Linearity of CMOS active Gilbert Mixer

Hello everybody,
I am designing a CMOS active Gilbert Mixer and running into the linearity problem. The problem is that the switching pair degrade my linearity quite a lot (for iip3 roughly about 10dB). I can not understand how this happens? Is it because of the non-linearity of the on-resistance? If so, how to improve that?

Thanks

i think u need to creafully design them , so they can fully switch on and off

khouly

Hi Khouly,
Thanks for reply. Can you shine more details about how? my understading is that you still want to keep the switching pair in saturation for a active Gilbert cell, in CMOS. by doing this, we probably can not have a very big LO swing, is that right?

my understading is that you still want to keep the switching pair in saturation for a active Gilbert cell, in CMOS. by doing this, we probably can not have a very big LO swing, is that right?[/QUOTE]

yes that's right. in gilbert mixer transistors must be at the saturation.this limits your swing.

Why not use a passive mixer, if you care the linearity a lot, even for the cellular Tx applications, passive mixer is popular too