Lecture 20: Passive Mixers by Prof. Ali M. Niknejad presented: Voltage switching mixers, MOS switching mixer, MOS device feedthrough, summary of MOS switching mixer MOS passive mixer is very linear. | EECS 142 Lecture 20: Passive Mixers Prof. Ali M. Niknejad University of California, Berkeley Copyright c 2005 by Ali M. Niknejad A. M. Niknejad University of California, Berkeley EECS 142 Lecture 20 p. 1/32 Voltage Switching Mixers LO +RF −RF LO IF RIF Instead of switching currents, we can also switch the voltage. In the above circuit, during the +LO cycle, switch S1 activates and feeds the RF to the output directly. In the LO cycle, switch S2 activates and feeds an inverted RF signal to the output. This circuit requires good switches that turn on hard (low on-resistance) and turn off well (good isolation). A. M. Niknejad University of California, Berkeley EECS 142 Lecture 20 p. 2/32 MOS Switching Mixer LO +RF −RF IF RIF A practical implementation uses MOS devices as switches. The devices are large to minimize their on-resistance with a limit determined by isolation (feed-through capacitance). We see that the RF signal is effectively multiplied by ±1 with a rate determined by the LO signal. LO A differential RF signal is created using a balun or fed directly from a balanced LNA. A. M. Niknejad University of California, Berkeley EECS 142 Lecture 20 p. 3/32 MOS Device Feedthrough G G Cgs Cgd Cgso Cgdo Cgb S Cjs Rbs Rch Cjd Rbd D S Cjs Rbs D Cjd Rbg Rbd B B When the device is “on”, it’s in the triode region. Due to the low on-resistance, the coupling through the substrate and LO path is minimal. When the device is “off”, the RF and LO leak into the IF through the overlap and substrate capacitances. A. M. Niknejad University of California, Berkeley EECS 142 Lecture 20 p. 4/32 Summary of MOS Switching Mixer MOS passive mixer is very linear. The device is either “on” or “off” and does not impact the linearity too much. Since there is no transconductance stage, the linearity is very good. The downside is that the MOS mixer is passive, or lossy. There is no power gain in the device. Need large LO drive to turn devices on/off Need to create a differential RF and LO .
