City
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wcheng,
This is what I (actually badly) tried to explain. In the multiplication mode the input can be sinewave but with a low amplitude so that you stay in a linear region (which has nothing to do with the operating region of the transistor, I'll argue later on ...) where the system will not produce any additionnal harmonics. So the only multiplication terms come from sin(wlo*t)*sin(wRf*t). If you use high amplitude signals that forces your system to go into nonlinear regions, that will chop your RF signal depending on the drive again.
Let me take an example: Consider a Tree mixer with a tail current source, an input differential pair, 4 transistors in a crosscoupled configuration and resistive loads with a filtering capacitor (I suppose we are downconverting) and for simplicity say we work with BJTs or MOSTs in weak inversion. The RF is fed at the differential pair input and the LO is fed at the 4 four crosscoupled transistors. Now, I you drive the LO amplitude << than 4*Ut, the transistor stays in a linear mode because to a deltaLO corresponds a deltaIout which can be described with deltaI=deltaLO*G right ? If the RF is also small enough (again << 4Ut) then the output voltage in the bandwidth of the filter can be written as Vout(t)=VRf*VLo*Rl*G*sin(wLO*t)*sin(wRF*t) which is the case that was described to you in the other forum.
If you increases the LO amplitude >> 4Ut (RF still small), the crosscoupled transistors will successively turn ON and OFF (i.e switching mode) and if you probe the collector current, you'll see a the input RF input Vin*sin(wRF*t)*Gm,in chopped at the LO rate which gives you a current in the form Vin*Gm,in*sin(wRF*t)*Fourier(square_wave(wLO*t)). This is the chopping mode.
So this does not depend on squarewave drive or not, it depends if you forces your system into linear or nonlinear regions. Of course, a squarewave in this case is the most efficient but it can be prove that a sinewave with high amplitude can also do a good job ... but at high voltage supply of course.
So what is the difference between both? Well you can see that even if the circuit is identical in multiplication and chopping region, gain will be different for both of them. Also Noise will be VERY different and very bad for mulitplication mode as described earlier. This is why those cells are used in the chopping mode and are called MIXERS rather than MULTIPLIERS, even if we can use them in both configuration.
Hope it was more clear.
City
