A High-Power Low-Loss Continuously Tunable Bandpass Filter With Transversely Biased Ferrite-Loaded Coaxial Resonators

This paper presents a technology for high-power low-loss continuously tunable RF filters demonstrated by the example of a two-pole coupled-resonator filter. The resonators are shortened coaxial cavities loaded with ferrite inserts, where an externally applied transverse dc magnetic bias controls the center frequency. The filter is analyzed by simulations that take into account the inhomogeneity of the bias field. A prototype is manufactured and tested, exhibiting a tuning span of 41% in which the unloaded Q factor varies in the range from 1223 to 2425. Good correspondance is observed between simulation and measurement results. Furthermore, the power-handling capability of the filter is discussed and the ionization breakdown limit under normal conditions is estimated to be 800 W. The filter's linearity is characterized by a standard two-tone test, where the level of the third-order passive intermodulation product is observed to be -53.1 dBm at an input fundamental tone level of 2 × 43 dBm.