The working frequencies of telecommunication systems is rapidly increasing and sub-THz applications will soon be required. In that context, the current article presents the synthesis of a tunable ring filter. The considered filter introduces a new degree of freedom that allows to meet targeted specifications in terms of central frequency and bandwidth. Thanks to a capacitor displacement, the equivalent characteristic impedance can be tuned independently of its equivalent electrical length. The existing filter synthesis is modified to take this concept into account and thus provide a synthesis tunable in central frequency and bandwidth. The proof-of-concept of the design procedure is achieved in SiGe BiCMOS 0.25µm technology with the implementation and measurement of 8 non-tunable circuits in the D-band.