[help]single end mixer problem

hi,i have some problem at my project. it's about single end mixer used antiparallel diode on cmos.the diode that i connect drain and gate of FET together and choose the small size.the mixer that i hope it can used as up and downconverter. when i simulate that i find it is easy to saturate cause when i change the IF power from -40 to -20dBm ,my conversion gain from -5 down to -25 (I use it as upconverter).so i confuse it .
i think what's the problem :
1.size is not right
2.diode is connect wrong.(IN GaAs diode used as conecting drain and source )
3.my setting in ADS is wrong.
which one ?

i got more question?
when i sweep lo power from -20~20dBm ,my Convgain is posive at 12dBm ,is it possible? but i don't use active device and no bias
thanks

i think that your mixer has a very low IP3.

I think you must have made some mistakes in your simulation . your mixer is probably have low P1dB.And i think it's impossible to have positive power gain using passive structure.