VNA Compact Cal Kit from Rosenberger

[sixtimesseven]

Just got my Rosenberg SOLT Kit - Much earlier than expected 

Went for the throu version. And it looks like it is indeed an airline. Looks really nice.
The throu is also not screwed on, but is attached via an internally fixed bolt and can be rotated by 90 degree. Which is great!


All in all very happy so far. Almost a bit scared to attach my Succoflex SMA cables to this 

[rf-messkopf]

Congratulations to your Rosenberger cal kit.

If somebody else is also pondering over the purchase of a professional cal kit: Don't overlook the possibility of obtaining a cheap cal kit and having it measured on an accurately calibrated VNA, and using the measured S-parameters directly for the correction of the kit.

In this way you can transfer almost all of the accuracy of a professional cal kit to a much cheaper one, including the return loss of the loads. A parametric error model of the cal standards will do that only to some extent, and even then most of the time the loads will have to be assumed ideal. Of course, you are left with errors due to the stability of the cal standards and the repeatability of connectors.

A number of VNAs support the use of Touchstone files for cal kit error correction. And even if your VNA doesn't, you still could do the vector error correction of your measured results offline. That would of course be an inconvenience, but then again you will not always need the full accuracy of your cal kit.

[TheSteve]

Looks great!

Now you can save up for the male version 

[TheUnnamedNewbie]

Congratulations to your Rosenberger cal kit.

If somebody else is also pondering over the purchase of a professional cal kit: Don't overlook the possibility of obtaining a cheap cal kit and having it measured on an accurately calibrated VNA, and using the measured S-parameters directly for the correction of the kit.

In this way you can transfer almost all of the accuracy of a professional cal kit to a much cheaper one, including the return loss of the loads. A parametric error model of the cal standards will do that only to some extent, and even then most of the time the loads will have to be assumed ideal. Of course, you are left with errors due to the stability of the cal standards and the repeatability of connectors.

A number of VNAs support the use of Touchstone files for cal kit error correction. And even if your VNA doesn't, you still could do the vector error correction of your measured results offline. That would of course be an inconvenience, but then again you will not always need the full accuracy of your cal kit.

The repeatability question got me wondered - how hard would it be (and would it be usefull) to home-brew a kind of sliding-match to do sliding-match calibration?

[rf-messkopf]

The repeatability question got me wondered - how hard would it be (and would it be usefull) to home-brew a kind of sliding-match to do sliding-match calibration?

A sliding load relies on the precision of the airline along which the load slides. Just like the airlines that are used for TRL and other purposes, these are manufactured to very tight tolerances. I doubt that it will be feasible to DIY them, unless you have a very sophisticated machine shop and the necessary skills available. And to manufacture a good load element that slides along the center conductor and makes seamless contact to the outer conductor will be all but trivial.

I don't think that using connectorized pieces of rigid coax of various lengths will be good enough to replace a sliding load, mainly due to the connector repeatability issue, and also due to the precision of the rigid coax. I do not remember which variability to expect due to connector repeatability, but I remember some publications in IEEE journals where this was investigated experimentally. I can have  look for references in my notes, but I will have virtually no time the next 10 days or so.

That said, sliding loads are more or less obsolete today since one can correct even a mediocre but stable load with the help of S-parameter correction data. This is the way that Ecal units work. I remember having read that their performance can hold up to that of a sliding load, and may only be very slightly worse. That will depend on the frequency range as well, of course.

[sixtimesseven]

My SMA-T Connector arrived and I was able to do the T-Check.

I loaded the cal. coef. manually. I tried to use the "VNA Cal Kit Manager 2" to share the cmk file but ran into problems :\
https://www.eevblog.com/forum/rf-microwave/vna-cal-kit-manager/

[John_ITIC]

Hello, as this is the only thread related to this Rosenberger cal kit, i'm reviving it.

I just got my 8720C VNA operational after various repair work and i'm now looking at cal kits. I got a quote for the 3.5mm SOLT kits from Rosenberg. Both the female and male versions are slightly under $1,200 (each) and i'm considering buying both versions, for future flexibility.

I have checked and my 8720C has features for adding user-defined cal standards. The open can be defined via C1-C4 constants and offset delay/loss. So far so good.
I however noticed that the Rosenberger cal kit spec card also specifies the L1-L4 for the short standard. My 8720C however does not have any menu option for entering this information. Only offset delay/loss. I'm suspecting the short inductance can only be entered on newer VNAs.

I'm wondering how much inaccuracy will result when I can't enter the short standard L1-L4?

See the attached PDF for the cal constants. Zoom in on the cal card. Note that they are showing an incorrect cal card as it only goes to 12 Ghz. OPs card displayed above goes to 26.5 GHz. So, they goofed up when taking the picture. Or just photoshopped an old 12 GHz cal kit picture for the 26.5 GHz kit and forgot to update the card.

Thanks,
/John.

[joeqsmith]

I however noticed that the Rosenberger cal kit spec card also specifies the L1-L4 for the short standard. My 8720C however does not have any menu option for entering this information.

I would have guessed it supported this.  I assume you looked if there was any upgrades available.

[John_ITIC]

Quote
I however noticed that the Rosenberger cal kit spec card also specifies the L1-L4 for the short standard. My 8720C however does not have any menu option for entering this information.

I would have guessed it supported this.  I assume you looked if there was any upgrades available.

No, I already have the latest ROMs (1.05).

Would it be correct to assume that the calibration standards have higher precision as they go to 26.5 GHz and the L1-L4 would not matter for the 20 GHz 8720C?
Or, would it be possible to correct for the difference in post-processing on the PC-side, in case more precision is ever needed?

Thanks,
/John.

[rf-messkopf]

I'm wondering how much inaccuracy will result when I can't enter the short standard L1-L4?

I wrote a small Octave script that calculates the S11 response of open and short cal standards for a given set of parameters (in Keysight format): https://www.mariohellmich.de/lostfound/files/ck-model. You have to modify the script (enter your parameters, frequency range, number of points) and run it. The script will generate two Touchstone (.s1p) files for the open and short. Then you can plot the S11 response of the shot, both with the specified parameters, and with the offset inductances set to zero. You can use an external Touchstone plotting software, or modify the script to use Octave's own plotting functions. Check the phase difference of both cases. I'd bet they will differ by at least a couple of degrees at 26.5 GHz when the short is not fully corrected.

Another question is what the impact on the measured S-matrix of a DUT would be, but I think it is safe to assume that phase accuracy would be uncertain by at least be the same amount.

Or, would it be possible to correct for the difference in post-processing on the PC-side, in case more precision is ever needed?.

Not exactly by post-processing, but you would have to do the calibration offline. That is, measure the cal standards with your test setup (open, short, load at both ports, and a thru for a standard SOLT), and then the DUT. Then export the data to a PC (that makes eight Touchstone files), and do the math as well as the plotting. Of course, that could be automated via GPIB.

[rf-messkopf]

Check the phase difference of both cases. I'd bet they will differ by at least a couple of degrees at 26.5 GHz when the short is not fully corrected.

Okay, I became curious, and I'm too tired to do something useful. See the attachment. At 20 GHz, the short will differ by about 8.5 degrees when the offset inductances are set to zero. That is already quite appreciable, and much more than the accuracy specification of <3 degrees up to 26.5 GHz.

[joeqsmith]

I  don't know why you couldn't just use the VNA to collect the raw data and do everything on the PC side.   A lot of work but certainly possible.

[John_ITIC]

Thank you both. I suspect the deviation due to not taking the short L1-L4 into account is acceptable for my needs. I mainly want to characterize differential serial communications channels and i suspect I will not need absolute accuracy. I have just started learning how to operate my VNA so unfortunately not yet at all competent in the field.

Edit: Apparently, there is a way to include the short L1-L4 compensation automatically when using the external METAS VNA Tools application over GPIB. I will look into this closer.
https://www.eevblog.com/forum/testgear/kirkby-calibration-kit-alternatives/msg1436372/#msg1436372

Thanks,
/John.