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Measuring the Coss of a ldmos fet.

Question asked by paulh on Dec 11, 2007
Latest reply on Dec 12, 2007 by Dr_joel
Hi just need a sanity check on my setup.

I am using the 8753A with a dual directional coupler, measuring S11
of the D.U.T

In measuring the COSS of the device I have the device biased at 12.5vdc
which is on a small pcb. The dc is decoupled and then fed to the drain of
the device via a 4.7Uh choke. The drain of the device is connected via a 
short microstrip line to a N connector. I calibrated S11 to take into account
the test jig. 

My questions are.

Are the ports on the 8753A dc isolated.
Will my biasing arrangement of the device effect the measured results
or would there be a better method to bias the device to measure COSS

My working frequency is 80Mhz

Thanks for any insight.. Paul.