Study And Design Of Microwave Filter Based On Defected Ground Structure

The defected ground structure(DGS)is a specific slot etched on the surface of the transmission lines like microstrip Line and coplanar waveguide,and then changes the distribution inductance and capacitance of the transmission lines.Filters based on DGS can be designed in simple way to realize miniaturization of devices and high suppression in stop-band,and have attracted the attention of many scholars at home and abroad.Traditional dumbbell DGS low-pass filter performs well in pass-band,but the rectangular coefficient of insertion loss near cutoff frequency is not ideal,and stop–band is quite narrow.In this paper,dumbbell DGS and its equivalent circuits have been analyzed first.Based on the characteristics of SRR DGS,a cascaded SRR DGS filter is designed to increase the equivalent capacitance of DGS and obtain a steeper filtering characteristic.The measured data shows that the cutoff frequency of this DGS low-pass filter is 5.39 GHz,and the insertion loss in low-pass band is less than 0.6Db.The band gap is about 600 MHz and bandwidth of-20 dB suppression is 6.7GHz.In band-pass filter,the loaded DGS structure can provide frequency adjustment and coupling compensation.Firstly,a five-order open loop band-pass filter is designed according to the performance targets,and the performance in changed size is analyzed later.With corresponding DGSs etched below the open ring resonators,and the upper /lower cutoff frequency of the filter move to the low frequency around 200 MHz,and the bandwidth is widened to 500 MHz.The measured results show that the transmission characteristics of this DGS band-pass filter and the open loop filter in former size are basically identical.Finally,the bending characteristics of the flexible filter are simulated and analyzed based on cascaded DGS filter structure.The bend and plane transmission characteristic of the DGS low-pass filter has been measured and compared with the simulation results.The bending of filter has a little influence on the cutoff frequency and band suppression,and the frequency bias is around 25 MHz.