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UBC Theses and Dissertations

A polarity coincidence spectrum analyzer for inputs with a wide dynamic range Bowering, Kenneth Wayne

Abstract

In the past, spectral analysis of Arctic Sea ambient noise has been carried out by octave band pass filtering and linear rectification, followed by analogue Integration. To relate this integral to the power spectral density of the noise, the amplitude distribution of the noise has been assumed to be Gaussian, giving rise to certain errors. A system is proposed which consists of a stage of variable gain followed by a polarity coincidence statistical wattmeter and measures the power spectral density of ambient noise after band pass filtering. The wattmeter will handle an input signal dynamic range of at least 20 dB and does so regardless of the statistical nature of the noise. This dynamic range is extended dynamically by controlling the gain of the driving stage. The gain level is automatically adjusted during a one minute "adaptive" time interval so that the noise delivered to the wattmeter is over the region of optimal system operation. Measurement of the power spectral density of the ambient noise is then made in the subsequent four minute interval. A prototype wattmeter has been constructed and tested. The gain level is determined by requiring that the noise not exceed fixed levels more than a certain percentage of the time. This automatic adjustment is carried out during a one minute adaptive time interval, and a relatively accurate measure of the mean square value of the noise is determined during the four minutes that follow. For purposes of testing the prototype, d.c. inputs and sinusoidal inputs of wide frequency and amplitude ranges were used. The actual root mean square value of the inputs was measured with a thermal milliammeter and a precision voltage divider. The results of these tests show the region of operation where the input-output relationship of the wattmeter is linear. From these results, suggestions are made.as to how the proposed system could be modified to replace the analogue system used for Arctic Sea ambient noise spectral analysis.

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