The Rough Prototype and Fine Tuning (RP-FT) Method for Multiband Bandpass Filter Synthesis

Nowadays, various new communication technologies operating in different frequency bands emerge constantly. There is a trend to integrate more than one of them into single system, for optimizing whole system performance and volume. As an important circuit component for frequency selection, the research on multiband bandpass filter (MB-BPF) receives much attention. This paper proposes a novel synthesis method for MB-BPF. It applies two frequency transformations on the specifications of the MB-BPF to be synthesized, so that a rough prototype containing the information of transmission and reflection zeros is constructed first. Then, optimization is applied for fine tuning until equal ripples are achieved in each passband. For simplicity, it is called the Rough Prototype and Fine Tuning (RP-FT) method, which demonstrate great advantage in flexibly controlling the performance of a MB-BPF such as passband number, performance of each passband, transmission zeros, etc. Furthermore, the filtering polynomials derived by the RP-FT method could be transformed into specific coupling matrixes for physical realization. For illustration, this paper derives a specific coupling matrix form, and applies it in the design of a microstrip quad-band bandpass filter. The measurement well verifies the validity of design process.