Five-pole parallel-coupled microstrip cascade quadruplet filters for high selectivity and flat group delay

The design of five-pole cascade quadruplet microstrip filters is presented. Based on a conventional parallel-coupled structure, good initial physical dimensions can be obtained by the well-known analytical method. Applying source and load to couple to the third resonator two cascaded quadruplets are formed. Two pairs of transmission zeros can be independently located on either real or imaginary frequency axes. Two filters are introduced in this paper that one have two pairs of real frequency transmission zeros for selectivity, and the other have one pair of real frequency transmission zeros for selectivity and another pair of imaginary frequency transmission zeros for group delay equalization. The flat group delay response is particularly sensitive to the unwanted cross coupling. The proposed structure solves this problem and shows a very flat group delay in the passband. The simulated and measured results of the two designed microstrip filters are well-matched.