Flat PassBand Method with Two RC BandStop Filters for 4Stage Passive RC Polyphase Filter in LowIF Receiver Systems This paper proposes a flat passband for a 4stage passive RC polyphase filter in a bluetooth lowIF receiver system; there the bandwidth is 8MHz, the center IF frequency is 4MHz, and the required image rejection ratio is u003c30dB. 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 CauchySchwarz inequality theorem at 160kHz (1.24dB) and 40MHz (1.24dB). Therefore, two RC bandstop filters are used to improve the passband of these local maximum values (improvement of the ripple gain of passband from 2dB into 0.47dB). As a result, a 4stage passive RC polyphase filter for a lowIF receiver is designed, where an image rejection ratio is 36dB, and the passband gain is flat (the ripple gain is 0.47dB).

Analysis and Evaluation Method of RC Polyphase Filter RC polyphase filter is a complex analog filter and has asymmetric frequency characteristics with respect to DC. It is composed of resistors and capacitors, and has complex analog input  output ports. Since this type of filter can partially realize Hilbert transform, it can be used for generating orthogonal signals. In the field of wireless communications, it can also be used for suppressing interference signals by combining the LowIF receiver topologies. In this paper, we introduce a technique to evaluate the mismatch characteristics between the real path and the imaginary path in the RC polyphase filter. These studies could lead the complex analog processing to apply for the wideband wireless communications.