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Hi Jef
One more comment. The load being 50 ohm it is not sensitive to a small displacement of the calibration plane.
You will soon see a perfect T-Check
Kind regards
Kurt

-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Jeff Anderson
Sendt: 15. december 2019 16:19
Til: [email protected]
Emne: [nanovna-users] Homebrew Female SMA standards and T-Check

A few years ago I made an open and a short female SMA standard for calibrating my HP 8753C VNA (for the load I used a very nice 85052 female 3.5mm load). I never used the open and short above about 30 MHz, so I didn't worry too much about their accuracy.

With my purchase of a NanoVNA I thought I could put them to use in lieu of the male standards the NanoVNA comes with. And so I thought I'd check their accuracy, using my 8753C and the Rohde & Schwarz T-check method.

Of course, I had not characterized my standards, and so I thought, as a first attempt at checking them, I would use the "stock" 3.5mm cal-kit definitions that are stored in the 8753C.

The key parameters of the "stock" 3.5mm definitions (to match HP's 85033C kit) are:

Open: C0 = 53, C1 = 150, C2 = 0, C3 = 0, and Offset Delay = 14.491 ps
Short: Offset Delay = 16.695 ps
Thru: Offset Delay = 0 ps

Much to my surprise, the results were very good, as you can see in the attached T-check plot.

But, I wondered, do the "stock" HP delays bear any resemblance to the actual delays of my SOLT standards?

Assuming HP's Offset Delays were not the same as my SOLT Offset Delays, why not change HP's "stock" Offset Delays (as defined above) to move the reference plane to the end of my standards (i.e. at the location of the actual open and short).

So, as an experiment, I decided to subtract 14.491 ps from HP's "stored" 3.5mm definitions. In other words,

o The Open Offset Delay changed from 14.491 ps to 0 ps.
o The Short Offset Delay changed from 16.695 ps to 2.204 ps
o The Thru Offset Delay changed from 0 ps to -14.491 ps

The results weren't quite as good as I expected (see attached T-check PNG).

Maybe, I thought, the Thru's delay change needed to be positive, not negative, so I changed its Offset Delay from -14.491ps to +14.491ps. Even worse results!

So I tried doubling the Thru's Offset Delay. That is, rather than making it -14.491 ps, make it -30 ps. The results now look pretty good.

But why does doubling the Thru delay give these results? And so my questions are:

1. Why did doubling the Open/Short Offset Delay delta of -14.491 ps to be the Thru's Offset Delay give the results it did?

2. Why are the Thru Offset Delays in HP's Cal Kits all spec'd to be 0 ps? (Clearly the length of the thru's are not zero, so what is HP referencing to determine that an Offset Delay is 0?)

Thanks for any insight provided!

- Jeff, k6jca