The main characteristics and advantages of the surface acoustic wave (SAW) distributed feedback resonator are discussed. A coupled mode analysis provides physical insight and simple formulas for the resonant frequency, the quality factor, and the input imped- ance. Those results are verified by means of a transmission lie com- puter model and by a number of measurements in the frequency range 30-250 MHz. On YZ LiNbO3 substrates, quality factors of the order of 5000 are routinely obtained. ST quartz), the reflector can be made out of shallow grooves, one or two percent of a wavelength deep. The fingers of the coupling transducer can be located in between the grooves (Fig. l(b)) or can lie in the grooves of the reflector. Strongly piezoelectric substrates, such as YZ lithium niobate, allow for an easier approach: the resonating transducer in which the transducer fingers act as reflectors (by shorting out the piezo- electric field at the surface of the substrate). Although reso- nating transducers are mentioned in several papers (l) -(4), the modes of the resonant cavity are not discussed, and the reason for the resonant behavior is not fully explained. In this paper we present an analysis of the surface acoustic wave (SAW) resonance phenomenon encountered in the structure shown in Fig. 1. In the next section a simple approximate theory based on the coupling between the forward and back- ward propagating waves is proposed. This allows one to ob- tain a set of simple formulas that describe the essential char- acteristics of the resonant modes of a DFR. In the third section an equivalent circuit for the transducer exciting the DFR is derived starting from the modal fields calculated in Section 11. Finally, the results of this approximate theory are checked in two ways: first by comparison with the impedance characteristic of a DFR as calculated by a transmission line model for the structure and, secondly, by comparing with measurements made on DFRs. 11. COUPLED WAVE ANALYSIS OF THE DISTRIBUTED FEEDBACK RESONATOR In this section the resonant modes of the DFR are calculated by means of the coupled wave theory. This method has al- ready been successfully applied to the analysis of SAW grating structures (g) -( 101 and, although approximate, has the ad-