Author Topic: NanoVNA V2+4 SMA Cal load  (Read 4812 times)

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Offline joeqsmith

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Re: NanoVNA V2+4 SMA Cal load
« Reply #75 on: December 16, 2020, 12:37:21 am »
1601 points is pretty common the the data base file. As long as the cal piece isn't changing quickly then interpolation works well. Rytting is the go-to guy for all things In VNA calibration.
Good info.  I really had no idea.   So the thought is what ever the calibration standards are, you would always have 1600 points?   Is this documented somewhere or is it just typical of what you have seen?   

From what you have found, do they commonly provide you with data outside of what the kit is rated for? For exampled 900K to 10.1MHz for a 1 to 10MHz standard? 

Our Automatic Calibration Modules (ACMs, same as eCal) use a 1601 point data base over the operating frequency. Data is only taken over the operating frequency range and it is actually measured with a golden VNA. There are more than 4 calibration artifacts used internally as you can see by the block diagrams. A least squares approach is used to perform calibration in the over-determined system of equations. This is helpful as it improves the accuracy of the calibration somewhat with some random variation in the measurements.

Thanks for the information.  I read the document and am a little unclear.  Maybe you can help.    The document lists a Max number of characterization points at 1601 but is seems to support between 2 and 1601.    They also state "The user characterization option is provided for saving new S-parameters of the Module after connecting adapters to its ports.".   Does these two statements suggest the user may create a 2 point characterization but the factory data is always 1601? 

In your particular case, is your process to setup the standard on one of your VNAs (the gold standard)  and to use it to measure the ecal with what ever adapters you have attached to it,  sweep it just over the range you plan to use it, and store that back into the ecal?   Then the ecal is then used on the other non-gold standard VNAs?   

When you collect  the data from the ecal over your range of interest, you perform the least squares fit to the raw data and that fit is what you write back?

When you perform the least squares, are you still using interpolation to get back to 1601 points over your range of interest? 

I did try and contact Kirkby Microwave today to see if they could provide any further insight as well.   

Thanks again.
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Offline virtualparticles

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Re: NanoVNA V2+4 SMA Cal load
« Reply #76 on: December 16, 2020, 02:55:20 pm »

Thanks for the information.  I read the document and am a little unclear.  Maybe you can help.    The document lists a Max number of characterization points at 1601 but is seems to support between 2 and 1601.    They also state "The user characterization option is provided for saving new S-parameters of the Module after connecting adapters to its ports.".   Does these two statements suggest the user may create a 2 point characterization but the factory data is always 1601? 

In your particular case, is your process to setup the standard on one of your VNAs (the gold standard)  and to use it to measure the ecal with what ever adapters you have attached to it,  sweep it just over the range you plan to use it, and store that back into the ecal?   Then the ecal is then used on the other non-gold standard VNAs?   

When you collect  the data from the ecal over your range of interest, you perform the least squares fit to the raw data and that fit is what you write back?

When you perform the least squares, are you still using interpolation to get back to 1601 points over your range of interest? 

I did try and contact Kirkby Microwave today to see if they could provide any further insight as well.   

Thanks again.

the "Golden" VNA is calibrated using primary standards in a temperature controlled dungeon. The ACM is then characterized and 1601 data points are stored internally for each calibration artifact. for a 100 kHz to 9 GHz ACM, point 1 would be 100 kHz and point 1601 would be 9 GHz.

"User Characterization" allows one to re-characterize the ACM with an added adapter. This requires a set of mechanical calibration standards with type and gender that match the other side of the adapter. The result is only as good as the mechanical standards which is typically not at all as good as the original ACM performance. I think the user can choose the number of points but 1601 would be advised since more is better. 2 points would obviously not be very good.

The Least Squares method is applied during the calibration process. The complex reflection coefficient of each calibration artifact is cubic spline interpolated at each user frequency point against entries in the 1601 point stored table. The set of known vs measured values are used to set up the over-determined system of equations and the 12 error terms are calculated and stored for each frequency point in the sweep.

I hope this answers your questions!

 

Offline joeqsmith

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Re: NanoVNA V2+4 SMA Cal load
« Reply #77 on: December 16, 2020, 06:41:09 pm »
Thank you for the write up.   

Quote
I hope this answers your questions!
Problem is I just have more questions.    :-DD  Based on the little bit I had found on primary standards for VNAs, I think Dave needs to interview you about your work as I have a hunch there is some interesting things going on.


https://www.euramet.org/Media/news/I-CAL-GUI-012_Calibration_Guide_No._12.web.pdf

Quote
The first fundamental step of the traceability chain of S-parameters is established with the characterization of calculable measurement standards [9, 10, 11, 12].  Known cal-culable coaxial standards are air-dielectric lines, offset shorts, flush shorts and offset opens. Calculable standards are parametrized and measured dimensionally. The me-chanical model should cover the whole standard including the connector interface to achieve  highest  accuracy  and  to  obtain  a  consistent  definition  of  the  measurement reference plane [13, 14].  Based on dimensional measurements and known material parameters the S-parameters of the calculable standards are determined with the helpof  analytical  equations  and  numerical  EM  simulations  in  combination  with  electrical measurements.  Calculable standards may be called primary standards.  The process of establishing traceability by using primary standards is called a primary experiment. Primary experiments are elaborate and require measurement and modeling capabilities at a level that can usually only be provided by national metrology institutes.
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Offline virtualparticles

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Re: NanoVNA V2+4 SMA Cal load
« Reply #78 on: December 16, 2020, 06:52:18 pm »
Problem is I just have more questions.    :-DD  Based on the little bit I had found on primary standards for VNAs, I think Dave needs to interview you about your work as I have a hunch there is some interesting things going on.

Euromet is the Swiss standards outfit. We generally use NIST over here. The primary standards that we have here in our ISO17025 lab are accredited through NIST, which in practical terms means that we occasionally send them over to NIST for a really expensive checkup. We purchase them and then have them accredited if memory serves. I'm sure it is possible to roll your own standards with precision machining but we don't do that.
 


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