An FPGA-based coded excitation system for ultrasonic imaging using a second-order, one-bit sigma-delta modulator

Abstract

Coded excitation and pulse compression techniques have been used to improve the echo signal-to-noise ratio (eSNR) in ultrasonic imaging. However, most hardware use phase modulated codes over frequency modulated codes because of ease of implementation. In this study, a technique that converts non-binary frequency modulated codes into binary frequency modulated codes is evaluated. To convert from a non-binary to a binary code, a second-order, one-bit sigma delta modulator is used. This sigma-delta modulated code is generated in MATLAB which is then stored in a double data rate synchronous dynamic random access memory. A field programmable gate array, which has access to the memory device, transmits the binary waveform and is recorded using an oscilloscope. The recorded data is then filtered using the pulse-echo transducer model of a linear array with a center frequency of 8.4 MHz and a fractional bandwidth of 100% at -6 dB. Pulse compression was then performed using a matched filter, a mismatched filter, and a Wiener filter. Image quality metrics, such as modulation transfer function and sidelobe-to-mainlobe ratio, were used to assess compression performance. Overall, echoes compressed when the excitation is the sigma-delta modulated coded waveform resulted in no measurable difference in axial resolution.