Notes on Switching Mixers

An area of interest for high performance receiver designers is switch-mode mixers. Mixers are usually the circuit element that determines the upper limit of intermodula- tion-free dynamic range, and switching type mixers have the potential for the highest per-formance. Mixers are considered to be inherently non-linear devices, but a mixer is more accu-rately described as a circuit that switches between two linear states. Those linear states are the transmission of the normal and inverted polarities of the signal. The rate of transition between the two states takes place at the offset frequency or “local oscillator” fre-quency. If the transition between the two states is instantaneous, there is only one non-linear transition. Of course, all switches have finite transition times, but as those times get smaller, the non-linear region—and its resulting distortion—is reduced.