Novel synthesis techniques for compact inline filters with transmission zeros
Modern communications systems are more and more requiring filters with high selectivity, low losses and small size. To cope with such contrasting requirements, use of transmission zeros in the filter characteristic is becoming unavoidable. Unfortunately, the introduction of transmission zeros through classical topological schemes based on crossed- coupling configurations, makes the implementing structures bulky and more difficult to realize with respect the simple in-line configuration used for all-pole filters. For this reason, in the last years several solutions have been proposed in the literature for realizing filters with a basic in-line topology but allowing the introduction of transmission zeros. In this talk we recall the most recent solutions (based on extracted-pole synthesis or use of frequency-dependent couplings). We also describe a novel synthesis technique for filters using the so called “Strongly-Coupled Resonators Pair” (SCRP) which provides both one equivalent resonator as well a direct path between the previous and following resonators. In this way, we may introduce a transmission zero in the filter characteristic. Exploiting the equivalence of a SCRP with a triplet topological block, an analytical procedure has been developed for synthesizing filters using SCRPs. After a detailed description of this procedure, we will show some examples of synthesized filters and discuss their practical implementation in coaxial technology.