I. Introduction
High-overtone Bulk Acoustic Resonator (HBAR) is a prospective acoustoelectronic device of sandwich piezoelectric structure operating up to microwave frequencies. A number of substrate materials as fused quartz and silicon [1], sapphire [1], [2], YAG [3], and diamond [4] have been already applied. HBAR based on synthetic diamond substrate has showed the excellent UHF resonant properties [4]. But one can observe the appearance of the additional resonant peaks which depends on the structure, dimensions and topology of thin film piezoelectric transducer (TFPT) as well as a diamond substrate thickness. As a result a deterioration of HBAR's fundamental resonant properties nearby the overtone's frequency takes place. The nature of such undesirable effect should be investigated for a more detail in order to design an improved HBAR at a given frequency band. Recall that such effect has been earlier observed for HBAR incorporating other crystalline substrates [2], and especially for FBAR and SMR [5]–[8].