RF-design of narrowband absorptive bandstop filters for UHF applications

This paper addresses the RF-design of absorptive bandstop filters (ABSFs) that feature narrow fractional bandwidth (FBW) and quality factors (Qs) of the order of a thousand. The aforementioned functionality is realized by means of a mixed implementation scheme in which one-port acoustic wave (AW) resonators and lumped-element impedance inverters are effectively combined in a compact geometry. The non-ideal effects-fabrication tolerances-of the utilized lumped-element components on the ABSF performance-theoretically infinite isolation and input reflection-are analyzed. A method to maintain the large stopband attenuation by adjusting the effective Q of each resonator with the aid of a tunable resistor is also reported. A filter prototype that makes use of commercially available surface acoustic wave (SAW) resonators and surface-mounted devices (SMDs) has been built and tested at 418 MHz for verification purposes. It exhibits a 0.02% 3-dB FBW and a maximum stopband isolation of 62 dB.