Improving the bandwidth of air coupled capacitive ultrasonic transducers using selective networks

One of the key limitations on using CUT (capacitive ultrasonic transducers) and cMUTs (capacitive micromachined ultrasonic transducers) in air is their relatively narrow bandwidth which although superior to that of current piezoceramic devices could be improved. Most air coupled capacitive devices could benefit hugely through the use of selective networks for bandwidth expansion, resonance reinforcing, or a combination of both. This work has investigated the application of pole/zero manipulation techniques to modify and enhance the transmission characteristics of capacitive transducers through front end mounted components. The main objective was to positively enhance the performance characteristics of capacitive transducers. A modified electrical equivalent circuit for CUTs to include the selective networks used was developed. The work assessed the effects of a tuned amplifier on the passband of the devices studied and then focused on more complicated network designs for enhancement. Simulations of the effects of the networks on the devices using equivalent circuit models were carried out and the response curves to pulsed operation were calculated and compared to experimental measurements from a pair of fixed CUTs with a combined centre frequency of 280 kHz and 3 dB bandwidth of 160 kHz. Increases in centre frequency of 25% and 3 dB bandwidth of 77% using a single tuned amplifier were obtained. Resonance reinforcing, resonance shifting and ripple suppression were also studied through the manipulation of the q factor and pole location of such an amplifier. Performance enhancements were studied for a number of CUT aperture sizes and membrane thicknesses and a comparative study of the theoretical and experimental effect of these variations was conducted. With the appropriately designed network, enhancement of peak resonance with a simultaneous bandwidth expansion was obtained at the expense of other operating parameters such as stopband ripple. Simulations of more complicated circuit designs using equivalent circuit models of capacitive devices showed that the maximum level of passband ripple observed for the bandwidth expansion method using a single tuned amplifier was reduced while achieving simultaneously the same 3 dB results. The implication for bandwidth expansion of a capacitive transducer through selective network design is significant, allowing increased resolution in imaging systems, ultrasonic ranging and non destructive evaluation. Significant improvements have been observed without additional signal manipulation, through digital means or otherwise, in certain transmission properties of the devices. Future work will expand on the enhancement of capacitive transducers through the use of hybrid resonator circuits and other related methods.