Arbitrary Prescribed Flat Wideband Group Delay Absorptive Microstrip Bandpass Filters

This article demonstrates the design of microstrip line symmetrical all-band absorptive bandpass filters (BPFs) with arbitrarily prescribed wideband flat group delay (GD) characteristics. The proposed BPFs consist of coupled line sections with series resistor connected stubs (short-circuited quarter-wavelength or open-circuited half-wavelength stubs). Analytical design equations are derived to obtain circuit parameters for arbitrarily prescribed flat GD characteristics. A detailed analysis shows that the absorptive sections (series resistor connected stubs) not only eliminate out-of-band reflections, but also determine the passband GD flatness characteristics. The proposed absorptive BPFs with arbitrarily prescribed flat GD can be easily extended to a higher order by cascading the first-order symmetrical quasi-absorptive BPF. For experimental validation, first- and second-order quasi-absorptive BPFs were designed with arbitrarily prescribed GD of 1.75 and 3.40 ns, respectively, and measurements were taken at a center frequency of 3.5 GHz. The results of the measurement are in good agreement with the predictions obtained through theoretical methods and simulation.