CMOS mixer design

Also, what's the best CMOS mixer for low power

which technology p.e:0.7um?

i think the passive one is better with respect to your requirements but if we consider the lo buffers power consumption there is no difference between the passive and active approximately however all of them depend on your design.if you need low noise the power consumption increases.for high linearity the passive mixers are excellent but it has conversion loss.and their flicker noise is lower than the active mixers.

http://www.mini-circuits.com/pdfs/LAVI-362VH+.pdf