Can one have a negative offset and capacitance for an "open" cal standard?

Question asked by drkirkby on Dec 25, 2012
Latest reply on Jan 16, 2013 by drkirkby
First of all, merry Christmas to everyone.

I've spent some of Christmas day messing around with my 8720D VNA - it is personally owned by me, hence in my home, so it was not quite as unsociable as it might appear!

I was interested in see if an unconnected male N connector could be used as an "open" calibration device, and if so what parameters would be best. Well, I know it can be done, as the QuickCal on the FieldFox range do this, but of course I needed the parameters to enter into the VNA. and I would not be surprised if they are proprietry to Agilent.

I noticed the following, after first calibrating with a proper 'N' calibration kit (85032B).

1) Much to my surprise, the open-N does not appear to be capacitive, as I would have thought due to the fringing capacitance. Instead, it appears inductive. Would it make sence to represent an inductance by capacitor of a value < 0 ? I found it is possible to enter negative values for all value C0, C1, C2 and C3. .

2) The centre conductor sits below the reference plane by about 1 mm. Given the velocity of light in a vacuum is 299792458 m/s, that would suggest the offset to be entered should be -0.001/299792458 = -3.335 ps. Does that make sense?

Perhaps it is worth lying about the offset by a few ps - making it even more negative so it is possible to enter the capacitance as a positive value. I'm not sure if that would be better or worst than having negative capacitors.

For what it is worth, my initial attempts to find sensible values for C0, C1, C2 and C3 by curve fitting were not too successful, but I suspect with more time it might be workabe. If anyone has ever tried this, or knows of a reference, I'd be interested.

You probably think I am mad, but it is Christmas day, so I have had a beer or two!

With that, all the best. I'm going to have an Irish Coffee! (That's quite popular in the UK - not so sure about the USA. I guess you could make it with Jack Daniels whisky.) If I'm honest, I make an "Irish Coffee" using a cheapish Scotish whisky. I think putting the more expensive Irish whisky in a coffee is not justified.

Dave