Spectrometer for Radio Astronomy

Since commercially available spectrum analyzers use frequency sweepers to obtain a broad-band spectrum, they are not sensitive enough for radio astronomical applications. Therefore, dedicated instruments have been developed for coherent spectral analysis. For radio frequencies one can identify three basic types of spectrometers: filter banks, acousto-optical spectrometer, and autocorrelators.

Early on, spectrometers were build using analog (bandpass) filters and detectors, which were then assembled into filterbanks. Later, acousto-optical spectrometers (AOS) were developed which allowed for wider bandwidth and a larger number of spectral channels (Schieder, 2003). Due to their design they they need a stable, thermally controlled environment, which put constraints on their operability.

The most commonly used spectrometers are digital autocorrelators (AC). Their technology is based on the Wiener-Khinchin-Theorem, stating that the power spectrum of a signal is the Fourier transform of its autocorrelation function (ACF), as illustrated in Figure 1. The advantage of this indirect method is the possibility to accumulate the ACF even before the Fourier transform produces the final power spectrum, which reduces the number of computations and enabled the calculation of the FFT in the early days. The ACF is computed by multiplication of the periodically time-shifted signals, followed by a summation of these. Limiting the sampler resolution to 1 or 2 bits, these three operations (shift, multiplication, addition) can be reduced to simple logical elements, and the ACF can be integrated in a customized silicon chip. This reduction to 1 or 2 bit (mostly 3-level) data sampling, however, results in a reduced sensitivity. Nevertheless, ACs have until recently been the only way to make possible the computation of CPU intensive Fourier transforms, so that bandwidths of order 1 GHz were manageable.