Center Frequency Tracking Scheme to Optimize Pulse-Echo Response of Ultrasonic A-mode Scanner Circuit System

This paper proposes a center frequency tracking scheme to optimize a pulse-echo response of ultrasonic A-mode scanner. A pulse-echo response of A-mode scanner represents an overall energy conversion characteristic from an actuation to an acquisition. A center frequency for optimized pulse-echo response can be varied depending on an actuation voltage, an acoustic medium, etc. Most of previous frequency tracking methods have focused on high-power continuous actuation applications, and have relied on a monitoring of phase difference between actuating voltage and current with auxiliary sensors. This work focuses on a monitoring of a peak envelope of echo without additional sensors. We employ a momentum based gradient ascent algorithm along with a finite state machine to track an optimum center frequency so that a peak envelope of echo becomes the maximum value. The proposed frequency tracking scheme was implemented on a field programmable gate array for real-time operation. We performed repetitive measurements to verify consistent frequency tracking performances for different acoustic mediums. The A-mode scanner adaptively tracked corresponding center frequencies for each of mediums within an average error of 61.7 kHz. When the A-mode scanner was in a steady state, an increment of signal-to-noise ratio (SNR) of echo was 7.4 dB, and an axial resolution was improved by 32.5 %.