Research Of Microstrip Filter Based On DGS

Defected ground structure(DGS) is a novel microwave circuit which has some "defects" etched on the metal ground plane. The DGS changes the current distribution on the ground and results in that certain frequencies of electromagnetic waves can not pass through the circuit. The outside-band performance of the filter can be improved by using DGS. DGS can also reduce the size of microwave circuit.The DGS adopted in the traditional filter and DGS directly designed as the low-pass filter in the cascaded system are two main ways of DGS application in the microwave filter.In this thesis, a three–unit cascaded low-pass filter is designed with dumbbell–shaped DGS. The gap position of DGS of low pass filter is slightly adjusted and the structure is used in the traditional parallel coupling band–pass filter design. The performance of parallel coupling band–pass filter is improved. The second harmonic, third harmonic and fourth harmonic in out–band are rejected effectively. The above parallel coupling band–pass filter is processed and measured. The center frequency of band-pass filter is 2.35 GHz. The return loss in center frequency is 2d B lower than the simulation result and S21 is below-40 dB from 3.2GHz to 10 GHz. In this regime the simulation result is about-25 dB. The out–band rejection in filter is improved notably.The method for design low-pass filter with DGS structure is simple and the performance of stop band of the filter is quite good, but the roll-off of the filter is not satisfied. A new DGS structure combining with the advantage of semi–circle DGS and split–ring DGS is presented in this thesis. Then the two and three of such DGS units are cascaded to compose the low-pass filters. The filters are processed and tested. The results show that the roll-off of the new DGS low pass filter is improved compared with the traditional semi–circle DGS low-pass filter. The new DGS low-pass filters with exellent roll-off feature can be designed conveniently.