alternate ground path that is responsible for the CM signal, but it looks like the Picotest and hand-made transformers are measured on different instruments. If so, then a close comparison loses some meaning. Please correct me if I'm mistaken.

It seems to me that a direct measure of the transformer common-mode inductance might be more repeatable, at least at low frequency. Or, is this what is actually being done and I missed the steps?

Cheers,
John

[joeqsmith]

These are interesting results, but I am a bit confused. How exactly is CM attenuation being measured? I looked back through this thread, but never found a very clear definition. In addition, it seems as if this would be dependent on the measurement instrument, since this would determine the alternate ground path that is responsible for the CM signal, but it looks like the Picotest and hand-made transformers are measured on different instruments. If so, then a close comparison loses some meaning. Please correct me if I'm mistaken.

It seems to me that a direct measure of the transformer common-mode inductance might be more repeatable, at least at low frequency. Or, is this what is actually being done and I missed the steps?

Cheers,
John

I'm not at all surprised by your confusion as I am still not clear about it myself.    As I understand it, three different VNAs were used following two different methods.  If we include insertion loss, I suspect five different VNAs were used.   ***  I used a one to measure common mode and another for insertion.  The Bode 100 is certainly not up to measuring 6GHz so we expect Picotest used a second VNA as well.   

Here we can see a picture of Brian's setup.   He works for Copper Mountain, so no surprise, is using one of their VNAs to perform the measurement.
https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3554237/#msg3554237

Of course when we compare Brian's results with what Picotest has published, they are very different.
https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3548742/#msg3548742

I wrote Picotest to try and understand how they conducted the test.   While their response makes sense, I don't believe it was the cause of the difference.  They are using the Bode 100 but it shouldn't matter.   Something else is off and I am not sure what.   I suspect it gets back my mom drowning all her dumb kids.  A bit of a smoke screen if you will.   I see this with other papers they have published where details are missing.  Just part of the business model I guess.

https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3561853/#msg3561853

You could see in that last post my attempt to replicate what Picotest explained.  I saw no difference between this method and what Brian showed.  I wasn't expecting to.    Brian's VNA is limited to about 9KHz where I can run down to 10Hz.   For the S21 plots where I overlay Brian's results with my own, I am attempting to run the same setup.  I would expect the results to be very similar. 

I have included a few pictures of the log sweeps down to 10Hz to provide viewers with more detail on how my transformers performed.

For insertion loss, I would expect Brian just calibrated the VNA and placed it between the two ports which is what I am doing. 

[JohnG]

I don't see any issues with insertion loss, since that should be more straightforward both to measure and to calibrate out.

But, I think that CM attenuation of a given transformer can really only be compared if the systems are the same, because it will depend on the impedance of the instrument ground path relative to the cable ground path, and this is really unlikely to be the same. However, the measurement description sounds more like a straight attenuation measurement, as if you sent a test signal through an inductor and looked at each of its terminals. In that sense, it should be basically valid at lower frequencies, but it's not really CM attenuation in a well defined sense, even if it correlates well. At high frequencies, you would at least want a fixture, same cables, etc.

Also at high frequencies (upper 10s of MHz and up), you would need to be very careful about intrawinding capacitance as you will get capacitive coupling between turns and this will lower the CM impedance. MgZn cores are a problem in this regard as they are not really a very good dielectric even at DC, and they get worse at high frequencies. NiZn cores are usually much better for this, though it can work with MgZn cores if you space the windings from the core. In either case, and make sure not to overlap windings and keep a sizable gap between the end turns.

Cheers,
John

[joeqsmith]

I don't see any issues with insertion loss, since that should be more straightforward both to measure and to calibrate out.

But, I think that CM attenuation of a given transformer can really only be compared if the systems are the same, because it will depend on the impedance of the instrument ground path relative to the cable ground path, and this is really unlikely to be the same. However, the measurement description sounds more like a straight attenuation measurement, as if you sent a test signal through an inductor and looked at each of its terminals. In that sense, it should be basically valid at lower frequencies, but it's not really CM attenuation in a well defined sense, even if it correlates well. At high frequencies, you would at least want a fixture, same cables, etc.

Also at high frequencies (upper 10s of MHz and up), you would need to be very careful about intrawinding capacitance as you will get capacitive coupling between turns and this will lower the CM impedance. MgZn cores are a problem in this regard as they are not really a very good dielectric even at DC, and they get worse at high frequencies. NiZn cores are usually much better for this, though it can work with MgZn cores if you space the windings from the core. In either case, and make sure not to overlap windings and keep a sizable gap between the end turns.

Cheers,
John

https://www.nonstopsystems.com/radio/frank_radio_coax-sw.htm

[joeqsmith]

I've ran a fair number of tests with the low cost VNAs and they seem to throw up some decent numbers.  I had changed back to using our local members firmware to solve a glitch I was seeing with the newer flavors of firmware.  This firmware allows measurements to 10kHz.       

Here the $50 NanoVNA is looking at the common mode attenuation of transformer eight.  I then used AppCAD to overlay this data with the data I had collected with my old HP.  Different cables and adapters were used.    Violet is the Nano, Red the HP and Blue showing Brian's data for the Picotest transformer.   

It would be interesting to run the Picotest transformer on my setup but based on Brian's experience and picture showing his setup, I have little doubt I would see a major difference.   

[joeqsmith]

The Nano doesn't support log sweeps so I fudge it by sweeping short spans of linear sweeps but calculating them based on a log.  So it's a sort of log linear sweep.   AppCAD doesn't appear to like the odd spaced frequencies.   

[JohnG]

https://www.nonstopsystems.com/radio/frank_radio_coax-sw.htm

Looks like a good link overall. Lots of useful measurement techniques. The definition of CM attenuation is lacking, though. You could have a lossless shield and still get attenuation, so damping is not the reason it works.

I guess the real problem is that the CM current is the one that passes through the instrument ground impedance Zg. The CM attenuation would be how much this current is reduced. So, his measurement will correlate to this, but again, since Zg is instrument-dependent, the CM current and it's attenuation can also be instrument dependent. It's also dependent on the cable setup, since it's the fact that the cable shield has non-zero impedance that is the root cause of the CM signal in the first place.

As a result, I don't think it is valid to compare measurements made on different instruments. This is testable by following the above measurement methods on two different instruments with the same transformer and comparing the result on a single graph. Unfortunately, I only have one instrument, and no transformer (or at least none with good rejection below a few 10s of MHz. I did get the cores I ordered, and travel has opened up, so maybe I will get a chance to test this in the coming months.

Edit: Ok, it looks like you did that. I'll have to look into this further to see if I'm missing something. How's it look as you get into 10s of MHz?

Edit2: If at some point you could provide a drawing or photo of the measurement and cabling setup, that'd be great. I would really like to understand better what is going on and what I'm missing. However, I realize I'm asking for free work, so if you don't have one handy, there's no need to do the extra work. I'm just really curious about this now.

He does show the cross-winding on the toroid in the same section, which is good for reducing the capacitance between input and output.

But, I guess it doesn't matter that much in the end. If it's good enough to make low impedance measurements into the milliohm range, that's what I'm after, and it does look like the basic method is suitable. At some point when my current workload lets up, I hope to pick this up again.

Cheers,
John

[coppercone2]

I got my reamer and lap in, I will try to make a spring winder now. I think I can drill a small hole in an aluminum welding rod of 1/8 thickness to make the shaft

[joeqsmith]

https://www.nonstopsystems.com/radio/frank_radio_coax-sw.htm

Looks like a good link overall. Lots of useful measurement techniques. The definition of CM attenuation is lacking, though. You could have a lossless shield and still get attenuation, so damping is not the reason it works.

Actually if you look at Brian's, the website linked and my own pictures of the setup, if the shield were lossless, there will no no attenuation.  Any device we place between the two ports that is lossless will have no loss...   

I guess the real problem is that the CM current is the one that passes through the instrument ground impedance Zg. The CM attenuation would be how much this current is reduced. So, his measurement will correlate to this, but again, since Zg is instrument-dependent, the CM current and it's attenuation can also be instrument dependent. It's also dependent on the cable setup, since it's the fact that the cable shield has non-zero impedance that is the root cause of the CM signal in the first place.

As a result, I don't think it is valid to compare measurements made on different instruments. This is testable by following the above measurement methods on two different instruments with the same transformer and comparing the result on a single graph. Unfortunately, I only have one instrument, and no transformer (or at least none with good rejection below a few 10s of MHz. I did get the cores I ordered, and travel has opened up, so maybe I will get a chance to test this in the coming months.

Edit: Ok, it looks like you did that. I'll have to look into this further to see if I'm missing something. How's it look as you get into 10s of MHz?

As we get above 1MHz the cable losses will minimize the ground loop and we can remove the transformer all together.   When I measured that 100uOhm resistor, it became inductive well below a MHz.   In the attached video, I was measuring some popcorn RF capacitors ESR which was in the 10s of mOhms and I was not using a transformer and we were using a two different sub $150 VNAs to do it.   Again, lets not forget that the thread is about making impedance measurements.   

Edit2: If at some point you could provide a drawing or photo of the measurement and cabling setup, that'd be great. I would really like to understand better what is going on and what I'm missing. However, I realize I'm asking for free work, so if you don't have one handy, there's no need to do the extra work. I'm just really curious about this now.

Beyond the photos Brian, the website and I provided?   I would sketch something up but I'm not sure what I could add that would make it any more clear.   

He does show the cross-winding on the toroid in the same section, which is good for reducing the capacitance between input and output.

But, I guess it doesn't matter that much in the end. If it's good enough to make low impedance measurements into the milliohm range, that's what I'm after, and it does look like the basic method is suitable. At some point when my current workload lets up, I hope to pick this up again.

Cheers,
John

Exactly, measuring impedance is the goal, for this thread anyway.   Getting into the 100uOhm range is certainly possible as I have shown using my home made transformer.   We can't get there with the Nano but I wouldn't be at all surprised to see this change.   I'm impressed with how well that V2Plus4 works.   Seeing it measure that ATC capacitor in the 30mOhm range was an eye opener.   

While the two transformers I put together work well,  they are a bit on the expensive side for the hobbyist.  Hopefully you can come up with something that outperforms what I have shown and is cheap.  I had already gone fairly cheap with connectors but thought about buying some of those $8 connectors from China and see how well they perform.  That's where a lot of my cost is sitting.


Starts about 12:00 in.

[JohnG]

I think we are miscommunicating on definitions of loss. I have been using loss in the sense of power dissipation, not attenuation. Damping, for instance, is a lossy (dissipative) phenomenon by definition, i.e. energy transformed to heat. A LC filter with ideal components is lossless in this sense.

I guess in the RF sense, the terms attenuation and loss are frequently used to mean the same thing, e.g. return loss, but it's been a while since I worked in the RF world. In the power world, you can have a "lossless" inductor if you make it out of a superconductor. But, it will still have attenuation if you use it in a filter.

In this same way, lossless cables can still have inductance and therefore shield impedance, and impedance can still result in ground loop problems, even if it is lossless in the dissipative sense.

I have looked through the thread. There is a lot of useful information, to be sure. But, I don't see a clear, meaning mathematical, description of what CM attenuation means. But, I figured out the answer to my own question. You are measuring attenuation in a 50 ohm system, whereas I was trying to understand what the CM attenuation is in a real system, for which the CM signal path is all over the map (or Smith chart). So, in your 50 ohm system, you can make measurements that compare data in a reasonable way, at least at low frequency. But, just about any real CM system will not be 50 ohms, nor is it likely to be real-valued and the same between different instruments.

Cheers,
John

[joeqsmith]

From the thread's title, we are using a VNA to make these measurements and yes VNAs (and most RF equipment) will commonly use 50 ohms.   Using a transformer like this to brake the ground look between the two ports is also common practice. 

Does the power world for you mean something to do with the AC power grid where you deal with frequencies in the sub kHz?   In this context,  most of your posts make a lot of sense but I wonder why the interest in this topic?  Something work related? 

[JohnG]

My career started designing RF amps, but most of it has been in power electronics over various sorts. Various sorts means from 50 Hz to about 100 MHz fundamental frequencies, with much of it in the range of 100 kHz to a few 10s of MHz, from standard PWM to class E and F amps. So, I am somewhat familiar with VNAs of both the low frequency type (like the current Omicron Bode 100) and the more standard RF type, as well as basic RF measurement and design. I've also designed some transmission line transformers, and the CM transformer is a sort of balun. In any case, in the power electronics world, loss means watts dissipated as heat, and attenuation means reduction in level. I also spent a fair amount of time diagnosing EMI problems, and CM EMI is often a big problem in EMI.

It's the series-shunt methods that I have less experience with. In my past job I had access to nice impedance analyzers, and I also used low frequency VNAs like the HP3577 in IV mode for impedance measurements. I had good isolation transformers of the conventional type, and used Pearson transformers for current measurement. The IV mode measurement means that the transformer frequency response can be pretty poor and you still get a good measurement. With this approach, I could get sub-milliohm measurements pretty accurately and repeatably. I also had access to an HP Q-meter. All this I used to measure filters, transformers, and inductors. Even if the fundamental switching frequencies were in the 100s of kHz, ringing and harmonics meant there was a lot of content at much higher frequencies, where inductors might be past a second or third resonance.

I am interested in this topic now because I am trying to characterize low value shunts up to 100 MHz or higher. I have some designs that appear to work ok, but I have nothing to compare them to. My total inductance budget is < 1 nH, so any conventional current sense solutions blow that out of the water. I'm also interested because I am trying to measure losses in PCB layouts over a wider frequency band. Finally, I am also trying to understand high frequency "ground" currents and ground bounce issues where there are very high, very fast current spikes. I don't have the same access to high end equipment that I once did, or least not RF equipment, so I need to figure out some other ways to do things. This is all work-related. The challenge is that I don't have the luxury of 50 ohm anything. Since they are power conversion circuits, a lot of amps are flowing around at low voltage, so the impedances are generally much lower than 50 ohms.

Cheers,
John

[coppercone2]

I found my spring bushing in a rotary joint inside of some old equipment, its a thing

[joeqsmith]

My career started designing RF amps, ..

... This is all work-related. The challenge is that I don't have the luxury of 50 ohm anything. Since they are power conversion circuits, a lot of amps are flowing around at low voltage, so the impedances are generally much lower than 50 ohms.

Cheers,
John

I was with you up to that last sentence.  There are many good educational videos on using 50 ohm systems to measure PDNs.  This thread hardly scrapes the surface.  In many videos, they will demonstrate simulation software available today.   You may enjoy watching some of them. 

My old HP3589A isn't good enough to look at that 100uOhm directly.  My goal is to eventually try and use the Nano to measure a circuit boards PDN.  Now that we have a transformer and DC blocks, it's back to software.   In my case, it's just for the fun of it.   

I have one of Pearson's transformers.  The patents were helpful in understanding how they pulled it off.   Years back, I read this article and tried to replicate their results.     
https://interferencetechnology.com/the-hf-current-probe-theory-and-application/

[JohnG]

I'm not measuring PDNs in the traditional sense, though there is a fair bit of overlap. I am trying to model small, very fast switching high-current circuits that might be coupled or attached to a PDN intentionally or otherwise. An imperfect analogy would be that PDN analysis tends to look at things macroscopically, and I need to look more microscopically.

Also looking at simulation software, but the expense is significant so I need to make sure it will meet current and anticipated future needs, and I have very little 3D simulation experience. The other problem is that to really get good at such a tool, it helps to have an experienced employee to go with it.

Cheers,
John

 

[joeqsmith]

I am trying to model small, very fast switching high-current circuits that might be coupled or attached to a PDN intentionally or otherwise. An imperfect analogy would be that PDN analysis tends to look at things macroscopically, and I need to look more microscopically.

Sorry but you would need to provide more details for me to follow.  I consider macro as being something large and micro as small.  You wrote how you want to look in the mOhms where I have posted looking in the uOhms.  Oh wait, it was an imperfect analogy.    

[coppercone2]

you can try to measure the same value 10 times and graph the plot in all the measurements to see the spread, it might have to do with the confidence interval

[JohnG]

Sorry but you would need to provide more details for me to follow.  I consider macro as being something large and micro as small.  You wrote how you want to look in the mOhms where I have posted looking in the uOhms.  Oh wait, it was an imperfect analogy.   

I'll try to make it better. A PDN is more of a system level concern, basically a network with a lot of objects (sources and loads, loosely speaking). The objects could be a capacitor, an IC with bypassing, a switch-mode converter, etc. I'm looking at switch-mode power stages, which might feed a PDN. Such a stage will have caps, controls, active and passive parts, so tends to have complex behavior on its own. So a person looking from a PDN point of view tends to view these as given impedances on the network, but I zoom in on these details and others, so to speak. The other details relate to power dissipation, power processing density, cost, safety and reliability, etc.

Cheers,
John

[joeqsmith]

This clears things up.  If you decided to make a blog about it, post a link or for that matter, maybe even post it here.  I would like to follow along.

[JohnG]

If my employer is ok with it, I may able to publish something. They are mostly ok with publications, but first I need to get together enough material to make a reasonable publication. That's a ways off. It's one of those background tasks that keeps getting bumped until results are needed yesterday... I keep chipping away at it, though.

Cheers,
John

[joeqsmith]

I watched about half of this video today where Eric Bogatin, currently the Dean of the Teledyne LeCroy Signal Integrity Academy shows us how to connect a scope to a breadboard.   There is some magic voodoo going on here.       

[Kosmic]

I watched about half of this video today where Eric Bogatin, currently the Dean of the Teledyne LeCroy Signal Integrity Academy shows us how to connect a scope to a breadboard.   There is some magic voodoo going on here.       

I guess he is using a ground spring ? but I haven't seen any.

The signal was probably pre-recorded anyway and he is "lip synching" the manip 

[YetAnotherTechie]

To Joe:
Why not ask VirtualParticles to open it up and see how it's made?

[joeqsmith]

I guess he is using a ground spring ? but I haven't seen any.

The signal was probably pre-recorded anyway and he is "lip synching" the manip 

Lip synch,      I would expect him to use the spring or wire wrapped around the tip with that 6" removed.  He may have very well used the springs but I couldn't see it.  If there's no spring, I wonder what the reference was.  I would have thought the little board he shows was isolated. 

To Joe:
Why not ask VirtualParticles to open it up and see how it's made?

There are a few reasons.  I'm not so brash that I would ask anyone to pull their equipment apart.   He stated he does work with them and I think helping reverse engineer their product would be a pretty unprofessional move.  Brian collected the S-parameters for us which is really all I needed to get an idea what it was.   Last, I thought that it may be potted.  I was actually thinking I could borrow one to X-ray if it came down to it. 

[coppercone2]

making those spring bushings is really hard to get right. at least I have an assortment of spring wire now

[PartialDischarge]

I was thinking if it would be a good idea in order to measure the ESR of a capacitor in shunt mode, to intentionally lower the resonance frequency by adding in series a low ESR inductor, ie a power inductor for example.
The main reason is that ceramic capacitors do have a high resonance freq and maybe the real use mode is going to be at much lower switching frequencies, where the real ESR is going to be lower too.

So for example a 100pF capacitor in series with a 1mOhm 4.7uH would have resonance at only 7.3MHz instead of at hundreds of MHz, a region in which it would be easy to make measurements even with an oscilloscope.

[joeqsmith]

ESR of a capacitor in shunt mode

Starts about 12 minutes in. 

[PartialDischarge]

ESR of a capacitor in shunt mode

Starts about 12 minutes in. 

I know the method, maybe I didn't explain very well. I'm referring to lowering the resonance point on purpose with an L.

[virtualparticles]

Interesting idea. This would probably work well as long as the inductor resonance was high enough to not get in the way.

I can think of an alternative though. Let's imagine you accurately extract the S-parameters of the shunt capacitor alone without the traces or connectors. You would need to use automatic fixture removal to do this properly or adjust gating very carefully. (See https://coppermountaintech.com/automatic-fixture-removal-plug-in/) Once you have the S-Parameters of the Capacitor alone you could examine the real (ESR) and imaginary parts (X) of the impedance using shunt Z conversion.

[JohnG]

The method you are proposing is the operating principle behind the "Q-meter" of old. A good practical reference on this would be the manual for the HP 4342A Q-meter: https://manualzz.com/doc/6376599/hp-4342a-operating-and-service

John

[virtualparticles]

It is! Now there's a blast from the past. 

I remember creating extremely high Q coils for certain G-jobs using copper tubing and polishing the tubes to eek out a bit more Q. Good times.

[nctnico]

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.
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

I have a few reservations about the graph and the article. I think there is an error in the graph as well. It doesn't make sense that the accuracy for shunt and series measurement are different.

Secondly, AFAIK an error margin is a tolerance. Every piece of test equipment should perform within that tolerance so the measurement result has a defined uncertaintity level. I don't follow why the author of the article converts the tolerance into a 3 sigma value. IMHO that just doesn't apply to measurement uncertainty as you don't know what the error is of the unit that sits in front of you. You have to assume the worst.

Anyway... I went ahead and put an Excel sheet together to make my own graph:



I used an error of 0.2dB (just like the article does) and you can see both shunt and series errors level out towards 2.33% which is the linear error that 0.2dB represents. I have attached my Excel as well. For simplicity and to verify the article, I went for a numeric approach to calculate the numbers instead of deriving a function. The Excel sheet calculates S21, applies the error, calculates Z from that and last but not least calculates the error. For the shunt part the error seems to be limited to 100% but that is due to to formula used for the percentage being unsuitable for that range of the impedance but since that area is not interesting anyway, its not relevant to calculate correctly anyway (so meh).

I also found an interesting article about making impedance measurements using a VNA or a 2 port as well: https://www.picotest.com/Rohde-Schwarz/latin-america-microwave-conference-paper.pdf

This describes a neat trick to shift the measurement range for 2 port shunt or series by adding series resistors for shunt measurements and shunt resistors for series measurements. In my Excel sheet it is possible to change Z0 (the reference impedance) to accomodate for these extra shunt / series resistors. What the resulting measurement range will become depends on the noise floor of the VNA / spectrum analyser that is being used. The Excel sheet also shows the S21 in dB.

Forum member oz2cpu was so kind to make a PCB design availabe that allows to fit these extra resistors as well:
https://www.eevblog.com/forum/projects/component-tester-board-for-sa-na-impedance-caps-inductors-filters/msg4026844/#msg4026844

[gf]

I don't follow why the author of the article converts the tolerance into a 3 sigma value. IMHO that just doesn't apply to measurement uncertainty as you don't know what the error is of the unit that sits in front of you. You have to assume the worst.

Whenever random errors are involved which are drawn from an unbounded probability distribution (like e.g. Gaussian, Chi-Square, etc.), then it does not really make sense to specify a maximum error, because the maximum error were always infinite in this case. It does not help you either to known that the worst error is infinite. A practical compromise is therefore to choose a confidence level (say 95%, or 99%, or 99.9%,...) and to specify the corresponding confidence interval instead of a maximum error.

[nctnico]

I don't follow why the author of the article converts the tolerance into a 3 sigma value. IMHO that just doesn't apply to measurement uncertainty as you don't know what the error is of the unit that sits in front of you. You have to assume the worst.

Whenever random errors are involved which are drawn from an unbounded probability distribution (like e.g. Gaussian, Chi-Square, etc.), then it does not really make sense to specify a maximum error, because the maximum error were always infinite in this case. It does not help you either to known that the worst error is infinite. A practical compromise is therefore to choose a confidence level (say 95%, or 99%, or 99.9%,...) and to specify the corresponding confidence interval instead of a maximum error.

I get that but I don't see how this would relate to test equipment. Typically test equipment has hard boundaries for the accuracy of the results. Outside those boundaries test equipment is considered 'broken' and needs to be repaired / adjusted. IOW: statistical outliers don't exist. For example: nobody wants to buy a multimeter of which 0.3% of the units may have an unknown tolerance.

[coppercone2]

well its bounded by being off scale (the error is so big it hits a rail), so if you have a 99% chance of being at ADC span, that is a bound.

[nctnico]

well its bounded by being off scale (the error is so big it hits a rail), so if you have a 99% chance of being at ADC span, that is a bound.

Ofcourse there is also a limit to the measurement range. However within the specified measurement range the results should be within the tolerance limit.

[virtualparticles]

It's good to question things. If you don't do so, you'll never really "own" the answers that you're looking for.

The accuracy for shunt and series measurements are different because the partial derivatives of the impedances with respect to the reflection coefficients are different. I used standard metrological methods for evaluating uncertainties. The reason why there are 3 sigma standard deviations is that metrology is a statistical science. There are no fixed, "defined" accuracies, only a confidence interval within which one can expect to find a particular measurement. This was explicit in the article. You can't arrive at the correct answers by banging away at it numerically in Excel. I used 3 sigma statistics in particular because almost all manufacturers of test equipment use this confidence interval when setting data sheet specifications. With a 3 sigma confidence interval, we can say that 99.7% of the time, your measurement should fall within a certain error tolerance. It goes without saying that 0.3% of the time, your measurement may be outside of the data sheet tolerance.

There is an error in the chart. As I stated in the article, the expected measurement error does not go to 0 for the shunt measurement. I was simply uninterested in the value at 50 ohms and only concerned about the crossover points with the two other curves.

Adding a series resistor for the Shunt-Thru measurement to extend measurement range is potentially useful, but there are limitations. Calibration must be performed with the resistors in place in order to compensate for them. This creates a problem for for the reflection measurements on each side. If the attenuation of the series resistors looking into the 50 ohm cal kit is greater than about 15 dB, the raw S11 and S22 measurements will suffer and the corrected S21 will have greater inaccuracy. If the attenuation is closer to 20 dB, then the S21 measurement cannot be trusted at all.

For reference, the most relevant doc for VNA uncertainties is "Guidelines on the Evaluation of Vector Network Analyzers (VNA), EURAMET calibration guide No. 12, Version 3.0
https://www.euramet.org/Media/news/I-CAL-GUI-012_Calibration_Guide_No._12.web.pdf

Best,

Brian

[cdev]

It is! Now there's a blast from the past. 

I remember creating extremely high Q coils for certain G-jobs using copper tubing and polishing the tubes to eek out a bit more Q. Good times.

Would you be up to tell us a bit more about this. What was the intended application?

I want to build an Ethernet-connected HF filter board, maybe with some antenna switching too. Kind of like the one in the Hermes Lite 2.

The filters in it will be socket-ed and interchangeable for flexibility. They will be constructed in a DIP format, which should be fine for HF.

[gf]

I used 3 sigma statistics in particular because almost all manufacturers of test equipment use this confidence interval when setting data sheet specifications.

Can you cite some references that promiment manufacturers are really assuming 3 sigma, when they specify +/-U (without augmenting this specification with a coverage factor or percentage)?
Searching the web, I did not really find clear statements. Some search result rather indicated, that 2 sigma / 95% were quite common. So I'm still puzzled
EDIT: Or does any official standard exist, which requires 3 sigma?

[virtualparticles]

You probably won't find anyone stating that kind of information. Part of it has to do with expected production yields, proprietary information. It isn't practical to "cherry-pick" products which meet a tight specification and pitch the failures. Once a production process has been optimized and the product is stable, the final specifications can be determined statistically. These specifications must exceed the goals set forth in the product plan and allow for small process drifts. To be profitable, It is important that a very high percentage of products meet all specifications without re-work. A 5% fall-out rate would not be very exciting. Therefore, 3 sigma or greater compliance to specification is a common requirement.

This applies to the production of hardware. The noise figure of the receivers, the raw source and load match or the directivity of the bridges. But more importantly, each measurement is affected by random factors. Reflection measurements are limited by residual directivity. The leakage signal adds to actual reflections either in or out of phase depending on the distance to the reflection. Transmission measurements like S21 are affected by receiver noise and Raleigh statistics govern the interaction. With a knowledge of the noise figure it is possible to predict that S21 measurements will fall within a certain range. It is not that the VNA is part of a 0.3% that doesn't meet specifications. A series of measurements will fall within a Raleigh distribution. Our data sheet specifications are based on 3 sigma statistics and since equivalent hardware from the other guys have the same specifications, I can state that they are doing the same.

There are two videos which quantify these uncertainties.

https://coppermountaintech.com/video-vna-transmission-measurement-uncertainty/

https://coppermountaintech.com/reflection-vs-transmission-accuracy-in-vector-network-analyzer-measurement/

Hopefully this is helpful

[virtualparticles]

I needed a very high-Q inductor for an avionics filter. (land based).

[jmw]

Reviving since I'm building a setup for PDN measurement and built my own common mode transformer. The core I used was the Vac T60006-L2090-W518, with a 3D printed former to help with the split winding and maintaining minimums on bend radius for conformable RG-402 coax. Better performance than the J2102B in the HF range and UHF range, but worse in the VHF range. The ground loop error is only significant at lower frequencies though, correct? My measurements were done just like the picture on page 3 with pigtails with the shields shorted via a copper sheet. I only have a TG+SA so no vector measurement of S21.

[joeqsmith]

Yes, the cables have more loss as you start going beyond a few MHz. 

Xrunner had found a version of firmware for the H4 VNA that looks to be stable enough to use.   I think my H4 had better performance than my original NanoVNA at these lower frequencies.   I could attempt to remeasure the three standards I made up and see if it does any better. 

Since sorting out the common mode transformers, a few people here sorted out the keys for the Agilent PNA.  It would be easy enough to repeat these test now to get a better idea how mine compares with the data Brian collected from the Picotest transformer above 6GHz. 

My friend flipper is also very interested in this.  We were hoping after a member here had posted about getting the new LiteVNA that they would post some data.  Flipper was tired of waiting and we have new hardware on the way.   This would provide yet one more data point.   

[joeqsmith]

Attached images show the latest LiteVNA 3.1 hardware being used to measure our shunt standards (0.1, 0.025 and 0.001 ohms). 

Capture8 showing my original NanoVNA measuring these same standards.  It's still not great but for this measurement, it by far out performs any of the other low cost VNAs I have looked at. 

Capture10 showing my original NanoVNA with the 0.025, 0.001 and 0.0001 ohm standards.   

[joeqsmith]

Attempting to measure our shunt standards with the H4 with the firmware Xrunner provided (Version: 1.2.08 [p:401, IF:12k, ADC:384k, Lcd:480x320]).

H4_2k_10M_RStandards1:  2kHz to 10MHz with 0.1, 0.025 & 0.001 ohm standards attached.    The H4 was only rated to run to 50kHz and you can see the performance is pretty poor as we move below this.   

H4_2k_10M_RStandards3: 20kHz to 1MHz with 0.025, 0.001 & 0.0001 ohm standards attached.   It's much worse than my original NanoVNA.  When you consider that original NanoVNA was the lowest cost of these, there's a reason I recommended it.