Optimal design of an ultrasonic low-noise chain of reception

This paper describes a detailed investigation of the noise generated by an ultrasonic chain of reception carried out on a piezoelectric receiver of standard characteristics amplified by a low-noise amplifier (LNA) of optimum design. The objective is to provide the conditions which will allow the best signal-to-noise ratio of the entire chain. The power of the transducer's noise is proportional to the real part of its electrical impedance. Initially we simulated then measured the influence of the emission surface and the acoustic load of the medium on the impedance at the antiresonance frequency at which the transducer is most sensitive. Simulations were carried out using the electroacoustic Redwood model.The second part of our paper is devoted to the theoretical study of the noise chain factor. We show that the noise factor is minimal if the noise characteristics of the LNA and the gain resistance of the first amplification stage are jointly matched to the transducer's internal resistance. By respecting these conditions we designed a two-stage amplifier optimized for transducers with a resonant frequency of between 1 and 10 MHz with an internal resistance of approximately 390 Ω. Experimental measurements of the noise output spectral density were carried out with a variety of source resistances. Both estimation and measurement results are in good agreement.Finally, we have confirmed, by experiment, the direct correlation between the real part of the transducer's impedance and the spectral density of the noise which it generates by measuring the influence of temperature.