Broad-band polyphase filters

In the double-quadrature low-IF architectures with adaptive image rejection (see Figs. 2.33 and 2.34) passive Resistor-Capacitor Polyphase Filters (RCPF) are used for I/Q signal generation. There are many possible ways to generate I/Q signals. The RCPFs are chosen because their implementation is straightforward and they are convenient for high-frequency applications where a high level of integration is required. The achievable maximal IRR in the double-quadrature low-IF architectures is directly dependent on the performance of the RCPFs. In order to be used in a multi-standard receiver, the frontend architecture has to accommodate requirements from different standards. Hence, the capability of RCPFs to provide a required performance in a wide frequency range is crucial for the application of the double-quadrature low-IF architectures in multi-standard receivers. It is therefore instructive to investigate the performance of broad band RCPFs before proceeding with the system and circuit level design (see Fig. 1.7).

This chapter is organized as follows. In section 3.1 a topology of a passive polyphase filter is described. Next, in section 3.2, RC polyphase filters are investigated. Design aspects related to RCPFs are discussed in section 3.3. Finally, the summary is given in section 3.4.