In this dissertation, the synthesis of the triplet absorptive topology is presented in detail. The coupling matrix of this topology is derived. The synthesis theory extends to arbitrary phase of the transmission line used in the topology. A new FSL that yields the state-of-the-art performance is proposed. It employs the triplet absorptive filter topology, which enables absorptive response in a wider tuning range, to achieve high isolation (70 dB) everywhere in its octave tuning range. This was not possible with any existing FSLs. This triplet filter topology also gives bandstop-to-all-pass response, which enables controlled attenuation, or variable-attenuation control. The filter is implemented in high-Q evanescent-mode cavity technology, which yields low insertion loss in all-pass response. The proposed FSL is integrated with feedback control loops to enable in-field operations. For one step further, the FSL system is redesigned for optimization of robustness and reliability without compromising the state-of-the-art RF performance.