Theory and Design of Frequency-Tunable Absorptive Bandstop Filters

Absorptive bandstop filters are a relatively new class of bandstop filter, which are able to achieve very high levels of stopband rejection with relatively low-quality-factor resonators, in contrast to typical reflective bandstop filters, whose stopband rejections are limited by the quality factors of their resonators. This paper performs an in-depth theoretical and practical analysis of this class of filter, presenting design principles for reducing the sensitivity to process variation, increasing the tuning range over which the filters can operate with good performance, and addressing the practical non-ideal effects of implementation, such as frequency variation of couplings and quality factor. Four varactor-tuned microstrip bandstop filters are presented to verify the presented theory. They illustrate the design tradeoffs between selectivity and tuning range, choice of coupling topology and tuning range, and show the benefits and drawbacks of cascading stages to create higher-order filters.