Signal-Interference Bandpass Filters Using Resonant Transversal Filtering Sections with Asymmetrical Transfer Function Characteristics

This paper introduces a new class of planar bandpass filters (BPFs) with quasi-elliptic transfer functions obtained by cascading resonant transversal filtering sections (TFS) with asymmetric transfer function characteristics. Each TFS includes a parallel-type resonator in one of its two RF signal paths which in turn leads to two transmission zeros (TZs) without increasing the circuit size. Furthermore, it enables high-order filtering transfer functions to be realized by directly cascading multiple TFSs without the need for inter-stage impedance inverters. Theoretical examples of spectrally asymmetrical BPF responses using linear-circuit analysis illustrate the resonator-based TFS BPF concept's operating principles. Additionally, a microstrip prototype was manufactured and measured at 1 GHz to demonstrate the proof of concept. The prototype exhibited the following RF performance characteristics: a center frequency of 1 GHz, a 3-dB bandwidth of 150 MHz, a minimum in-band insertion loss of 0.8 dB, and in-band group-delay variation: 4.2-7.5 ns.