Studies And Design Of Microstrip UWB Bandpass Filter

With the increasing of new communications systems, the limited spectrum resources bring more conflicts. Not only the utilization of the original frequency needs to be improved, new technology with higher capacity and faster rates is also needed. Ultra-wideband technology with unique excellent performance is placed high hope by all sectors, and has become the most potential and fastest growing communication system in the past decade. As one of the front end components of the UWB system, UWB filters also draw much attract, whose performance will have a significant direct impact on the entire system.This paper focuses on the research and design of ultra-wideband microstrip filters. Multi-mode resonator and artificial materials have been applied to two miniaturized ultra-wideband microstrip filters, respectively. First, a four-mode resonator with a third-order SIR loaded short circuit stub is designed, in order to increase the degree of freedom to adjust the resonant frequency, four short open stubs are loaded symmetrically. Considering that the UWB filters require large external coupling, input-output coupling structure uses interdigital line, and rectangular DGS structure is etched on the ground. Specific physical dimensions are obtained by simulation and optimization. Finally, a wideband bandpass filter with 110%(3.0 to 10.6GHz) relative bandwidth is designed, fabricated, and measured. The measured results are in accordance with the simulated ones. The insertion loss is 0.6dB, and stopband rejection has reached 15 d B from12 to 25 GHz. As indoor ultra-wideband frequency band contains the working frequency band of WLAN and satellite communications, it is necessary to design ultra-wideband filters with notched bands. A notched band of 5.2GHz is designed by asymmetric coupled line, and two notched bands are by loaded resonator. The second filter is designed based on composite right/left handed transmission line(CRLH-TL), then,a low-pass filter based on defected ground structure is cascaded. The left-hand series capacitor is realized by the interdigital capacitor structure, and shunt inductor by the grounded microstrip lines. Back-to-back complementary split ring resonator is used to produce low-pass characteristic. The entire filter is just 18 mm × 9.8mm, with good out-band characteristics.