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

[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