Transversal Coupled Triple-Mode Spherical Resonator-Based Bandpass Filters

In this letter, we present an innovative filter configuration employing transversal coupled triple-mode spherical resonators while capable of realizing transmission zeros (TZs). Transversal coupling involves simultaneous source and load-controlled couplings to all the three modes of the spherical resonator. Spherical resonators inherently have very large quality factor (Qu) and exhibit a farther spurious response thus yielding bandpass filters (BPFs) with lower insertion loss and wider spurious-free window. The design methodology for transversal coupling is explained and is employed for designing a third-order bandpass filter with TZs. In addition, a sixth-order bandpass filter using cascaded trisection is also presented. A third-order BPF with symmetric TZs is fabricated for verifying proof of the concept. The filter designed at 6.545 GHz with a bandwidth of 120 MHz has an insertion loss better than 0.1 dB and a spurious-free window better than 1.7 GHz. The proposed filter promises to be useful in aerospace applications that demand high Qu, low volume, and wider spurious-free window.