Spectrum Analysis

The signal spectrum is analyzed with a commercial spectrum analyzer that is controlled by a 386 PC. The analyzer operates from 3 to 37 MHz with an instantaneous bandwidth of 10 kHz. The spectrum is sampled at 128 points every three seconds. The detected output signal from the analyzer is linearly integrated at each frequency step for about 20 ms, then A/D converted and sent to the PC that controls the system and acquires the data. Every 30 minutes the system makes a full scan of the spectrum, from 3 to 37 MHz, with 10 kHz resolution. It then finds any 'holes' between interference transmissions in each 265.6 kHz-wide section of the spectrum and, for the next 30 minutes, it observes only in these 128 'holes'. After 30 minutes the system re-adapts itself to the constantly changing interference environment. The data are displayed in frequency-time coordinates on a monitor with signal intensity on the colour axis. The data are also written to disk every 30 minutes and, at the end of each day, they are compressed and automatically transferred to tape for off-line processing and long-term storage.

Without these adaptive procedures to avoid most of the interference, observations below 20 MHz would be practically impossible and, in spite of these procedures, high interference levels from over-the-horizon sources remain. In addition to man- made interference, wide-band spikes caused by lightening, both local and distant, are common. An off-line filtering program has been developed which reduces the response of the system to steady narrow-band signals and to instantaneous wide-band spikes. As illustrated in Figure 2, the output of this filtering program produces records of solar emissions that are virtually interference-free.

© Copyright Astronomical Society of Australia 1997