Estimation of Passive Mixer Output Bandwidth Using Switched-Capacitor Techniques

Passive mixers have become an essential component of SAW-less receivers. It is well known that the passive mixer behaves as a switched-capacitor circuit (SC) but to the authors knowledge, there is no reported analysis of the mixer impedance that truly accounts for the SC behavior. In this paper, we present for the first time a closed form of the passive mixer output impedance based on SC techniques. We prove that the fundamental lower limit of the mixer impedance is proportional to the well-known switched capacitor resistor 1/(f(LO)(C)) and different from the previously reported mixer switch ON resistance. We also explain that the equation is useful in estimating output bandwidth of passive mixer based front-ends with general LNA load impedance. We finally show that our bandwidth estimation matches measured results of two receiver front-ends.