A Compact Ultra-Wideband Multimode Bandpass Filter With Sharp-Rejection Using Stepped Impedance Open Stub and Series Transformers

This brief presents the analysis and design of an ultra-wideband bandpass filter (UWBPF) with four transmission zeros and seven poles. The UWBPF model consists of series transformers, coupled lines, and a shunt-stepped impedance open stub. The even-odd mode analysis method and scattering parameters theory are used to extract the closed-form equations for transmission zeros and poles of the proposed UWBPF. For validation, a prototype with a center frequency of 2.49 GHz is designed, simulated, and tested. The measured 3 dB fractional bandwidth is 103.2%, and the fabricated prototype occupies an area of $0.165\lambda _{g}^{2}$ . The circuit, full-wave simulated, and experimental results are in a good match.

I. Introduction

High data rate wireless communication systems are in great need of bandpass filter (BPF) circuits with wide fractional bandwidth, high selectivity, low insertion loss in the passband, good out-of-band characteristics, and compact size. To meet these requirements, several methods have been reported [1]–[21] to design high-performance wideband bandpass filters (WBBPFs). The series transmission lines of the conventional BPF is replaced by series transmission lines in [1] to increase the fractional bandwidth (FBW). However, the prototype occupies a large circuit area. Coupled transmission lines of different configurations are used in the design of WBBPFs [2]–[7]. Most of the realized filters based on coupled transmission lines have compact size, good selectivity with decent 3-dB fractional bandwidth (FBW). Ring resonators like dual-mode ring resonator [8], nested split-ring resonators [9], stepped impedance open stubs loaded ring resonator [10], and rectangular ring resonator [11] have been employed to develop WBBPFs. Microstrip WBBPFs based on multimode resonators (MMRs) are reported in [13]–[15]. In [16], a comb-slotted substrate integrated waveguide MMR and cross-shaped defected ground structure are used to realize a high selective WBBPF with good upper stopband performance. Even though the performance of the filters based on MMRs is good, they are occupying a large circuit area. Parallel coupled transmission lines and cross-shaped resonators are utilized in [17] to develop a WBBPF with reconfigurable bandwidth; the maximum 3-dB FBW achieved using this technique is 56.5%. In [18], [19], signal interference technique is used to realize high selective WBBPFs. A coplanar waveguide WBBPF with 68% 3 dB FBW based on non-periodic spoof surface plasmon polaritons with H-shaped cells and spiral-shaped DGS is reported in [20]. Lumped and distributed elements are used in [21], to develop a WBBPF cascaded with a low-pass filter. The manufactured prototype is compact, however, the selectivity factor is poor.