中文摘要：

分析了夹心式压电换能器的阻抗特性，讨论了基于相位方式的现有跟踪方案的不足，提出了一种新的频率跟踪方案，介绍了实现该方案的电路系统。该方案的特点是：采用先扫频后跟踪的策略，解决了频率范围设定难的问题；增加解锁控制，使得系统在死锁或误跟踪时自动回到频率搜索状态；采用直接数字合成器（DDS）作为频率调整和信号产生的器件，实现了全数字系统，调整方便，精度高；采用复杂可编程逻辑器件（CPLD）做相位比较和DDS控制，使频率跟踪速度快，并且跟踪精度和速度均可控制。

英文摘要：

This paper analyzes the impedance characteristics of the sandwich piezoelectric transducer, discuses the inadequacy of present frequency tracing schemes using phase locking method.It then proposes a novel scheme, and presents the circuitry that realizes the scheme. The distinguishing features of this scheme are as follows： utilizes a strategy of sweeping frequency before tracing and thus keeps away from the difficulty of settling the frequency tracing band; adds an unlock function which renders the system back to frequency sweeping state in the case of dead-locking or false tracing; uses a direct digital synthesizer（DDS） to adjust the frequency and generate driving signal, thus forms an all-digital system which is convenient to manipulate and can achieve high degree of accuracy; uses complex programmable logic device （CPLD） to detect phase difference and to control DDS, thus achieves high tracing speed and makes the tracing speed and the tracing accuracy controllable.