An inference monitor with real-time FFT spectral analysis for a radio observator

A system is described which uses a real-time fast Fourier transform (FFT) spectrum analyzer to monitor radio interference near 408 MHzs occurring at a radio observatory. Direction of arrival, frequency, intensity, and time of occurrence are recorded under the control of a microcomputer. A sensitive receiver can be connected to any one of eight directional antennas to establish direction of arrival. The receiver output is digitized to 8 b, and analyzed by the FFT spectrum analyzer which has a real-time bandwidth of 0.5 MHz. A total bandwidth of 20 MHz is analyzed in segments of 0.4 MHz. The analyzer uses the modified periodogram method developed by P.D. Welch (IEEE Trans. Audio Electroacoust., vol.AU-15, p.70-3, June 1967), and a Kaiser-Bessel windowing function is applied to ensure low sidelobes. Dynamic range is 40 dB, and the interference monitor obtains high sensitivity to very weak interfering signals by time averaging. Several general design principles which arise out of certain constraints are elucidated. Some new results are presented concerning the detectability of small signals by FFT processors