Synthesis and design of tunable bandpass filters with constant absolute bandwidth using varactor-loaded microstrip resonator

Abstract

This article presents two new types of tunable filters with constant absolute bandwidth using varactor-loaded microstrip resonators. First, the second- and third-order Butterworth tunable filters are designed based on the parallel coupled-line J inverters. Second, a fourth-order Chebyshev tunable filter is designed based on the alternative J/K inverters, in this design, two adjacent resonators are coupled with each other through a short-circuited transmission line as the K inverter. The proposed two topologies can be easily extended to high-order tunable filter. Three tunable bandpass filters with J and alternative J/K inverters, respectively, are built with a tuning range from ~1.8 to ~2.3 GHz. The measured second-order filter has a 3-dB bandwidth of 160±6 MHz and an insertion loss of 2.4-3.8 dB. The third-order filter shows a 3-dB bandwidth of 197±5 MHz and an insertion loss of 3.8-4.8 dB. The fourth-order filter shows a 3-dB bandwidth of 440±5 MHz and an insertion loss of 2.1-2.6 dB. For all the designed filters, the measured results are found in excellent agreement with the predicted and simulated results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:681-689, 2014.