Minimization of the Noise Figure of RC Polyphase Filters

The sideband suppression ratio of RC polyphase filters is independent of source and load impedances. This property is valid for any number of stages and for any detuning between the stages. Due to this property, noise figure minimization can be done independently. Two formulas for noise figure of passive two-ports are obtained, and noise figure of polyphase filters are minimized. The noise figure of an RC polyphase filter is strongly dependent on the source resistance and capacitance. For a two-stage filter providing better than -25 dB sideband suppression over two octaves, a minimum noise figure of 10.18 dB was found.