Evaluation of impedance matching schemes for pulse-echo ultrasonic piezoelectric transducers

In order to optimize the broadband response of a piezoelectric transducer, different impedance matching procedures are used, looking for a compromise between efficiency and bandwidth. In this paper a general methodology for the evaluation of acoustic and electrical matching schemes is presented. Broadband acoustic impedance matching schemes are evaluated in the frequency domain using the transmission line analogy. Electrical matching networks at the reception stage are evaluated using the transmission matrix approach. In practical cases of ultrasonic imaging and NDE, electronic "spike" generators based on capacitive discharges are used. The special topology of such generators (including semiconductor devices) conditions the electrical driving pulse at the transducer terminals. This driving waveform (determined by the interactions among the spike generator, the electrical matching network and the transducer) is approximately evaluated in the frequency domain for the low Megahertz range. Several examples in emission and reception illustrate the methodology, including a comparative study of criteria proposed for the,optimum design of quarter-wave transmission line matching layers.