Digital Modeling of Microwave Filters With Coupled-Line Sections

A digital-modeling technique for the transmission scattering parameters of frequency-periodic RF/microwave passive filters has been recently proposed. By means of the direct matching of the ideally-synthesized transfer function of the filter and that of its associated discrete-time linear time-invariant (LTI) system, the corresponding coefficients ak and bk are identifiable. Hence, zero-pole diagrams in the Z-plane are derived, which enables a reinterpretation of these frequency-periodic microwave filters. This work demonstrates that this digital matching is also possible for frequency-periodic filters based on coupled-line sections. In particular, this discrete-time modeling is applied to a classic third-order coupled-line bandpass filter and to a signal-interference ultra-wideband (UWB) bandpass filtering section. In addition, their zero-pole diagrams are analyzed and discussed in detail.