Flat Pass-Band Method with Two RC Band-Stop Filters for 4-Stage Passive RC Polyphase Filter in Low-IF Receiver Systems

Abstract

This paper proposes a flat pass-band for a 4-stage passive RC polyphase filter in a blue-tooth low-IF receiver system; there the bandwidth is 8MHz, the center IF frequency is 4MHz, and the required image rejection ratio is <-30dB. Based on the superposition principle, the transfer function of this filter is derived. As the input signals are the wanted signals, there are two local maximum values which are calculated based on Cauchy-Schwarz inequality theorem at 160kHz (1.24dB) and 40MHz (1.24dB). Therefore, two RC band-stop filters are used to improve the pass-band of these local maximum values (improvement of the ripple gain of pass-band from 2dB into 0.47dB). As a result, a 4-stage passive RC poly-phase filter for a low-IF receiver is designed, where an image rejection ratio is -36dB, and the pass-band gain is flat (the ripple gain is 0.47dB).