[TowerTalk] Measuring Common Mode Choke with Anritsu VNA

[Jim Brown]

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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