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Design >> RF Design >> Theory of Mixer Operation
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Message started by aaron_do on May 10^th, 2006, 12:15am

Title: Theory of Mixer Operation
Post by aaron_do on May 10^th, 2006, 12:15am

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

Title: Re: Theory of Mixer Operation
Post by mikki33 on May 17^th, 2006, 6:07am

See the classic:
Ho-Jun Song and Choong-Ki Kim, ’An MOS Four-Quadrant Analog Multiplier Using Simple Two-Input Squaring Circuits with Source Followers’, IEEE Journal of Solid-State Circuits, Vol 25, No 3, pp 841-848, June 1990.
(I didn't find it in the Internet, you can use the library). This worked 10G with 0.13 process.

Another good one is
http://citeseer.ist.psu.edu/cache/papers/cs/12470/http:zSzzSzwww-scf.usc.eduzSz~sangyoubzSzPaperszSzCktzSzMixerzSzCmoszSz33ssc06-hsiao.pdf/hsiao98parallel.pdf