G. S. Nikonova, S. A. Doberstein, B. A. Arzhanov

The paper deals with the basic tendencies in the developments of advanced low-noise single-mode (single-frequency) SAW oscillators. It is noted that special topology SAW delay lines (DL) used in these oscillators for providing a single-frequency mode are inefficient due to increased insertion losses. Acoustoelectronic low-loss filters (ring structures and double-conversion resonator filters) are suggested for use in state-of-the-art SAW oscillators allowing low- noise single-mode oscillators to be realized. Topological structures and measured frequency responses of 250 MHz SAW ring and resonator filters on LiNbO3 and LiTaO3 substrates developed for oscillators are presented. The designed ring filters do not require matching elements as they provide specified active impedances 50-200 Ohm within the passband by self-matching when IDT static capacitance is compensated by SAW radiation susceptance. The filters provide low insertion losses (about 1 dB) and phase shift ±180º within a fractional bandwidth of 1.5-5 %. The filter phase response is close to a linear one within the passband with a slope of about 90°/MHz. The designed double-conversion resonator structure filters also due to self-matching do not require matching element and within 75-100 Ohm paths provide the low insertion loss 1-2 dB and the phase shift ± 180º within a fractional bandwidth of 2-6 %. The filter phase response is close to a linear one within the passband with a slope of about 35°/MHz. The paper gives the basic recommendations on the expediency of using these two filter types in SAW oscillators of different frequency ranges from hundreds of megahertz to units of gigahertz. Conditions for providing the single-frequency mode in oscillators with such acoustoelectronic filters are considered, recommendations on the theoretical analysis of SAW structures and SAW oscillators using S-parameters are given. Characteristics achieved by state-of-the-art SAW oscillators from world leaders are analyzed. It is noted that designs suggested by the authors of this paper are not inferior to foreign analogues.