Linearity of CMOS active Gilbert Mixer

[nxing]

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

 

[khouly]

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

khouly

 

[nxing]

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?

 

[victoriya]

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.

 

[buckaroo]

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