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Model a connector in HFSS to use as a calibration standard

Question asked by drkirkby on Jun 3, 2013
Latest reply on Jun 22, 2013 by drkirkby
This is probably more for Ken Wong, but anyone else with knowledge is welcome to answer. 

I'm interested in using an open N connector as a calibration standard for a SOLT calibration.  I know it will not be as accurate as a true open circuit standard, but it is clearly possible - the "Quickcal" on the FieldFox range do this. 

I've long since thought HFSS may be a way to tackle this, and found Ken Wong's paper 

K.H. Wong, “Characterization of Calibration Standards by Physical Measurements,” 39th ARFTG Conference Digest,  June 1992

which I found very interesting. Ken used a combination of very accurate physical measurements and computer modelling using HFSS - obviously this pre-dates Agilent's EMpro, which has broadly similar capabilities to HFSS. 

What it is not clear to me from Ken's paper is how the data about the calibration standard is derived from the HFSS model. What I was thinking of doing, is:

1) Set up an HFSS model of an open male N connector. This would have some coax attached. A reasonable length of coax would be 60 mm from the reference plane of the N connector.  The exact length should not matter too much, although one would not want to make it too long as it just wastes computer resources, but one also needs the fields to have settled down, so I don't think it is sensible to make it too short. 

I think it would be reasonable to make the coax an airline - at least initially anyway, as a first go at this problem. 

2) Add a waveport at on the end of the coax

3) Run HFSS and let it determine the S-parameters at the waveport, which would be 60 mm away from the reference plane of the N connector. 

4) Use post-processing in HFSS to find the S-parameters at the reference plane of the N connector. 

Step 4 is a problem for me though. In HFSS, it is possible to move the reference by 60 mm in post processing. I don't have a copy of HFSS here, but I know one can do it. So in principle one could move the reference plane for the simulation up to the reference plane of the N connector, and so find the properties at the reference plane. But I don't think a simple movement of the reference plane would be valid in this case, as I think HFSS would assume the transmission line is uniform in cross section. But the N connector would not be - the male pin tapers. Hence I don't see how to estimate the S-parameters at the reference plane of the N connector, which would be needed to use it as a calibration standard. 

It's hard to tell exactly how Ken used HFSS from that paper, but I suspect HFSS was used to determine the S-parameters at the reference plane of 1.85, 2.4 and 3.5 mm connectors. I'm interested in how that was done. 

To me it seems quite easy to model a connector with a bit of coax and determine the phase angle. It is less obvious how one could use that information to find the fringing capacitance at the reference plane, which is what I think is needed in order to characterise a calibration standard using a combination of accurate physical measurements and computer simulations. 

*General comment on scientific papers written in the computer age.* 

When I was at school we were told in science experiments to write up the experiment in a way someone else could repeat it to verify the results. With modern papers, making use of computer models, it is basically impossible to write up an experiment in a way someone else could reproduce it. The use of modern propriety software like HFSS, which will be obsolete in a few years, running on hardware which will also be obsolete in a few years,  will make it impossible for the results to be repeated by anyone else. 

Recently I found a paper of interest, where someone had used Fortran and offered to make his code available. I would have like to have got the code, but had concluded the author could not be contacted, and was quite possibly dead. 

This is not a criticism of Ken's paper - my own Ph.D. thesis has computer simulations which nobody could reproduce. 

Dave  

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