Case Studies
Arbitrary Ultrasonic Generator
Ultrasonic generation pulses must be varied to accommodate different types of ultrasonic transducers. General-purpose highly damped contact transducers are usually excited by a wideband, spike-like pulse that is provided by many common pulser/receiver units. Lightly damped transducers, used for example in high power generation, require a narrowband tone-burst excitation from a separate generator unit. Sometimes, even the same transducer will be excited differently, for instance, to study the dispersion of a material's ultrasonic attenuation or to characterize ultrasonic transducers.
In order to construct a flexible ultrasonic generation unit, engineers have excited transducers with a gated sine wave amplified by an electrically matched high voltage amplifier. Varying the sine wave frequency with a signal generator and the gate width with an electronic gate, a relatively arbitrary ultrasonic pulse can be generated. Due to the abrupt action of the gate, however, pulse shape can vary enormously as a function of frequency, particularly for wideband signals. Finer control of ultrasonic pulse generation is necessary for detailed study of frequency-dependent behavior. Furthermore, separate function generator and gating circuitry can be awkward to automate.
GAGE'S SOLUTION
Gage's CompuGen 1100 is a 12 bit, 80 MS/s arbitrary waveform generator that is an add-on card installed on the ISA bus in a PC. The CG1100 can easily be turned into an arbitrary ultrasonic signal generator. The figure below schematically shows a CG1100 operating in a transmission-mode ultrasonic inspection system. Driving an ultrasonic transducer through an electrically matched high voltage amplifier, the CG1100 has simply replaced the traditional gated sine wave source. Signals with amplitudes of several hundred volts excite the transducer and generate an ultrasonic beam into a buffer medium (often water). A specimen is placed in the path of this beam and transmitted ultrasonic echoes are captured by a detection transducer as usual. Echo signals are shown to be digitized by a CompuScope 8012A -- Gage's A/D board most suitable for general purpose ultrasonics.
The advantage of the CG1100 is that any arbitrary ultrasonic with frequency components up to 40 MHz can be software selected for generation. The output ranges are also programmable between ±100 mV and ±5 V so that the 12 bit resolution on the ultrasonic amplitude can be fully exploited. Furthermore, software selectable 5, 10 and 20 MHz low-pass filters can be applied to the output signal in order to smooth the staircase-like appearance of extremely high-frequency signals. Through its MS-Windows DLL, the CG1100 can easily be programmed in virtually any Windows NT or Windows 95-based programming language.
Memory on the CG1100 board is available up to 16 Megasamples. New waveform patterns can be quickly uploaded to the CG1100 at 1 MS/s through the ISA bus. Complete patterns can be seamlessly looped forever or for a programmable number of times. Thousands of kilosample-sized generation pulses can be loaded into the memory. The board can generate on an external trigger, and a synchronized output is available whose state can be arbitrarily programmed.
Acting as a truly arbitrary ultrasonic generator, the CG1100 opens up new possibilities in ultrasonic instrumentation. As already suggested, the CG1100 can excite both narrowband and wideband transducers. The generation frequency might be varied to study ultrasonic dispersion. Different ultrasonic generations might be encoded systematically to indicate, say, transducer position. The generation signal might even be generated through feedback from signals digitally captured by the receiver transducers.
Together with ultrasonic transducers and analog amplifiers, the CompuGen 1100 D/A board and the CompuScope 8012A A/D board in a Gage industrial PC provide a flexible and complete ultrasonic test and measurement system. With minor modifications, this Gage system could be adapted to a wide variety of stimulus/response experiments.
PRODUCT RECOMMENDED
CompuGen 1100
12 bit, 80 MS/s Arbitrary Waveform Generator Card for the ISA Bus
CompuScope 8012A
12 bit, 100 MS/s A/D and Scope Card for the ISA Bus
GagePC 586
Industrial Grade PC
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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