# Design of a Compact Ultra-Wideband Microstrip Bandpass Filter
## 1 Introduction
[13], liquid crystal polymer (LCP) technology was used to realize the integration of wideband coupling patch, embedded quarter-wave resonators, bandpass periodic units and defected ground structure (DGS), and a design of an ultra-wideband bandpass filter with high selectivity was completed. In Ref. [14], a UWB filter with 180% fractional bandwidth at the center frequency of 10.5 GHz, which was integrated by multilayer liquid crystal polymer technology, was reported. Liquid crystal is mainly used in the design of reconfigurable filters, and its manufacturing process is complicated. To realize the miniaturization of the filter, a novel multimode double-ring ultra-wideband bandpass filter based on high-temperature superconducting material was proposed [15]. The filter, with a size of 21.95 \(\times\) 16.46 mm\({}^{2}\), was realized on a MgO substrate with a thickness of 0.5 mm. Its 3 dB FBW reached 125.3% and the minimum insertion loss in the passband was less than 0.42 dB. Filters made of high-temperature superconducting material film have small insertion loss, steep transition band and good out-of-band suppression characteristics, but the manufacturing process is complicated. In Ref. [16], a miniaturized on-chip bandpass filter based on GaAs was presented. By introducing coupling capacitor and series resonance circuits, transmission zeros were generated, and out-of-band rejection was improved. Its size was 0.8 \(\times\) 0.5 mm\({}^{2}\), and the relative bandwidth was 39.02%. In Ref. [17], a method of preparing lithium niobate single-crystal film on multilayer oxide film was proposed to obtain a bulk acoustic wave filter with oxide Bragg reflector. The filter had a broadband bandwidth of 10.3% at 3.128 GHz, but its size was reduced to only 0.6 \(\times\) 0.4 mm\({}^{2}\), which was expected to be applied in 5G systems. In addition to the new materials mentioned above, graphene is also a material for designing novel filters. By adjusting the structure of graphene layer, its chemical potential and other characteristics are changed, so that the center frequency of filters and antennas is reconfigurable [18]. At present, existing graphene-based filters and antennas have been designed and applied in conventional ultra-wideband and terahertz frequency bands [19], and they also have broad design and application prospects in the future. The technology of multilayer printed circuit board (MPCB) can also be used to realize compact multilayer microwave circuits, which is a low-cost method based on dielectric materials with relatively low dielectric constant and good via support. Compared with LTCC, LCP and other new material technologies, devices made of PCB can be much more easily integrated with other electronic devices and packaged into existing microwave systems in chips. At the same time, the process of multilayer PCB can also enable filter design of multilayer structures in three-dimensional space, which meets the requirements of miniaturization and structural diversification of new filters. Some works in the literature reported the design of UWB bandpass filters based on MPCB. In [20], a dual-band bandpass filter with two transmission paths was designed by using substrate integrated coaxial lines and a reusable semi-open transmission line. The filter adopted a design of vias and blind holes on three layers of RO4350 substrate and achieved a relative bandwidth of 58%. In [21], [PERSON] et al. proposed a filter with complementary split-ring resonators that was based on the folded substrate integrated waveguide structure. The filter used two layers of Rogers5880 dielectric substrates (Chandler, AZ, USA), with a relative bandwidth of 35.5% and good stopband characteristics. In [22], a packaged bandpass filter based on MPCB was proposed by the [PERSON] team, which was suitable for 5G millimeter wave applications. The filter had a fractional bandwidth of 27.9% in the band of 28 GHz, and its out-of-band harmonic suppression level was higher than 24 dB up to 110 GHz.