Design Of Coplanar Waveguide Bandpass Filters Based On Defected Ground Structure

With the development of modern communication,more and more radio frequency devices work in two or even three frequency bands,such as work in 5G band,wire local area network(WLAN),wordwide interoperability for microwave access(Wi MAX)and microwave relay systems.As one of the main components,filters have wide range of functions in the radio frequency front end.Therefore,it is significant to design a multiband bandpass filter with compact size,excellent performance,and low cost.In recently,coplanar waveguide(CPW)has been extensively studied for the design of miniaturized circuits.The design methods of CPW bandpass filters with spiral-shaped defected ground structure(DGS)are focused in the thesis,and a single-band,multi-band,and a passband tunable filters are designed.The main work is as follows:Firstly,A CPW bandpass filter based on spiral-shaped DGS is proposed.Two pairs of spiral-shaped DGSs are loaded on the two ground planes of a CPW main transmission line.A cross-shaped notch is added in the central strip of the CPW main transmission line to generate the passband,while two m-shaped DGSs are brought in to improve the passband performance of the filter.The measured central frequency is 3.54 GHz,with the10-d B fractional bandwidths of 10.1%.Then,in order to meet the development needs of modern communication for radio frequency devices to work in two or three frequency bands at the same time,three types of multi-band bandpass filters are designed in this thesis.The first one is CPW dual-band bandpass filter based on asymmetric spiral-shaped DGSs.Two pairs of spiral-shaped DGSs with different sizes were loaded on the two ground planes of a CPW main transmission line,and the gap was adopted in the central strip of the CPW main transmission line to form the two passbands.The measurements show that the central frequencies located at 3.35 GHz and 5.02 GHz,with the 10-d B fractional bandwidths of25.6% and 2.0%.The second CPW dual-band bandpass filter is based on symmetric spiral-shaped DGSs.The CPW single-band bandpass filter with symmetric spiral-shaped DGSs is designed to achieve the first passband performance,and then the L-shaped slots are loaded in the central strip of the CPW main transmission line to achieve the second passband performance.The measured central frequencies are 3.59 GHz and 5.87 GHz,with the 10-d B fractional bandwidths of 5.8% and 5.4%.The third CPW tri-band bandpass filter is also based on CPW single-band bandpass filter with symmetric spiralshaped DGSs.Additional L-shaped slots with different lengths are added to form the third passband.The measured central frequencies are 3.45 GHz,5.24 GHz and 7.52 GHz,with the 10-d B fractional bandwidths of 9.5%,3.0% and 1.3%.Finally,a tunable filter with switchable passband is based on the first single-band bandpass filter with spiral-shaped DGSs and m-shaped DGSs is proposed.A PIN diode is placed on the cross-shaped notch.By controling the on and off state of PIN diode,the filter can be converted from single-band bandpass to dual-band bandpass.