Design of an Active 2nd-Order Polyphase Filter for Image Rejection in Subsampling Receivers

Receivers that work with real subsampling, have the disadvantage that they can not transform a bandpass signal directly to baseband. By using complex subsampling, direct conversion into the baseband is made possible. For this purpose, the bandpass signal must first be converted into a complex or analytic signal. This signal, consisting of real and imaginary parts, is filtered by a polyphase filter. This type of filter allows the transition of positive frequencies and filtering the corresponding negative frequencies. In order to ensure the requirements of the receiver, a high image rejection between negative and positive frequencies must be achieved. This can be done by cascading several first-order filters. To obtain a cheap cost alternative, a novel second-order polyphase filter is described, employing only half the number of OpAmps compared to known cascaded first-order filters. For this reason, this paper focuses specifically on the design and verification of a second-order polyphase filter on a printed-circuit-board (PCB).