Direct-coupled resonator filters based on foreshortened coaxial transmission line resonators

In this paper we present certain canonical filter structures that appear to have previously been overlooked. We systematically discuss direct-coupled series- and shunt-resonator-based coaxial filter topologies and elucidate the progression from lumped elements to foreshortened transmission-line resonators with their respective advantages and disadvantages. This finally leads to an unconventional direct-coupled foreshortened seriesresonator design. This design may also be interpreted as the series-coupled foreshortened series-resonator "dual" equivalent to the comb-line filter (which is a parallel-coupled foreshortened shunt-resonator filter). Consequently, both topologies exhibit similar advantages, such as compact size, low loss and higher passband suppression. In addition, aspects of practical design of coaxial filters are covered in this paper. We provide the impedance and admittance slopes for A/8 resonators, which turn out surprisingly simple. We propose a novel variant of impedance inverter, which allows for a large range of coupling coefficients in series-resonator filters. This enables narrow as well as medium-to-large bandwidth filters and additionally offers freedom to choose parameters for simple and low-cost implementation. Based on this concept, an optimized high-power bandpass filter for the 900 MHz mobile telephone downlink bands is presented, which can be fabricated at low cost.