Electronics > RF, Microwave, Ham Radio

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

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joeqsmith:

--- Quote from: JohnG on May 22, 2021, 08:34:58 pm ---
--- Quote from: joeqsmith on May 21, 2021, 10:39:56 pm ---
https://www.nonstopsystems.com/radio/frank_radio_coax-sw.htm

--- End quote ---

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.
--- End quote ---

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...   :-DD


--- Quote from: JohnG on May 22, 2021, 08:34:58 pm ---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?
--- End quote ---

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.   


--- Quote from: JohnG on May 22, 2021, 08:34:58 pm ---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.
--- End quote ---

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.   


--- Quote from: JohnG on May 22, 2021, 08:34:58 pm ---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

--- End quote ---

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

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