Tunable Filter Structure – Analysis and Design

A simple filter application of a new efficient hybrid method is presented in this paper. The method combines the discrete finite difference method in frequency domain (FDFD) with analytical mode matching (MM) technique and orthogonal expansion method. The procedure allows for the analysis of electromagnetic wave scattering by cylindrical objects with arbitrary cross-section located within the rectangular waveguide junction. The analyzed structures are the constructive elements of a direct-coupled resonators filter being under investigation. The size and location of the posts in a waveguide junction can be optimized to obtain the required frequency response characteristic of the filter. Moreover, an inhomogeneous shape of the investigated objects allows one to utilize them as tuning elements to correct any inaccuracies created during fabrication by means of their simple rotation. Filters are widely used components in modern microwave communication systems. Since the modern communication develop rapidly the spectrum is becoming more and more crowded, and the requirements for more sophisticated filters are increasing. Nowadays filters should be compact, have low loss, high selectivity, high power-handling, should be capable of being manufactured in a great amount at low cost. In literature one has met many different filtering structures such as filters employing direct-coupled microwave resonators of various shapes [1]-[4], combline filters or dualmode filters [5]-[7]. In order to meet the above mentioned requirements a high precision of manufacture is required. This on the other hand contradicts the low cost requirement. Here we propose the approach of utilizing the resonators with arbitrary cross-section to both design the filter and tune it after manufacturing in limited precision. The validity of the presented approach is verified via commercial software QuickWave 3D (FDTD) [8] and own measurements of the fabricated structure.