A Varactor-Based Tunable Combline Bandpass Filter Using Suspended Integrated Stripline (SISL)

In this article, a varactor-based tunable combline suspended integrated stripline (SISL) bandpass filter with a tuning range of 2.3-3.9 GHz is designed and built. The filter is realized with five different substrate layers to implement an air cavity around the filter layer, creating the suspended stripline. Surface mount varactors are soldered onto the filter to allow for frequency tuning by reverse biasing the varactors at different dc voltages. The filter design concept is explained, detailed models are provided, and simulated results are given. The fabricated filter is shown and the measured results are compared with the simulated results and analyzed. The resulting filter demonstrates a tunable S-band filter implemented using SISL to allow for integration within a substrate.