A Novel Synthesis of Parallel-Coupled Microstrip Line Bandpass Filters with Chebyshev Responses and Accurate Bandwidths

This research is engaged in the new synthesis methods of parallel-coupled microstrip bandpass filters (PCBPFs) with accurate bandwidths and the Chebyshev responses. In a conventional design, the equivalence of a coupled stage is established by using two quarter-wave transmission lines with a J-inverter in between. Since the J-inverter is independent of frequency, the conventional equations are accurate only for filters with relatively narrow bandwidths. When a large bandwidth is designed, filters synthesized based on the conventional method will have a fractional bandwidth less than specification. For recovering the bandwidth decrement, the insertion loss (IL) functions are proposed for synthesis of PCBPFs with accurate bandwidth. This proposal is divided into three parts. (1) The IL functions are based on the composite ABCD matrix of all coupled stages instead of modeling each stage with the J-inverter equivalent circuit. (2) Synthesis equations are established by matching the coefficients of IL function with the Chebyshev functions. The under-determined conditions leave several degrees of freedom in choosing the circuit dimensions. (3) By properly utilizing these degrees of freedom, the problem resulted from the tight coupled-line dimensions can be resolved by gathering all difficulties to the end stages and employing tapped input/output to replace the end stages. This research proposes using the IL functions for synthesis of PCBPFs with accurate bandwidths and the Chebyshev responses. In addition to the formulation and simulation, we will also fabricate and measure practical circuits to demonstrate the significant improvement.

Project ID:PB9508-3989
External Project ID:NSC95-2221-E182-012