Thread: Can I determine electrical delay, C0, C1 and C2 of a home-made cal kit?

I have an 8720D 20 GHz VNA and an Agilent 85033E Mechanical Calibration Kit DC - 9GHz 3.5mm. Obviously the cal kit can't make best use of the VNA, but at this moment I can't afford the better 85052B 26.5 GHz kit.

Is there any way I can calibrate the VNA with the official 9 GHZ kit, then find the parameters of the electrical delay (ps), C0, C1 and C2 of my homemade 3.5 mm open, so that I may use it for less critical measurements, perhaps upto 3 or 6 GHz.

It's fairly easy to calculate the input impedance of a transmission line of length L, terminated in some arbitrary impedance Z. So I should be able to find an equation of the form.

Z (frequency) = f (frequency, delay,C0, C1, C2)

I could do similar with the short, and find the delay, L0, L1 and L2.

Then I would have thought a non-linear fit of the measured values of Z on my open, over the range of say 0-6 GHz, would allow me to calculate values for the delay, C0, C1 and C2 for my home-made open, and the delay, L0, L1 and L2 for my short.

Finding a suitable fitting routine does not seem easy. There does not appear to be one in the Sage open source maths program

http://www.sagemath.org/

nor in Mathematica

http://www.wolfram.com/mathematica/

but I've been told something suitable exists in the GNU scientific library.

http://www.gnu.org/software/gsl/

Before wasting my time trying to do this, I'd appreciate if others feel this is practical or not.

I've got several reasons for wanting to do this.

• A friend has a HP VNA, but only an Anritsu cal kit of unknown performance.
• I'd like to buy the 26.5 GHz 85052B, sell the 9 GHz 85033E, then use something cheap/homemade for non-critical measurements, to save wear on the expensive 85052B.
• Although I don't yet have an N cal kit, I do intend buying the very expensive 18 GHz 85054B N cal kit. Again, it would be good if I could have something cheap and cheerful to save wear on the expensive kit.

Any thoughts?

Dave

Edited by: drkirkby on Aug 27, 2012 6:50 AM

But if I did make a home-made cal kit, would the method I suggested by the best one to find the coefficients? Or would another approach be better?

I noticed the foam inside the plastic containers for the centre conductor of the airline is my verification kit is breaking up. But I think it should clearn off the airline OK.

It's interesting what you say about the load. I bought a very cheap APC-7 cal kit (HP 11866A), which I think is only rated to 2 GHz. But I was wondering how well that would work if the load was replaced with a better quality one. It would seem to me, that with APC-7, there's a lot less variables than with N. I don't have much use for APC-7, but the kit was cheap. It is not supported by my VNA anyway.

Dave

Thank you Dr_Joel. There is only one 7mm cal kit in the firmware - the 85050, which is an 18 GHz kit.

The 7mm kit I have, which I bought for very little money, is an 11866A. The kit has an HP 909C 2 GHz load. Are the coefficients available for that old model avaiable? Looking at the manual for it, there is very little information indeed. I don't even think it mentioned what models it worked with, or what the frequency range was.

I was wondering how good/bad it would be to substitute the load for an 18 GHz one, and just use the 85050 cal data in my VNA. I don't see APC-7 as something I am going to worry about much, but I did have some brainwave idea about making measurements of permittivity of PCB material by pressing a dielectric against the end of an APC 7.

I would be very interested in the papers you have about getting calibration data for standards. I appreciate it is a non-trivial project, but for some reason it interested me. I think if I worked in academia again (which I don't), it would make an interesting student project to try to do that.

I would add, I was recently made unemployed, so my income is zero. So whilst I intend setting up a company soon, at the moment everything I buy is coming from my own finances. So I need to save money were I can.

For me, only N and 3.5 mm is that important. Things like 7 mm, and 7/16 are less intersting. I have used 7/16 on an an amateur radio amp I have partially completed, but it's not something I am going to be putting a VNA on.

Dave

My problem is not with a cal kit, but the centre conductor of the airlines for a 85055A verification kit. The centre conductors are stored in plastic tubes, which have foam rubber at each end. That is breaking up, and has desosited some of itself on the airlines's centre conductor.

I've not used the verification kit yet, as I don't have an N calibration kit. But I need to sort out something with the 85055A verification kit before using it.

I suspect if I put alcohol on the plastic tubes, it might damage them. I just need to find a suitable "foam" of some sort to put inside the caps to stop the ends being damaged, whilst at the same time not permitting rubbish to get on the airline.

Dave