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Re: [TowerTalk] Measuring Common Mode Choke with Anritsu VNA

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring Common Mode Choke with Anritsu VNA
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sat, 30 Dec 2017 13:25:45 -0800
List-post: <mailto:towertalk@contesting.com>
Hello Maximo.

The necessary setup places the choke as the series element of a voltage divider between the VNA output and input (with the VNA input Z as the load), with calibration performed at the point where the DUT is connected and a jumper equivalent to test leads needed to connect the choke for Thru calibration. A test jig is likely to be required for good accuracy. W6GJB built a very nice one for me in his shop. Stray (parasitic) capacitance can be the cause of significant measurement error for large Z mag, because parasitic capacitance of the choke itself is a major component of its impedance, combining with the choke's inductance to form a parallel resonance. It is the resistive component of the choke around resonance that makes the choke most effective.

Not knowing the Anritsu unit but knowing the measurement, it is likely that you WILL need to transfer the data to a spreadsheet where the S21 data can be used to compute the Z using complex circuit analysis.

You might want to look at the DG8SAQ VNWA built and sold by SDRKits in the UK. It's a fully built and tested unit, not a kit. The control software includes a very capable math function that can, among other things, do the math (for the equation that you enter) to compute and plot complex Z of the choke. The software then can be set to display  Z mag, Rs, and Xs vs frequency.

https://www.sdr-kits.net/index.php?route=web/pages&page_id=11_11

https://www.sdr-kits.net/index.php?route=product/product&path=66_67&product_id=99

73, Jim K9YC

On 12/30/2017 12:41 PM, Máximo EA1DDO_HK1H wrote:
Hi all,


I have in my hands a very nice device called "VNA Master" from Anritsu, model 
MS2025B.

The thing is I am trying to measure a Common Mode choke with this VNA, using S21 mode on 
the braid of a coax cable with different toroid combinations, but my knowledge is limited 
and I am not sure if this device is able to show the common mode on screen (I think is 
called "complex impedance").


The options available for the trace are:

Log Mag

Phase

Real

Imaginary

Log Mag/2

Log Polar

Real Impedance

Imaginary Impedance


(Plus some more; SWR, Group delay, Linear Polar, Smith Chart, etc.)


Is any of the above types useful to show Common Mode Choke on screen?


Then , the Marker has some more options to show additional information.

Apart from above list, there are some extra options:


Impedance (Z)

Admittance (Y)

Normalized Impedance (NZ)

Normalized Admittance (NY)

Polar Impedance (PZ)


So, I am not sure if any of those options are good to shown the "choke 
impedance" on screen, without additional calculations.


What I can do, and in fact I do, is to export the measure in .s2p Touchstone 
format, then apply some maths on a excel sheet to convert LogMag and Phase to 
complex R, complex I, and finally complex Z. All of that on Excel.

But to get that directly on screen would be much better.


Thank you very much.

Happy New year.


73, Maximo

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