Highly sensitive simple homodyne phase detector for ultrasonic pulse-echo measurements

Abstract

We have designed and built a modern versatile research-grade instrument for ultrasound pulse-echo probing of the elastic properties of a wide range of materials under laboratory conditions. The heart of the instrument lies in an AD8302 microchip: a gain and phase detector from Analog Devices, Inc. To construct the device, we have implemented a schematic that utilizes the homodyne principle for signal processing instead of the traditional superheterodyne approach. This design allows one to measure phase shifts with high precision and linearity over the entire range of $0-360^\circ$. The system is simple in construction and usage; it makes ultrasound measurements easily accessible to a broad range of researchers. It was tested by measuring the temperature dependence of the ultrasound speed and attenuation in a KTa$_{0.92}$Nb$_{0.08}$O$_{3}$ (KTN) single crystal at a frequency of $\sim$ 40 MHz. The tests were performed in the vicinity of the ferroelectric transitions where the large variations of the speed and attenuation demand a detector with outstanding characteristics. The described detector has a wide dynamic range and allows for measuring in a single run over the whole temperature range of the ferroelectric transitions, rather than just in limited intervals available previously. Moreover, due to the wide dynamic range of the gain measurements and high sensitivity this instrument was able to reveal previously unresolvable features associated with the development of the ferroelectric transitions of KTN crystals.