Quasi-elliptic Microwave Filter Design Using the Reflected Group Delay Method

Abstract

Microwave filters are essential components in a microwave system, as they define the usable and allowable bandwidth and as they reject unwanted signals from passing through them. The design and required optimization of microwave filters is an involved process and getting the desired filter response can be difficult, especially for filters with a higher order and those with narrow bandwidths. In most instances, a fabricated microwave filter will need to be tuned to get the desired response, so it can be very important to achieve accurate simulation results to limit the required post-fabrication tuning. As the electromagnetic simulations can take significant computation time when optimizing the physical dimensions of the filter, improvements in the design and optimization process are always being sought. In this thesis, the group delay method of designing filters is investigated and for the first time it is implemented in designing non-Chebyshev sequentially coupled filters and quasi-elliptic filters requiring cross coupling. The author first becomes familiar with the group delay method by designing simple lumped element filters. The group delay method is then used to design a filter with a Butterworth response, which is a non- Chebyshev sequentially coupled filter A design method is then developed for using the group delay method for designing quasi-elliptic filters that require cross coupling. The design process involves creating optimization curves from a lumped element filter and using these curves to design a microstrip filter using a circuit simulator. The design steps are documented as the proposed design method is followed. The limits of the proposed design method are investigated by determining the maximum absolute bandwidth a filter can have using this method. To validate the proposed design method, two microstrip filters are designed, fabricated and tested, one filter with higher losses and one filter with lower losses. The filters are fabricated using standard printed circuit board technologies. The simulation and test results are similar with differences occurring due a solder mask layer used in the fabrication process. Simulations are performed to account for the effects of the solder mask and recommendations are made to further characterize the solder mask properties.