Arif Muhammad, Ali Muhammad Asim, Shaikh Muhammad Mujtaba, Freear Steven
Department of Biomedical Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan.
Department of Electrical Engineering, Sukkur Institute of Business Administration, Sukkur, Pakistan.
Ultrasound Med Biol. 2017 Aug;43(8):1690-1702. doi: 10.1016/j.ultrasmedbio.2017.03.005. Epub 2017 May 5.
Non-linear frequency-modulated (NLFM) chirp coding was investigated to improve the pulse compression of the second harmonic chirp signal by reducing the range side lobe level. The problem of spectral overlap between the fundamental component and second harmonic component (SHC) was also investigated. Therefore, two methods were proposed: method I for the non-overlap condition and method II with the pulse inversion technique for the overlap harmonic condition. In both methods, the performance of the NLFM chirp was compared with that of the reference LFM chirp signals. Experiments were performed using a 2.25 MHz transducer mounted coaxially at a distance of 5 cm with a 1 mm hydrophone in a water tank, and the peak negative pressure of 300 kPa was set at the receiver. Both simulations and experimental results revealed that the peak side lobe level (PSL) of the compressed SHC of the NLFM chirp was improved by at least 13 dB in method I and 5 dB in method II when compared with the PSL of LFM chirps. Similarly, the integrated side lobe level (ISL) of the compressed SHC of the NLFM chirp was improved by at least 8 dB when compared with the ISL of LFM chirps. In both methods, the axial main lobe width of the compressed NLFM chirp was comparable to that of the LFM signals. The signal-to-noise ratio of the SHC of NLFM was improved by as much as 0.8 dB, when compared with the SHC of the LFM signal having the same energy level. The results also revealed the robustness of the NLFM chirp under a frequency-dependent attenuation of 0.5 dB/cm·MHz up to a penetration depth of 5 cm and a Doppler shift up to 12 kHz.
为了通过降低距离旁瓣电平来改善二次谐波啁啾信号的脉冲压缩，对非线性调频（NLFM）啁啾编码进行了研究。还研究了基波分量与二次谐波分量（SHC）之间的频谱重叠问题。因此，提出了两种方法：方法I用于非重叠条件，方法II采用脉冲反转技术用于重叠谐波条件。在这两种方法中，将NLFM啁啾的性能与参考线性调频（LFM）啁啾信号的性能进行了比较。实验是在水箱中使用一个2.25 MHz的换能器同轴安装在距离1 mm水听器5 cm处进行的，在接收器处设置了300 kPa的峰值负压。模拟和实验结果均表明，与LFM啁啾的峰值旁瓣电平（PSL）相比，方法I中NLFM啁啾压缩后的SHC的PSL至少提高了13 dB，方法II中提高了5 dB。同样，与LFM啁啾的积分旁瓣电平（ISL）相比，NLFM啁啾压缩后的SHC的ISL至少提高了8 dB。在这两种方法中，压缩后的NLFM啁啾的轴向主瓣宽度与LFM信号的相当。与具有相同能量水平的LFM信号的SHC相比，NLFM的SHC的信噪比提高了多达0.8 dB。结果还表明，在频率相关衰减为0.5 dB/cm·MHz、穿透深度达5 cm以及多普勒频移高达12 kHz的情况下，NLFM啁啾具有鲁棒性。
