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UBC Theses and Dissertations

An interference monitor for a radio observatory Romalo, David N.


This thesis describes the design, construction, and testing of a radio-frequency interference monitoring system for use with the synthesis array telescope at the Dominion Radio Astrophysical Observatory near Penticton, B.C. The system is designed to provide continuous, automated surveillance of the radiospectrum around 408 MHz. Interfering signals are characterized and catalogued according to strength, duration, frequency, and direction. Although the monitor is presently a very useful tool for detecting and finding sources of interference, it is ultimately intended to communicate directly with the telescope's control computer, so that sporadic bursts of interference can be removed automatically. The system can detect a weak interfering signal that is within 5 dB of the smallest signal that can contaminate the astronomical observations. The smallest signal was calculated based on the following conditions, considered to be the case for which a synthesis telescope is most sensitive to interference, i.e., the worst case: a) observing at high declination (towards the North Pole), so that the fringes of the synthesis telescope are too slow to reduce the effects of the interference, and b) with the interference present continuously during the observation. These weak signals can be detected in the presence of other signals, nearby in frequency, which are up to 40 dB stronger, i.e., the dynamic range of the monitor is 40 dB. The monitor consists of an antenna system, a computer-tunable radio receiver, a fast Fourier transform (FFT) spectrum analyzer, and a microcomputer for control and data analysis. Everything except the microcomputer hardware was built as part of the project. A thorough survey of the literature on the design of dedicated FFT machines was required. It was discovered that there had been no investigation of the design details for fixed-point FFT machines which are required to do long integrations. In such situations, fixed-point errors limit the performance of the machine. A computer simulation of the Welch process was developed to analyze the effects of these errors and to optimize the design. Some new results concerning the detectability of small signals are presented. The FFT spectrum analyzer is used to estimate the power spectrum of 500 kHz-wide sub-bands using Welch's method of modified periodograms. It computes 256-point transforms in real-time with a resolution of 3.91 kHz (corresponding to one FFT every 512 μsec). This is comparable to the speed of a large array processor but at a fraction of the cost. Since the FFT is equivalent to a bank of contiguous filters, it can analyze the spectrum in much less time than the single swept filter found in most commercial spectrum analyzers, i.e., it is much more sensitive. The analyzer was specially designed and built using recently-available digital integrated circuits. The design draws upon several high-speed architectural concepts including pipelining, parallel arithmetic, and hard-wired control. Except for expensive array processors, the analyzer is much faster than any commercial FFT processors or FFT-based spectrum analyzers. As part of the antenna system, an array of helical antennas was designed and constructed, its characteristics were investigated and found to be suitable for the present application, and a method of remotely switching them on and off was devised. One more note - the radio spectrum is becoming more and more cluttered with man-made signals. Unprotected radio astronomy bands are being adversely affected and radio astronomers are turning to FFT spectrometers to cope with the relatively large interfering signals. The work herein on FFT-based design is applicable in such cases.

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