The design of ultrasonic waveguide system for time delay applications Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ms35tc688

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  • A waveform-preserving, variable time-delay system capable of several milliseconds delay is described in this thesis. These considerably long delays are achieved by judicious use of the relatively low sonic velocity of gases. Initial system design criteria specified a time delay of ten milliseconds and a signal bandwidth of 0-10 KC. Due to limiting parameters of available system components, optimum performance may not be obtained for delays in serious excess of five milliseconds and maximum signal frequency which may be passed through the system is two KC. Acoustic waveguides in the form of circular tubes are used to contain the gaseous transmission media. Energy conversion is achieved by electroacoustic transducers placed at each end of the waveguide. With air as the delay medium, a time delay of 0.885 milliseconds per foot of waveguide propagation distance may be obtained for an ambient temperature of 20 degrees C. Since the attenuation properties of acoustic waveguides are very non-linear functions of frequency, intelligence transmission is best accomplished by frequency modulating an ultrasonic carrier. An FM carrier of 40 KC was used to accommodate available transducers. A unique FM modulator based on the principles of mutual inductance and current division was designed for use in the system. These principles allow it to possess a very large frequency-voltage transfer ratio at low carrier frequencies in contrast with other types of sinusoidal FM modulators. Detection is performed by means of a capacitive pulse averaging device. Strongest limitations of the system with respect to feasible delay time are imposed by reflection coefficients of waveguide terminations and sensivity of reception apparatus. These factors serve to determine the longest practical propagation distance which may be used for a given modulation index and signal frequency while maintaining percent distortion in the output below a desired level. System bandwidth is limited to two KC by transducer passbands. Very good reproduction of delayed signals may be obtained by operating the system under its optimal modes of performance as dictated by these limitations. Several possible applications of ultrasonic waveguide systems are discussed, among them the implementation of time delay generators in correlation function computation systems.
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