A New Synthesis and Design Method for Wideband Bandpass Filters With Generalized Unit Elements

A new synthesis and design method for wideband bandpass filters has been proposed using the concept of generalized unit elements (GUEs). The synthesis starts with a lumped-element LC circuit, which is obtained by some well-known circuits and classic methods, such as the ladder-type circuit and the coupling matrix method. A GUE is an equivalent circuit, which is represented by either a coupled LC circuit or a section of the transmission line coupled by stubs. In this view, an LC circuit is transformed into the transmission line circuit directly via a general network of a GUE. This contributes to the key idea behind the synthesis procedure. Detailed analysis on the GUE and its equivalent circuits are discussed in terms of the equivalent characteristic impedance and the extended Richard's transformation. Based on these two variables, frequency and constant mapping functions are proposed to explicitly obtain characteristic impedances in the transmission line circuit. In the final part, three prototype filters, namely, the classic Chebyshev ladder-type filter, the general box-shaped Chebyshev filter of symmetrical filtering responses, and the cascaded trisection filter in an asymmetrical transmission zero distribution are designed and fabricated, respectively. All the experimental results agree well with the predicted ones and thus it verifies the proposed synthesis and design method.