Synthesis and investigation of novel filter topologies and application to space systems

Resumen

Resumen de la tesis: The development of accurate algorithms to design microwave filters with transmission zeros located at precise frequencies, is important to satisfy the increasing requirements of the current communication systems for space applications. This is one of the important steps before the physical implementation of the filter. However, the whole process to implement a specific filter is more complicated. In general, we will have to go through several stages. This research work is concerned with the different aspects of this process, providing significant new insight in fundamental properties of passive filters. The following stages can be distinguished: ¿ First of all, a specific technology must be chosen according to the appropriate characteristics of a particular application. The technologies readily available include waveguide, dielectric resonators, evanescent-mode filters, coaxial lines and printed filters in microstrip, among others. ¿ Second, we have to fix a suitable coupling configuration that our technology can easily implement. Among the different suitable coupling configurations, one of them must be chosen. This selection can be made according to different criteria, such us the easiest topology to be implemented by the technology, the least sensitive scheme, the topology with best handling capabilities, or any combination of these factors. The evaluation of the different topologies in terms of sensitivity and power handling capabilities, is one of the key aspects investigated in this thesis. On one hand, a close relationship between time average stored energy and sensitivity in microwave filter networks has been highlighted for the first time. Besides, a new approach for the sensitivity analysis of microwave filters, overcoming the limitations of the standard method of sensitivity calculation, is presented. It has been shown that, when the sensitivity calculation is expanded to also include sensitivities of detuned filters, accurate tolerance predictions can be made even for large geometry variations. The new sensitivity has been called Expanded Sensitivity. Besides, it has been demonstrated for the first time that, for in-line topologies, sensitivity can be predicted directly from the group delay. Thanks to the new techniques developed, it is possible to compare different filter topologies as well as different transfer functions in terms of their sensitivity, in order to select the best among the different available options for filter implementation. The usefulness of the results obtained has been proved by obtaining the maximum degradation of the in-band performance for different filter examples. Once the filter topology has been decided, we need to find the associated coupling matrix to represent the required amount of coupling between the different parts of the filter (input/ output ports, resonators and non-resonating nodes). This is usually done with analytical techniques able to synthesize specific frequency responses. To carry out this stage, the N + 2 coupling matrix can be used. A review of the different lossless and lossy synthesis techniques to obtain the N + 2 coupling matrix is included in this thesis. Besides, the possibility to synthesize filters with a prescribed insertion loss value including the effect of having a finite Q value at the same time, has been proposed for the first time. These filters are called Filters with Prescribed Insertion Loss and Finite Q Response. They may be useful when there are stringent flatness requirements in the transmission response of the filter in order to relax the required QR in the resonators, maintaining at the same time a high flatness in the passband. The possibility of reconfiguring the transversal lossy coupling matrix into more adequate topologies has also been described in this thesis. ¿ Finally, in order to obtain the final filter in the desired technology, we will have to obtain the appropriate dimensions of the geometry, starting with the obtained coupling matrix. This last step depends on the technology, as we have to find the relationship between the coupling terms of the matrix and the real dimensions of the structure. Related to this stage of the design process, novel implementations of microwave filter structures have been presented, giving to the second part of this PhD a more practical aspect. First of all, a novel hybrid structure has been proposed for the implementation of high selective microwave filters. This novel structure is based on the combination of two well-known technologies: the microstrip and the waveguide, providing both single and dual-bandpass operation. Different designs have been illustrated in order to show the validity of the proposal. Furthermore, three of the designs have been manufactured and tested, showing good agreement with respect to the predicted results. A fully canonical transversal filter of order three has been for the first time implemented using the new hybrid structure. Second, novel topologies for implementing bandpass elliptic filters employing structures composed of inductive windows and dielectric objects have been proposed. A variety of design examples have been included in order to show the different possibilities offered by the novel structures. A prototype has been manufactured and tested with very good results. Third, novel broadside coupled filter structures employing both resonating and non resonating nodes (NRNs) have been introduced. A new filter topology has been introduced by combining for the first time a trisection block with a NRN to implement two transmission zeros on both sides of the passband. The usefulness of the new broadside coupled structures has been confirmed by means of several practical design examples, obtaining good results. Finally, several second and third-order novel microstrip bandpass filters have been designed, manufactured, and tested. The different included examples show the flexibility of the proposed structures going from simple second order transversal topologies to more complex structures. The results of the research work are expected to be useful for the development of more compact filtering structures, with smaller mass and volume but capable of operating under high power conditions and reduced sensitivity as needed for space applications.