Tunable Combline Filter and Balun: Design, Simulation, and Test

Reconfigurable filters are an attractive solution for many military and commercial applications due to their ability to alter the partitioned frequency band in an RF system without requiring a bank of fixed filters. The onset of this technology has the potential to revolutionize the RF industry by allowing for agile devices which consume less size, weight, and power while providing greater performance. However, at the present state, reconfigurable filters present a reduction in performance when compared to filter banks. This has led to exciting research in the field of RF tunable filters. For many applications, planar reconfigurable filters have been utilized due to their low cost of manufacturing and ease of implementation in a system. One topology that has proven to be versatile in design is the combline filter which employs line resonators loaded with a capacitor to obtain a predetermined response. To implement a varying center frequency reconfigurable combline filter, the resonator is loaded with a capacitance that can be tuned either digitally or continuously by presenting a DC voltage. Due to their ease of use and availability, varactors are a common choice as they provide a continuously tunable capacitance by presenting a reverse bias voltage to the device. To continue the trend of lowering size, weight and power while maintaining high vitality in performance, consolidation of RF components may prove to be a good next step. Tunable balun filters have already been presented as a viable option for consolidation of components and show good performance. However, those designs which have been presented do not demonstrate a topology that can implement higher than a second order filter. This project, for the first time, investigates the consolidation of the Marchand balun and the combline filter into a single topology which allows for quick adaption of higher order filters while maintaining vigor in performance. A design is presented which achieves 25% tuning bandwidth centered at 1.5 GHz with less than 5 dB insertion loss, a phase balance of 180 ± 1° and an amplitude difference of ± 0.6 dB