Design of microstrip quadruplet filters with source-load coupling

Quadruplet microstrip filters with source-load coupling are proposed to achieve similar skirt selectivity and/or in-band flat group delay as that of a sixth-order canonical form or an extracted pole microstrip filter. The diagnosis method of unwanted effects such as asynchronous resonant frequencies and unwanted couplings, which often occurs in the microstrip's open environment, is described in detail. A systematic design flow to implement a quadruplet microstrip source-load coupled filter with proper filter response is also provided. Two trial filters exhibited quasi-elliptical and flat group-delay response are designed and fabricated. Both theoretical and experimental results are presented.