Switched Allpass-to-Bandstop Absorptive Filters With Constant Group Delay

A new reconfigurable bandstop filter class is proposed, which has the unique property of possessing multiple insertion loss states with a common group delay state. These filters switch between a low-insertion-loss allpass state and a high-rejection bandstop state. The theory, synthesis, and design of these filters are presented. As a demonstration of the concept, a fourth-order constant-group-delay switched bandstop filter microstrip prototype was designed, built, and tested. The prototype has a measured bandstop-state center frequency of 1000 MHz, a 3-dB bandwidth of 119.15 MHz, a stopband rejection of 32.52 dB, a maximum allpass-state insertion loss of 4.08 dB, and a negligible group delay variation between the two states.