aaron_do
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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
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