a design of the novel coupled-line bandpass filter using defected ...

CHAPTER 5CONCLUSION AND SUGGESTIONWe have introduced in this thesis the limitations of classical microstrip linebandpass filter based low pass and band pass filters and the way to improve the designand response by using the defected ground structures. New investigations arepresented the dumbbell shape for the DGS structure as an element for the BSF. Wehave also introduced the ultra-wideband band pass structure in DGS and used it forthe development of a new compact BPF.5.1 ConclusionA parallel coupled-line bandpass filter cascading with defected ground structureof microstrip line is introduced and demonstrated in this thesis. The conventionalparallel coupled-line bandpass filter is firstly designed and its response proposedunwanted passband. The cascading DGS with that parallel coupled-line BPFintroduces attenuation pole contributing the suppression of unwanted secondpassband. The wideband BPF is then fabricated and evaluated. The filterperformances are validated and exhibits |S 21 | more than -2 dB, |S 11 | less than -15 dB,center frequency of proposed filter is 4.1GHz with operating bandwidth 49% and cansuppress unwanted passband to below -15 dB.5.2 Problem and suggestion for future workThere are some residual errors between simulation and measurement due to fivereasons. First is the incorrect PCB engraving, second is the embedded parasitic effectof SMA connectors, third is error in fabrication process, fourth is changed ε r in highfrequency and the last problem come from low quality of FR4.The first, second and third problem can be overcome by considering otherprecise realization process and the parasitic effect of SMA connector is anotherembedded problem that can be de-embedded by using on wafer-measurement systemwhich is unaffordable for this study.The changed ε r can use piecewise simulation method (varies ε r by Figure 3-2)to solve it. By separate each simulation of range to 1 GHz for simulation and theresult show in Figure 5-1.