Investigations on wideband microstrip bandpass filters using defected ground structures (DGS)

In the front-end trans-receiver of a communication system, a microwave filter is required to suppress some unwanted frequencies while pass some of the desired frequencies. One of the filter types is a bandpass filter. There are numerous types of bandpass filter design methods. One of the bandpass filters is a microstrip based parallel coupled line bandpass filter, which offers reasonable matching bandwidth.;This thesis presents the investigations results of the wideband microstrip bandpass filters using defected ground structures (DGS). The advantage of DGS is that it improves the matching bandwidth performance of the filter without adding any extended length to the filter design. The proposed bandpass filter using DGS on low loss Roger's substrate (epsilonr = 2.2, tan delta = 0.0004) is excited using two edge launchers 50 O¸ coaxial probes. The reference parallel coupled bandpass filter shows a fractional bandwidth of 62% (S11 ≤ -10 dB) with the passband frequency range of 2.9 GHz to 5.6 GHz and insertion loss better than -0.5 dB. Various shapes of defected ground plane structures were investigated and their effect on the filter performance was recorded. In comparison to the reference filter, implementation of DGS with the filter offers an improved passband frequency range of 2.65 GHz to 5.75 GHz and insertion loss and reflection coefficient magnitude better than -0.3 dB and -15dB, which accounts to a fractional bandwidth of 74%. The effect of employing electromagnetic bandgap (EBG) structures with the defected ground plane based filter was also studied, which shows a fractional bandwidth of 80%, the insertion loss and reflection coefficient magnitude are better than -0.3 dB and -15 dB, respectively. So, clearly the performance of the filter is significantly improved by employing the DGS and EBG structures.;This filter was fabricated on a low cost FR-4 substrate (epsilon r = 4.5, tan delta = 0.04) while physical dimensions of the filter remained unchanged. The prototype filter's experimental verification was performed using a vector network analyzer. This DGS filter on FR-4 substrate exhibits an insertion loss, S21 more than -3 dB, and reflection coefficient magnitude, S11 less than -10 dB with the center frequency of the proposed filter 2.4 GHz and thereby an operating bandwidth of 64%. For comparison, the operational bandwidth of the bandpass filter without DGS is only 44%.