Improved range resolution and side lobe level of ultrasound echoes using nonlinear frequency-modulated excitation signal and modified compressing filter

Linear frequency modulated (LFM) pulses are used as excitation for ultrasonic imaging systems. Temporal sidelobes, due to echo compression, are generated and impart a degradation of the range resolution. This work proposes a compensated frequency-modulated (CFM) excitation signal that improves the range resolution of compressed pulses, keeping low the sidelobe levels. An experimental pulse-eco system setup was used employing a 2.25 MHz transducer (-3 dB relative bandwidth of 45%) excited with a CFM signal, with a duration of 20 μs, immersed in a water tank containing a flat aluminum reflector. The spectrum magnitude of the CFM signal was determined as the nth power of the pseudo-inverted pulse-echo system transfer function magnitude. Its phase was calculated from the group delay determined from the CFM power spectrum. The echoes were compressed with a modified Wiener filter and the best range resolution of 0.47 mm, measured in -30 dB level, was achieved. This result is superior to the latest best published result, which is 42% larger than the one from this present work.