Fabrication of high frequency (25-75 MHz) single element ultrasonic transducers

The design and fabrication of high frequency single element ultrasonic transducers present a multitude of challenges for the transducer engineer, from size constraints to electrical impedance matching. This paper discusses the trade-offs involved in procedures used to fabricate lithium niobate (LiNbO3) and lead titanate (PbTiO3) transducers in the 25MHz to 75MHz range. Transducers of varying dimensions were built according to an f-number range of 2-3.5. Desired focal depths were achieved with use of either an acoustic lens or a spherically focused piezoceramic. Silver epoxy backing with an acoustic impedance of approximately 5.9 MRayls was used in all designs. All transducers were designed around a 50 Omega send and receive circuit. Electrical tuning of the transducer to the receive circuitry was achieved by using an RF transformer and/or a length of coaxial cable. All transducers were tested in a pulse-echo arrangement using a Panametrics 5900PR pulser, a Wavetek function generator and a LeCroy digital oscilloscope. The bandwidth, insertion loss, and depth of focus were measured. Numerous transducers were fabricated with -6dB bandwidths ranging from 40% to 74%, and two-way insertion loss values ranging from -14dB to -28dB.