Design of Polyphase Cascaded Integrator Comb Interpolator Filter for Ultrasonic Phased Array Time Delays

Abstract

Time-delay accuracy plays a crucial role in beamforming algorithms of ultrasonic phased array. This paper presents a novel design for a polyphase cascaded integrator comb (CIC) interpolator filter, which can achieve high time-delay accuracy for an ultrasonic phased array focusing-delay system, and the beamforming principle and time-delay algorithm implementation are studied in detail. Firstly, a novel design for the CIC filter is proposed, which improves traditional designs (The data rate increases with the increase of interpolation coefficient, when the direct interpolation mode is used) by using polyphase interpolation. Secondly, according to the principle of interpolating and filtering, CIC polyphase interpolation formulas are deduced by using the exhaustive method, and the formulas can simultaneously implement interpolation and polyphase decomposition with the data rate of each phase same as the original rate. Finally, the algorithm feasibility is validated by simulation analysis and field programmable gate array implementation. Experimental results reveal that a time-delay accuracy of 1 ns at a 100 MHz sampling frequency can be achieved using the improved 10-fold polyphase interpolation CIC filter, and the maximum relative error of the time-delay accuracy is typically only-0.08% for 5 MHz ultrasonic echo waves. This design can be used for many important practical applications for phased array instruments with excellent performance.