Design and implementation of magnetically–tunable quad–band filter utilizing split–ring resonators at microwave frequencies

In this article, we present a magnetically–tunable quad–band filter with high tunability in the frequency range of 2.1–3.9 GHz. A multi–band filter with four stop–bands comprises of a microstrip line coupled to four frequency–selective split–ring resonators (SRRs). We achieve tuning of individual frequency bands using magnetic reed switches connected in between the capacitive gaps of each split–ring resonator. Application of magnetic field tunes this capacitance affecting its resonance frequency. The measured reflection spectrum of the proposed device matches well with the simulation results. The results show more than 25% tunability for each of the four bands with bandwidth values in the range of 30–70 MHz with over 100% overall tunability in the 2.1–3.9 GHz frequency spectrum.