Analysis and measurement of BAW devices with interdigital transducers. Application to spectrum analysis and delay function

This paper deals with devices using InterDigital Transducers (IDT) for bulk wave generation and detection. A theoretical study of the coupling between IDT´s deposited on two non parallel faces of a piezoelectric substrate has been carried out. This coupling has been characterized by the mutual admittance of the transducers. Its computation has been adapted for two kinds of devices. The first is a spectrum analyzer. Devices using bulk acoustic waves for spectrum analysis have already been proposed by F. Sabet-Peyman et al. (1987). We present a new type of device in which the three functions transduction-diffraction-focusing are achieved by a chirped input IDT deposited on one face of a LiNbO₃ prism. A set of output transducers analyzing regularly spaced frequencies is deposited on another face of the prism. A prototype analyzing a 500 MHz bandwidth with a resolution of 5 MHz has been designed and manufactured. Experimental data are compared with simulations. The second application is a one port reflection delay line. In this case two parallel connected IDT´s are both deposited on the upper face of a LiNbO₃ plate. They both generate bulk acoustic waves and detect them after propagation and reflection on the lower face. A theoretical study of the acoustical reflection has been carried out. A 4 GHz center frequency, 200 MHz bandwidth and 0.6 μs delay one port reflection line has been designed and manufactured. The comparison between simulated data and measurements demonstrates a very good agreement concerning both the insertion loss, the center frequency and the bandwidth