Corrugated Microstrip Coupled Lines for Constant Absolute Bandwidth Tunable Filters

This paper presents corrugated coupled lines for miniaturized fixed and tunable microstrip bandpass filters. The novel approach uses microstrip corrugated coupled-line concept to synthesize a coupling coefficient, which maintains a nearly constant absolute bandwidth across the tuning range. A miniaturized two-pole varactor tuned filter is demonstrated with a frequency coverage of 1.32-1.89 GHz and an insertion loss < 3 dB with a constant 1-dB bandwidth of 70 ± 4 MHz across the tuning range. In addition, a three-pole comb-line 4.7% fixed filter at 1.94 GHz shows a 3:1 resonator spacing reduction over the conventional approach with an insertion loss of only 1.1 dB. This technique will allow the design of miniaturized small bandwidth fixed and tunable microstrip filters.