Hi,


for fully differential, you can use a 4-port network analyzer, or a 2-port network analyzer with a pair of baluns. You may need AC coupling if the 50-ohm impedance of the network analyzer would draw DC from your circuit. The capacitor needs to be large enough that the high-pass corner formed by the capacitor and the 50-ohm load of the network analyzer is significantly lower than your measurement frequency. You can buy an SMA DC block which should be good enough if you want to measure RF. It will indicate its frequency range.


regards,
Aaron

P.S. Actually in theory I think you can just use a 2-port network analyzer as long as you are able to terminate all unused ports in 50-ohm for the measurement. You then have to convert several sets of 2-port s-parameter data into 4-port s-parameter data.
However, one technician from Agilent of R&S (can't remember which) told me that this only works well for passive components and that for active components, both the positive and negative ports need to be stimulated at the same time.