Calibrating HP 8753 S11 with 7mm OSL, smith chart shows 9 pSec open delay - why?

[wb0gaz]

Experimenting with HP 8753A VNA calibration, APC-7/7mm rudimentary cal kit (basically just an open, short and load; the load is a Midwest Microwave and good return loss thru 3 GHz, and all associated connectors have been cleaned with 99% isopropyl and non-lint swabs) and the VNA's port extension feature. The open does not have a "collet pusher" feature (a plastic element that presses the test set's APC-7 connector's collet flush when inserted.)

The VNA has built-in cal coefficients for APC-7/7mm (that's the default at power-up). Only other relevant parameter is sweep range (power-up default 3-3000 MHz.)

After doing S11 (single port) calibration and displaying in smith chart format and re-attaching the cal components sequentially, short is a dot (far left), load is a dot (center), and open is a short line around outside perimeter, terminating at the far right extreme (representing about 9 pSec offset from zero-delay open). This artifact is fully repeatable and stable (and I've got other hands-on experience handling APC-7/7mm connectors, so I think I'm handling the connectors correctly.)

I had expected for open a dot at the far right of the smith chart, seeing as 7mm cal components all have 0 offset delay. The 9 pSec delay/line seen with the open cal component is unexpected and what I'm asking about. Is this a result of no "collet pusher" in the open? 9 pSec seems like quite a bit (representing 3mm in round-trip delay).

I thought this (unexpected delay in open) might be a test set issue, however, identical results with an 85046A and 85044A (both connected - separate times - to the VNA's OUT, REF and A-IN ports with separate short, identical-length coaxial cables.)

What am I seeing (or am I seeing artifact of a defect somewhere?)

Thanks!

[MRIKen]

My thought on this would be the midwest load you are using has a different internal length then the HP open/short you are using.  For my coil work I used a set I made from BNC.  Since they were different internal lengths there was always a delay at higher frequencies.  If it really bothered me I would just add a small delay.  But then again the highest freq we used was 600 MHz

[Miek]

If you re-measure the standards after calibration, then they should match whatever the calibration kit definition specifies (which won't necessarily be a single dot).
It will depend on what is set in the kit definition on your VNA but, as far as I could see, the default for the HP 85031B's APC 7 open should be zero delay but also a capacitance of around 93 fF at 3 GHz. That will look a lot like a delay of about 9 ps.

[wb0gaz]

Miek -thank you, that explains I am sure my observation! I did not consider how the fringing capacitance would show up in terms of delay, but now I can compare C0/C1/C2/C3 parameters for APC-7 vs. offset  3.5mm calibration kit.

Your explanation is very clear.

Dave

[ahbushnell]

9 ps is about 0.1 inches at the speed of light in free space.