Development of negative-group-delay circuit for high-frequency ultrasonic transducer applications

Compared with low-frequency (<15 MHz) ultrasonic transducers, high-frequency ultrasonic transducers provide higher spatial resolutions while reducing the echo-signal amplitudes and bandwidths owing to the size reduction of the piezoelectric materials. Therefore, performance improvement of high-frequency ultrasonic transducers can enhance the performance of the whole ultrasound instrument. To improve the echo-signal quality, such as the amplitudes, signal distortions, and bandwidths, negative-group-delay circuits for high-frequency ultrasonic transducers have been applied. The components of the negative-group-delay circuit can be constructed using the measured magnitude and phase characteristics of the electrical impedances of the ultrasonic transducers. Therefore, the relationship between the negative group-delay circuit and ultrasonic transducers must be analyzed to identify the adequate components of the negative-group-delay circuit. To confirm the proposed idea, the performances of negative-group delay circuits were measured and compared. The differences in the measured phase angles of the electrical impedances between the ultrasonic transducer and the combined resistor limiter, negative-group-delay circuit, and preamplifier (including a coaxial cable) were 0.66 degrees, 1.31 degrees, and 9.45 degrees at the resonant, center, and anti-resonant frequencies, respectively (<10 degrees). The calculated and measured maximum negative group delays with the resistor limiter and preamplifier were -21.64 ns at 31.65 MHz and -17.41 ns at 27.38 MHz, respectively. To verify our concept experimentally, we performed a typical pulse-echo test using the designed negative-group-delay circuit integrated with a preamplifier and using the preamplifier alone. The peak-to-peak amplitude, pulse width, -6-dB bandwidth, and total harmonic distortions of the echo signal for the negative-group-delay circuit with the preamplifier (0.232 Vp-p, 0.36 mu s, 81.12%, and -60.72 dB, respectively) were improved compared with those for the preamplifier alone (0.154 Vp-p, 0.74 mu s, 51.98%, and -28.61 dB, respectively). Therefore, the negative-group-delay circuit is useful for improving the echo signal quality of high-frequency ultrasonic transducers. The proposed approach is simple and effective for improving the echo-signal quality of ultrasonic transducers without increasing the performance of the power amplifiers and preamplifiers. (C) 2019 Elsevier B.V. All rights reserved.