This paper presents analysis and design of a compact ultra wide band (UWB) filter using three parallel folded coupled lines in a defected ground structure in multi-layer structure. Defected ground has been incorporated under the coupled lines of the filter to improve the coupling over a wide bandwidth. The closed form expressions for even and odd mode impedances for the coupled lines with defected ground have been obtained to design a filter of desired bandwidth with the proposed structure. Based on circuit models, an UWB filter for 3.1-10.6 GHz has been analyzed and the results have been compared using full wave simulations. Analytical results are satisfactorily matching with simulations. Filter exhibits a constant group delay of ±0.08 ns in the pass band. Size of the filter is 6.2 mm×3.4 mm×2.8 mm.
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