Novel analysis of passive mixer output impedance using switched-capacitor techniques

Passive mixers became an essential component of SAW-less receivers. It has been 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 using SC techniques. We prove that the fundamental lower limit of the mixer impedance is proportional to the well-known switched capacitor resistor 1/(f_LOC) different from the previously reported mixer switch ON resistance. We also explain that the equation is useful in estimating output bandwidth in voltage mode mixers and estimating blocker filtering in both voltage mode and current mode passive mixers.