Using SHORT cal standard on older VNA without inductance parameters? - Page 1
Forum section: electronics / rf-microwave

author: mark03
author_rank: Frequent Contributor
timestamp: May 15, 2019, 09:08:16 pm
content: I've built up a set of female-SMA open/short/load standards for my HP 4396B VNA (100 kHz to 1.8 GHz). I intend to use IN3OTD's optimization tool to fit the model coefs for my homemade standards, based on S11 measurements on a calibrated VNA. (Seehttps://www.qsl.net/in3otd/electronics/VNA_calkit/calkit.html,also discussed here in some older threads.) To that end, I'm planning to borrow a nice OSL cal kit (Copper Mountain S911) from a friend, program its data into my 4396B's user-cal-kit slot, warm-up, calibrate, and then measure my homemade OSL standards.The S911 datasheet provides a model of the short standard which includes nonzero L0/L1/L2/L3 as well as offset delay. My VNA, like the 8753, doesn't support Lx for the SHORT definition; I'm told the reason for this is that below 6 GHz, the parasitic inductance is well modeled with offset delay alone.Presumably, using Copper Mountain's specified offset delay / offset loss while ignoring the nonzero Lx's will result in a suboptimal calibration. Is there a straightforward way to massage their short definition into a best-fit model with Lx=0 and a different offset delay, over a specified frequency range? It sounds like a job for MATLAB if someone could point me to the relevant math.PS: The Rosenberger cal kit being discussed in another thread has the same issue, so this may be of interest for anyone looking to buy one for an 8753.

author: yl3akb
author_rank: Regular Contributor
timestamp: May 16, 2019, 08:00:08 am
content: One way would be to calculate S - parameters from S911 electrical model coefficients and then use Your fitting software to fit new model. I added old MATLAB script which does similar calculation, it may help

author: mark03
author_rank: Frequent Contributor
timestamp: May 16, 2019, 04:37:49 pm
content: Thanks! After seeing no replies yesterday, I actually did the same thing in Matlab last night, calculating the reflection coef per HP app note 1287-11. I get the same results as your script, which is a nice sanity checkI must say I'm surprised that the Lx coefs have so little effect, even all the way up at 6 GHz. I was going to tweak the offset delay to re-optimize as you suggest, but the total phase error at 1.8 GHz, my upper limit, is only .05 degrees. This can be reduced to +/- .008 degrees over the 0-1.8 GHz band by changing offset delay from 31.785 ps to 31.823 ps, so yeah, why not, but I have to assume unit-to-unit variation is of a similar magnitude?The nice folks at Copper Mountain have offered to measure an S911 on their "golden" VNA and send me the typical S parameters. This would provide yet another avenue, i.e. running IN3OTD's optimization script on the S-parameter data and deriving my own set of model coefs. Might be interesting to see how they compare.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 16, 2019, 05:40:24 pm
content: Many kits won't specify the inductance of the short because it makes such little difference - I've got a ZVRE which supports the inductance coefficients, but even on the 26.5+GHz cal kit profiles they have loaded, the inductance coefficients are left at zero. Some of it also has to be the given phase error specification of the unit, if you can really get almost an order of magnitude improvement of phase error by adjusting the offset delay by 50fs.... I'm impressed the unit can even discern that fine of a phase difference, but I wouldn't be surprised if thermal drift from ambient operating temperature in the specified range could vary that measurement widely. I think phase stable cables generally can't touch that level of tolerance - often specifying a degree per GHz.

author: mark03
author_rank: Frequent Contributor
timestamp: May 16, 2019, 06:09:16 pm
content: Quote from: DaJMasta on May 16, 2019, 05:40:24 pmThis is entirely in simulation, looking at S11 computed from the provided model (with nonzero Lx) and then playing with the offset delay on a modified model where the Lx are zero. Unaccounted for are:* unit-to-unit variation of the cal kit (they are not individually measured)* fitting error, for whichever specimen they actually did measure, to arrive at the datasheet coefs* instrument noise* thermal effects* mechanical phase stability of the test apparatus* connector wear* ??It does make you wonder why they provide the Lx coefs to five significant digits!

author: ejeffrey
author_rank: Super Contributor
timestamp: May 16, 2019, 06:29:41 pm
content: QuoteFixed that for you

author: Bud
author_rank: Super Contributor
timestamp: May 16, 2019, 06:43:19 pm
content: They would be happy to provide to 10 digits but 5 was limitation of their EM simulation software.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 16, 2019, 06:46:41 pm
content: Quote from: mark03 on May 16, 2019, 06:09:16 pmGotta appease those mm wave gods or they will screw with your measurements! Simulation makes much more sense, haha!

author: in3otd
author_rank: Contributor
timestamp: May 17, 2019, 08:52:31 am
content: yep, Lx seems to have only a little effect; I tried to do some calculations to see how Lx could be approximated with an increased offset delay.For a short-circuited transmission line the input impedance is \$Z_{in}=jZ_0 \tan(\beta l)\$; when the line is short this can be approximated with \$Z_{in}=jZ_0 \beta l = j2\pi f Z_0 \frac{l}{c}\$ so this looks like an inductance of \$L_{sc}=Z_0 \frac{l}{c}\$. The last term is the transmission line time delay \$T_d\$, so \$L_{sc}=Z_0 T_d\$.So to approximate the short-circuit inductance \$Lsc\$ you can increase the offset short delay by \$T_d=\frac{L_{sc}}{Z_0}\$.Taking the numbers from the Copper Mountain Tech N911 CalKit Offset Short, the main inductance term \$L_0 = 3.3998 \times 10^{-12} H\$ can be approximated by increasing the offset short delay by about \$0.068\,ps\$. This will change over frequency due to the others \$L_n\$ terms but the effect remains rather small at not too high frequencies, as you already saw.

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 29, 2019, 12:34:32 am
content: Glad I could help with the S911 parameters. :-)