A radio interferometer measures the mutual coherence function of the electric field due to a given source brightness distribution in the sky. The antennas of the interferometer convert the electric field into voltages. The mutual coherence function is measured by cross correlating the voltages from each pair of antennas. The measured cross correlation function is also called Visibility. In general it is required to measure the visibility for different frequencies (spectral visibility) to get spectral information for the astronomical source. The electronic device used to measure the spectral visibility is called a spectral correlator. These devices are implemented using digital techniques. Digital techniques are far superior to analog techniques as far as stability and repeatability is concerned.
The first of these two chapters on correlators covers some aspects of digital signal processing used in digital correlators. Details of the hardware implementation of the GMRT spectral correlator are presented in the next lecture.