Analysis of the digital signal processing influence on time-of-flight estimation

Abstract

Application of digital filters in ultrasonic data acquisition system has been investigated. The work analyses the digital signal processing influence on a time of flight (TOF) estimation when applied on ultrasonic signals. The ToF estimation was done using the direct correlation technique of the sampled signal. The subsample estimation was done using parabolic interpolation of discrete direct correlation. The investigation covers the ADC and digital filter rounding noise influence on amplitude and TOF estimation performance. The theoretical equations for interpolation-influenced and quantization-influenced random errors assessment have been suggested. The acquisition system has been developed, including Xilinx Spartan 3E Field programmable gate array (FPGA), high speed 10  bit ADC and USB2 high speed interface. FPGA fixed point FIR filters have been developed and implemented. AccelDSP development suit was used to create the filters simulation environment in MATLAB.
Numerical simulation was used to analyze the filtering performance, influence of electronics noise, rounding errors and interpolation errors on TOF. Experimental results are documented and presented in tables and figures. It has been concluded that a digital filter output has to be quantized according to the processing gain obtained; otherwise rounding noise is increased, causing the TOF estimation to degrade.