Design of 5 GHz-Compact Reconfigurable DGS-Bandpass Filter Using Varactor-diode Device and Coupling Matrix Technique

— In this work, a novel electrical reconfigurable second order microstrip bandpass filter is presented. The investigated filter structure consists of two neighbour coupled octagonal-DGS resonators. The capacitor and varactor are placed on the DGS-shape. According to the tunable characteristic of discrete lumped capacitors and varactor diode, the resonant frequencies can be tunable. The proposed filter occupiers an area of 2.5 x 1.5 cm2. The coupling method has been applied and leads to an extended coupling matrix that assumes multiple couplings between both filter resonators. The measured relative tuning range of the bandpass filter is 23.5% with a fractional bandwidth of 12%. Varactors loaded on the octagonal DGS resonator allow us to control the center frequency with quasi constant bandwidth as well as to have a large tuning bandwidth. Based on the detailed analysis, the varactor-tuned DGS bandpass filter is designed, in which the tuning bandwidth reaches 12 % of the center frequency at 5 GHz. The proposed reconfigurable bandpass filter was fabricated and tested. The measured results show a variation of the center frequency from 4.3 GHz(0V)-to 5.4 GHz(15V) tunable range, while the insertion loss varies from 5 dB to 3 dB. The measured performance of the tunable bandpass filter agrees well with the simulation results.