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Pulse Compression

Signal processing techniques such as Superheterodyne, Pulse Compression and Swept Frequency Multiplication are widely used in telecommunications to improve signal detection but have not generally been used for ultrasound. We have undertaken a study of these three techniques for use with ultrasound. Each technique has advantages and disadvantages, we have chosen to concentrate on pulse compression. This entails using a broadband chirp signal with a Hanning filter the received signal is then put through a bandpass filter and cross-correlated with a reference signal which gives the compressed pulse. The advantage of using a chirp is that it is a complex coded waveform, this gives good accuracy for time of flight measurements as it only correlates well at a single well defined time of arrival. Additionally it can be detected when the received chirp level is well below the level of any random noise.

chirp  after hanning window
Chirp signal of 50µs duration after application of a Hanning window
transmitted chirp in noise
Transmitted chirp embedded in noise before and after bandpass filtering
The use of cross-correlation of the bandpass-filtered noisy signal with the original chirp signal allows the efficient detection of the signal. A typical recified cross-correlation output is shown below.
compressed pulse
A compressed pulse signal at 30 µs

Pulse compression has been successfully used in a number of ultrasonic applications including testing of solid rocket motors and food packaging. The technique is also very useful in one of our newer research areas, namely Acoustic Cargo Screening. We are also looking at other methods of increasing signal to noise ratios, as many of our measurement and imaging experiments use small signals in the presence of high noise levels.

Some related papers

K.S. Ho, D.R. Billson and D.A. Hutchins, “Ultrasonic Lamb wave tomography using scanned EMATs and wavelet processing”, Nondestructive testing and Evaluation 22, 19-34 (2007).

K.S. Ho, T.H. Gan, D.R. Billson and D.A. Hutchins, “Application of pulse compression signal processing techniques to Electromagnetic Acoustic Transducers for non-contact thickness measurements and imaging” Review of Scientific Instruments 76, paper 054902-1-8. (2005).

T.H. Gan, D.A. Hutchins, R.J. Green, M. Andrews and P.D Harris, “Non-contact high-resolution ultrasonic imaging of wood samples using coded chirp waveforms”, IEEE Trans. Ultras. Ferr. Freq. Contr. 52, 280-288 (2005).

T.H. Gan, D.A. Hutchins, D.R. Billson and D.W. Schindel, "The use of broadband acoustic transducers and pulse compression techniques for air-coupled ultrasonic imaging", Ultrasonics 39, 181-194 (2001).