1-I have designed an LNA amplifier at 5.5 GHz and a Dielectric Resonator at 5GHz.I was hoping to down convert the LNA output using a dual gate mixer (5.5GHz at RF and 5GHz at LO) but none of the dual gate FETs I came across operate at such high frequencies e.g BF998 2-While designing the FET mixer,can I use the S parameters for calculations?Aren't they specified for linear models while mixer is clearly non-linear.
Maybe for such high frequencies you would have better luck using an integrated mixer like the LTC5510 instead of rolling your own.
Excerpt from the datasheet:
EDIT (prompted from a comment)
I doubt you can use a BF909 or a 2N25139 up to 5GHz. Look at their datasheets. The BF909 is intended for operation up to 1GHz:
While the 2N25139 is specified up to 4GHz (even the non-linear model is valid up to 4GHz):
Here is an excerpt from the datasheet of the first of that list: