Tunable Q matching networks for capacitive ultrasound transmitters

Abstract

Airborne capacitive micromachined ultrasonic transducers (CMUTs) have predominantly large input capacitive reactance with small series radiation resistance. To maximize the acoustic power radiation at resonance we employ low cost LC matching networks between the low output impedance driving source and CMUT transducer. Conventional LC networks, including pi and T-network topologies are employed to provide a high-Q match. An important free parameter that controls the bandwidth of match in pi and T-networks is the center impedance. Since a simple pi-network can be formed from two basic L-sections, the center impedance of two L-sections provides control over the bandwidth of match. Hence these are tunable-Q matching networks. We use the lumped circuit model of CMUT to derive the input impedance of CMUT at resonance. At resonance the lumped equivalent circuit of transducer can be reduced to a series RC circuit consisting of radiation resistance and transducer’s equivalent capacitance only. From the input impedance of CMUT we determine the lumped inductance and capacitances of various LC matching networks. The aim of these matching networks is to cancel the large input capacitive reactance of CMUT cell and to match the cell’s radiation resistance to source resistance at resonance. We report significant improvement in the radiated power when CMUT is driven at resonance using the proposed matching schemes.