Range measurement using ultrasound FMCW signals

Low peak power frequency-modulated continuous wave (FMCW) radar is used in various fields, such as vehicle collision warning systems, airborne radio altimeters, and other applications. The authors proposed an ultrasound FMCW range measurement system based on the same principle as the FMCW radar. In the proposed system, the transmitter and receiver operate at very low voltage. This advantage suggests the possibility of designing a small and simple transmitter/receiver using low power ICs. Although pulsed ultrasound imaging is safe to patients, it´s always a good idea to find a method that can use a minimum ultrasound power for diagnosis. The sawtooth and isosceles sawtooth functions are selected as the frequency modulation signals for the ultrasound FMCW range measurement system to achieve sufficient range resolution in human body. An ultrasound FMCW signal is transmitted into human tissue and the reflection signal is received after the two- way propagation time in human body. Instantaneous frequency difference between the transmitted and received signals is proportional to the propagation time in the system. Therefore, the range information may be obtained from the frequency spectrum of the baseband signal which is generated by multiplying the transmitted signal by the received signal. The spectrum and range resolution were calculated based on the basband signal analysis. The calculation results suggest that the system has similar range resolution to conventional ultrasound pulse diagnosis systems. We verified the calculated results by the experiments using electrical delay line and an ultrasound phantom. We measured the baseband frequency spectrum versus electrical delay time with the sawtooth and isosceles sawtooth frequency modulation signals and observed that harmonics peak frequency moved to a higher frequency band as the delay time increased. The relationship between the harmonics peak frequency spectrum and the delay time agreed with the calculated results. - - Next, we measured the frequency spectrum in the baseband signal using an ultrasound phantom. We confirmed a linear relationship between the harmonics peak frequency and the delay time in a range of 0-170 micro seconds. These experimental data agreed well with calculated results. The 3 dB bandwidth of the baseband frequency spectrum was 1-3 kHz with the sawtooth modulation signal and it was equivalent to the range resolution of 0.5 mm ∼1.5 mm in human body. The measured frequency spectrum and its bandwidth satisfied the range resolution required in a medical diagnostic ultrasound system. We conclude that the proposed system has a possibility of decreasing ultrasound peak power to less than 1/100 of that of conventional pulse ultrasound diagnostic systems.