Frequency-Agile Coplanar-Waveguide-Fed Miniaturized Helical Resonator Filters

Abstract

A tunable helical resonator filter utilizing varactor diodes is presented for the first time. Coplanar waveguides are employed as a transition from the ports to the tapped connection, to the helical resonators. This novel configuration not only provides external coupling to the resonators but also allows for two transmission zeros (TZs), one on each side of the passband. A helical resonator with variable capacitors was fabricated and characterized as a building block for the proposed filter. The tunable helical resonator yields a measured unloaded quality factor (Q) of 192–227 with a variable center frequency of 150–230 MHz. Also, a 4% second-order bandpass filter was designed, fabricated, and tested to demonstrate the concept. The measured second-order bandpass filter achieves a variable center frequency of 170–230 MHz with an insertion loss of 0.9–1.3 dB.