Products > Test Equipment
Kirkby calibration kit alternatives?
hendorog:
--- Quote from: G0HZU on February 19, 2018, 10:38:34 am ---I think it's possible to make a cheap but decent DIY SMA cal kit for use up to maybe 3GHz. I did this many years ago and I still use this cal kit today for many applications. The short and load and open were all DIY using SMA end launchers and I worked out the correction factors myself for the user cal file.
I get very good results with this cal kit and I typically use it to help me model SMD components up to a few GHz.
--- End quote ---
Good point, I had forgotten that you had done that. It is very well done and is light years ahead of the eBay offering.
https://www.eevblog.com/forum/testgear/agilent-8753es-and-other-vnas-this-kind/msg1063141/#msg1063141
In comparison, the cheap eBay kit is shown in the pictures at about 10dB return loss on the load at 3GHz. It looks to be only usable below 500MHz perhaps.
Here is a useful page as well for DIY cal kits:
http://www.qsl.net/in3otd/electronics/VNA_calkit/calkit.html
--- Quote ---
Once above 3GHz I'd prefer to use a used but decent commercial cal kit, eg a healthy 3.5mm 85033E mechanical cal kit or a 3.5mm Ecal kit. I don't think I'd want to pay $600 for the Kirkby kit. Old/used commercial cal kits can suffer damage and wear so some caution would be needed but I'd still prefer to go down this route compared to a cheapo SMA based alternative no matter how much time and effort went into measuring the cal coefficients.
--- End quote ---
The issue with that is the availability of those used kits. They are very rare at reasonable prices, the sellers seem to think that even the boxes are worth a fortune.
Overall, I think that the simplest approach is to purchase some quality parts, get them tested by someone with a real cal kit, and then determine the coefficients using the script in the link above.
G0HZU:
Thanks. I had a quick look over the Kirkby website and there are a few things that give me slight concern.
--- Quote ---•Difference between the phase predicted by the mathematical model of the opens, and the actual phase of the opens is <4° from DC to 7 GHz
•Difference between the phase predicted by the mathematical model of the shorts, and the actual phase of the shorts is <4° from DC to 7 GHz
--- End quote ---
It would be nice to see some test setup info and some data on this. My first reaction is to run to the hills because this spec of 4 degrees doesn't sound good... maybe I'm misinterpreting what is meant here? .. or maybe it's a typo on the website and the spec is much better than this?
Also, the kit appears to use regular SMA F-F bullets with the standard screwed at the end. It looks cheap... The closeup images of the short and open don't inspire me. I wouldn't expect a regular SMA bullet to qualify as being part of a VNA cal kit that runs to 7GHz. Maybe these are special versions of the SMA F-F. I'm used to the Suhner ones at work. Also, what happens if the cap rotates? Do you tighten it again with a torque wrench and hope the cal is still good? Maybe it works a lot better than it looks?
An ex colleague of mine bought one of these kits recently and I could maybe try and borrow it and test it. At work, I've got access to new and UKAS calibrated PNA VNAs and various calibrated mechanical kits and several Ecal modules and some £££ Rosenberger test cables. I think the cables cost about £1500 each!
hendorog:
--- Quote from: G0HZU on February 20, 2018, 10:22:20 pm ---
An ex colleague of mine bought one of these kits recently and I could maybe try and borrow it and test it. At work, I've got access to new and UKAS calibrated PNA VNAs and various calibrated mechanical kits and several Ecal modules and some £££ Rosenberger test cables. I think the cables cost about £1500 each!
--- End quote ---
I vote hell yes!! Please do if you can swing it.
G0HZU:
Sadly, he's unlikely to lend it to me unless he goes on holiday because he uses it every day for his business. So it might take a while.
You posted up a link to that (fringe) cal coefficient tool using GNU Octave but it looks like it is still being developed.
For my homebrew cal kit here I embed keysight's polynomial for the fringe capacitance (for the OPEN) into an RF simulator and make all the coefficients variable. Then I use a simple tline model plus the polynomial result to predict the phase result of an ideal model. I then compare against the homebrew open after a mechanical or Ecal calibration. On the real homebrew standard I used a fine rotary tool to optimise the end of the open and I got very good agreement out to well beyond 3GHz. But I only made this kit for use with my first VNA and so it only had to be good to 3GHz. It's lasted many years and I still use it a lot. If I made another I'd expect to get better results for the short and open and maybe get it to work up at higher frequencies too. I'm not sure how valid my approach for the fringe capacitance was but it seemed to work OK on the simulator. Back then I would have measured and optimised my open and short at work on an old 6GHz HP8753ES with an 85033E cal kit.
orin:
--- Quote from: G0HZU on February 20, 2018, 10:22:20 pm ---Thanks. I had a quick look over the Kirkby website and there are a few things that give me slight concern.
--- Quote ---•Difference between the phase predicted by the mathematical model of the opens, and the actual phase of the opens is <4° from DC to 7 GHz
•Difference between the phase predicted by the mathematical model of the shorts, and the actual phase of the shorts is <4° from DC to 7 GHz
--- End quote ---
It would be nice to see some test setup info and some data on this. My first reaction is to run to the hills because this spec of 4 degrees doesn't sound good... maybe I'm misinterpreting what is meant here? .. or maybe it's a typo on the website and the spec is much better than this?
--- End quote ---
A fair proportion of that 4 degrees will be the uncertainties in measuring the actual phase of the standards. According to the datasheet, the 8720D's uncertainty is close to 2 degrees in the 2-8GHz range for a 1.0 reflection coefficient.
--- Quote ---Also, the kit appears to use regular SMA F-F bullets with the standard screwed at the end. It looks cheap... The closeup images of the short and open don't inspire me. I wouldn't expect a regular SMA bullet to qualify as being part of a VNA cal kit that runs to 7GHz. Maybe these are special versions of the SMA F-F. I'm used to the Suhner ones at work. Also, what happens if the cap rotates? Do you tighten it again with a torque wrench and hope the cal is still good? Maybe it works a lot better than it looks?
--- End quote ---
In the Kirkby SMA kit I have, the female standards do indeed use an F-F adapter (and the male standards seem to use an M-F adapter). The F-F adapters have flats so they can be held by a wrench when being connected, so you shouldn't be holding them by the cap. Given the variation in SMA connector design, I'd guess the results using the male open on its own with varying female SMA connectors were unacceptable. Adding the adapters ensures a consistent interface to the actual male open and must give better overall results or Dr Kirkby wouldn't have done it that way.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version