János Ladvánszky and Gerhard Schultes
austriamicrosystems

This paper demonstrates that the sideband suppression of RC polyphase filters is independent of the source and load impedances under very general conditions. This property is valid for any number of stages and any detuning between the stages; consequently, noise figure minimization can be done independently of the adjustment of sideband suppression. A formula for noise figure of passive two-ports is obtained and applied for the case presented.

The noise figure is strongly dependent on the source resistance and capacitance. A formula for the lower bound of the noise figure of n stage polyphase filters is given. 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.