Varactor-Based Tunable Planar Filters and Post-Fabrication Tuning of Microwave Filters
Rezazadeh Sereshkeh, Alborz
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Post-fabrication tuning of filters is usually realized by adding number of elements for tuning the frequency and/or controlling the couplings between the resonators. The task of these tuning elements is to control resonators center frequency, inter-resonators coupling and input/output couplings. While the most common tool for the post-fabrication tuning is to use tuning screws and rods, it is not usually practical to tune a planar filter with these tools. This thesis introduces a novel method for global post-fabrication tuning of microwave filters by designing and adding a passive distributed-element circuit in parallel to the detuned filter. The idea, which is demonstrated by experimental results, has several advantages over traditional techniques for filter tuning that use screws. The quality factor of resonator reduces significantly after adding the tuning screws while the proposed method does not affect the Q of resonators. The most important advantage of the proposed compensator circuit is that it can be employed without knowing details of the detuned filters. Since the compensator circuit will be added in parallel to the detuned filter, it will not affect the elements of filter individually. So whether the filter is planar or cavity, the proposed circuit can be used for the tuning. The experimental results obtained demonstrate the validity of this method. The dissertation also presents a novel concept for designing a center frequency and bandwidth tunable microstrip filter by using GaAs varactors. The proposed isolated coupling structure which is used in this filter makes the bandwidth tuning possible by reducing the loading effect of coupling elements on the resonators. The center frequency of this filter can be also tuned by using a different set of varactors connected to resonators. A 3-pole filter based on this concept has been designed and simulated. The concept can be expanded to higher order filters.