Souly,
there is a bit of a problem with your problem description.
You say you are designing a conductivity probe.
So have you designed the probe? Is it for measuring liquids or solids. Is there a specified test voltage?
The AD is a vector analyser which applies a test signal to a 'network' and does a vector analysis on the received signal. From my experience, a conductivity measurement is generally a 'real part' measurement. It is possible you are working on microwave losses, but the AD is a 1 mHz max device.
So this is why I think the question you ask is a bit vague.
I have a conductivity meter for measuring liquids and the probe is a coaxial cell (like a short piece of coaxial cable) and is excited at 2000 Hz.