Hi all,

anybody know of any good simple explanations of Mixer Operation. I've read a couple of papers which gave some pretty good explanations but most people simply stick to saying its a simple multiplier. Just to give you an idea of what i'm talking about,


                           Rvariable
node X_________vvvvvvv_________output node
l            l                                       l
l            l             ------>                 l
l            >              I1                     l
(I)          > Ro                                =  Clarge
l            >                                      l
l            l                                        l
gnd      gnd                                  gnd


lets represent the mixer as a variable resistor and the output has a large capacitor connected to limit the LO-IF leakage. The input is comes from the LNA and is equal to gm.Vin, here just called I, and an output resistance r0. The way I see it, we can represent Rvariable as,

Rvariable=RDC+RLOsin(wLOt)

in other words, it has an average component and a varying component which depends on the LO swing (RLO<=RDC). So the explanation I can think of which seems to correlate well with simulation goes like this,


VX (RF) = gm.Vin (RDC//R0)                       // means in parallel
I1=gm.Vin.(RDC//R0)/RDC
Vout (IF) = gm.Vin.RLOsin(wLOt).(RDC//R0)/RDC

so from the final equation, we need to maximize RLO (the LO swing) and RDC to optimize the gain. The maximum gain is 1 because RLO<=RDC. This seems to correlate with simulation, but a better explanation would be welcome.

note as i understand it, RLO/RDC can actually be as high as 4/pi from the fourier expansion of a square wave or something like that...can't really remember.

thanks,
Aaron