BST-Based Tunable Helical Filter Operating in the Higher Order Mode

this paper addresses the optimum design of tunable bandpass helical resonator filters to realize a constant absolute bandwidth over a relatively wide tuning range. The impact of the location of the tuning element on the input/output group delay is studied for both the dominant mode and the higher order mode as the frequency is varied. The paper also investigates the impact of the orientation of two adjacent helical resonators on the inter-resonator coupling. The paper demonstrates that a much wider tuning range can be achieved if operating at the higher order mode. Barium Strontium Titanate (BST) varactors are used for tuning. For experimental verification, a 3-pole filter operating at the higher-order mode was fabricated and tested demonstrating a constant bandwidth and low insertion loss variation over the tuning range.