Mixed Lumped and Distributed Circuits in Wideband Bandpass Filter Application for Spurious-Response Suppression

Mixed lumped and distributed circuits (MCs) have been analyzed and applied in a wideband bandpass filter (BPF) synthesis procedure for spurious-response suppression. First, an MC is proposed and compared with the conventional commensurate line. With the even-odd mode analysis, the MC is decomposed into an inverter and the shunt stubs on the two sides, which finds its lowpass equivalent circuit. Then, constant and frequency mapping functions, simply formed by two basis functions, are derived to link the MC and its lowpass equivalent circuit. With this technique, the lowpass prototype filter can be mapped to its BPF counterpart. Finally, the BPF design variables are obtained by comparing the input admittances of the even/odd mode in the MC or the shunt stubs to the basis functions. A fifth-order ladder type BPF has been implemented following the proposed procedure. The experimental results accurately predict both wide passband and spurious-response rejection band.