Swept Frequency Multiplication (sfm) Techniques for Improved Air-coupled Ultrasonic Nde

A new technique has been investigated for improving the signals that can be obtained in air-coupled NDE. This relies on the broad bandwidth available from polymer-filmed capacitive transducers. The technique relies on a swept-frequency "chirp" signal, which is transmitted from a transducer in air, passes through the sample under test, and is then detected at the far side using a separate receiver in through-transmission. The new technique differs from existing time-domain correlation techniques, such as pulse compression, in that a single multiplication process is performed in the time domain to give a difference frequency signal. This can then be easily isolated in the frequency domain. It will be demonstrated that this new Swept Frequency Multiplication (SFM) approach gives excellent resolution between overlapping signals, and indeed has several potential advantages over pulse compression. Examples will be shown of air-coupled images, obtained using SFM processing, which demonstrate the wide application of the technique to air-coupled NDE. INTRODUCTION In many ultrasonic experiments, there is a need to improve the accuracy of an individual measurement, while also trying to improve the signal to noise ratio (SNR). There are many techniques available for signal enhancement, such as signal averaging and correlation techniques that could be used to recover a wanted signal from noise. This paper describes a new technique known as Swept Frequency Multiplication (SFM). This technique makes use of a swept frequency or "chirp" signal, and is a modification to a technique in optics known as Optical Frequency Domain Reflectometry (OFDR). The OFDR technique has been widely used to solve Fresnel reflections for optical fiber links and it has good time accuracy [1-3]. With this technique, useful information is obtained from the frequency domain so that time-domain signals can be extracted. The SFM technique is now applied to an ultrasonic system using two broadband capacitance transducers. This paper describes the basis of the technique, and the resulting characteristics and performance, when applied to the particular case of air-coupled testing. THEORY OF THE SFM TECHNIQUE Consider the case where the voltage drive waveform is a linear FM frequency sweep (or chirp). This is shown in Figure 1 as signal A, where the frequency is increasing CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti © 2003 American Institute of Physics 0-7354-0117-9/03/S20.00 620