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off the shelf center frequencies are not close enough to the desired measurement frequency. However, frequencies that lie within the range above the available center frequency, at -3 to -6 dB down from the center frequency, could be used if these frequencies could be excited electronically.
Figure 2., shows a diagram of a plot of the frequency response of a broad band transducer with its center frequency Fc at 17.5 MHz. However, we would like to use a higher frequency, perhaps at 20 MHz. This point is shown in the diagram as FDesired. A small circle surrounds the intersection point at the energy level for the desired frequency. The intersection point is above the –3dB energy level. Therefore, at the desired frequency, we will still can transmit more then 70% of the energy found at the 20MHz center frequency.
When a spike wave or square wave electronic pulser (Figure 3.) is used, the transmitted output sound energy will occur at natural resonant frequency FC of the transducer, 17.5MHz. To excite the transducer to operate at the desired frequency of 20 MHz, we need to excite this transducer with an excitation sinusoidal frequency burst at a frequency of 20MHz. This can be done by using an electronic pulser that can drive the transducer with a selected output of 20MHz burst frequency. In Figure 4., the top trace shows a 4 cycle burst from a burst pulser. Although the trace shows a square wave burst, the broad band trans- ducer will respond by generating the output sound frequency as a fun- damental sine wave burst of 20MHz. This is due to the principle that the fundamental of a Fourier series component of a square wave will be a sine wave at the same base frequency as the square wave.
Optimizing Transducer Parameters with an
Electronic System
The following list points out the two critical measurement areas of concern that we might wish to improve:
Dimensional transducer parameters
Flaw detection transducer parameters
Fig. 2
Technical Papers
In this article, we will concentrate on the dimensional transducer parameters. Although, the other dimensional measurements such as OD, ID, ECC, and Ovality are very important, meeting the trans- ducer requirements for wall measurements will simultaneously cover the requirements for the remaining dimensional measurements. Flaw detection parameters typically have not caused much dif culty during setup times, when applying 12 or 15 MHz line focused angle beam transducers. The shear wave wavelengths, 0.0083 to 0.0067 inch, are acceptable for relatively small notches 0.25 in (6.35 mm) long by 10% of wall depth
Frequency and Wavelength
If you purchase a broadband transducer with a center frequency having a wavelength that is less than 3/4 of the thickness that you wish to measure, the only electronic assistance that will be required is a pulser with a fast enough rise time and a receiver with adequate bandwidth. For the tube specimen examined for the tests reviewed in this article,
Fig. 3
Fig. 4
ITAtube Journal No2/July 2018
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