Adaptive Bandpass Filter for Microwave Frequency Range

Abstract

The paper is dedicated to development and investigation of adaptive filters at the microwave frequency range. Two cases are considered that include static and dynamic control of bandwidth. The requirements for accuracy and efficiency of operation of a signal transfer system are growing and require new approaches to filter design which can be adapted to the environment impact and operating conditions. The ring resonators were used in the paper for this aim. The ring resonators are the class of structures that involve a planar design of metallic elements with different shapes over a dielectric substrate. These resonators are one of the most prospective elements for the microwave filters implementation due to their simple design and a possibility for their precise adjustment. The ring resonator operates as an ordinary resonance oscillating LC-circuit in the microwave representation. The considered filters have a unique feature for control the resonance characteristics: static and dynamics. Static control can be implemented with variation of geometrical parameters of the ring resonators. Thus, the appropriate experimental investigations were performed for changing the distance between the filter ring resonators in the range h = [0.37, 0.51, 0.66, 0.81, 0.96] mm. It allowed to achieve the bandwidth variation from 250 to 60 MHz over the threshold of 0.707. This approach is applicable to a filter adjustment for the defined frequency range at the constructive and manufacturing stage. Dynamic control can be achieved by a varactor diode utilizing. It should be inserted into the additional gap of the ring resonator. A varactor diode is a semiconductor, its capacitance can be controlled by bias voltage. Therefore, the presence of the varactor diode in the structure of the rig resonator allows changing the total capacitance of the ring resonator. In the experimental investigation it was shown that, the bandwidth variation up to 140 MHz was achieved with the bias voltage changing from 1 to 7 V. The dynamic control of the filter characteristics makes the device more flexible and allows fast adaptation of its frequency characteristics to the operation conditions without any hardware rebuilding. Additionally, the principle of the suggested filter functionality from the point of view of electromagnetic processes, taking place in the zone of reactive near-field interaction is considered in the paper. The reactive near-filed zone was under study that allows clear consideration of surface current distribution. This enables to demonstrate how the filter operates within and beyond the operational frequency range. This knowledge ensures more precise modelling of the filter functionality in the real conditions and provide the optimal characteristics during operation.