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We investigate the use of laterally-coupled High Overtone Bulk Acoustic Resonators (HBAR) based on AlN thin films sputtered on silicon and sapphire substrates in order to target an operation frequency between 2 and 5 GHz. Being tightly coupled, they generate an extremely narrow bandwidth, assimilated to a single resonance whose unloaded quality factors reach respectively 3,300 and 2,000 at 2.5 and 4.4 GHz on silicon substrates and 25,000 at both frequencies on sapphire substrates. The later lead to Q.f factors close to 1.1 1014 Hz, reaching after twenty years the record established by Kline and Lakin . More interestingly, for filters fabricated on Sapphire substrates, quality factors do not scale with frequency, indicating that viscoelastic losses in Sapphire are not yet the most limiting factor. Indeed, we demonstrate that a compromise needs to be drawn between insertion loss and quality factor in such structures.