Electronics > RF, Microwave, Ham Radio

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

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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:

--- Quote from: coppercone2 on April 10, 2021, 06:15:54 am ---...
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

--- End quote ---

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:

--- Quote from: coppercone2 on April 10, 2021, 08:46:49 am ---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

--- End quote ---

--- Quote from: coppercone2 on April 10, 2021, 08:46:49 am ---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

--- End quote ---
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

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