Software Correlator - I

(Hopefully, the first of many entries on this subject)

Mandar is working to create a program which will replace a bulk of old, creaking machines at GMRT and other similar synthesis-imaging instruments, which produce a stream of cross-correlations between different antennas that are separated from each other by a huge distance.

The first (baby) steps are to create a program to compute cross-correlations from a stream of numbers. Then, we develop the same algorithm for an input of quantized numbers. After correcting for source delays (fringe-stopping and fractional delay correction), we compute an FFT of the cross-correlation series, to obtain the power spectrum of correlations. Finally, we tie this bit with astronomical source co-ordinates.

Data Acquisition System

We would like to make radio maps using Earth-rotation aperture synthesis . This technique requires us to record raw voltages at two antennas and compute complex visibility (cross-correlation) for different frequency channels.

The block diagram of the receiver and acquisition is shown in the figure on the left.

Each of 4 antennas will have a heterodyne receiver system which will translate a signal at the radio frequency (RF) of 73.9-MHz to an intermediate frequency (IF) of 1.9 MHz.

Four such signals reach the data acquisition system marked in the diagram. Each signal will be converted to digital using a 12-bit ADC, and then sampled at the rate of 0.5 MHz using only the most significant 2-bits.

Aniket carried out feasibility studies and design of the DAS to acquire Nyquist-sampled voltages from a bandwidth-limited signal (0.25 MHz). These quantized voltages are stored on a computer disk. There is a provision of acquiring from 4 antennas, so total data rates are 0.5*4 = 2 M-samples/s. Each sample consists of 2 bits each. Hence it is 8 M-bits/s = 1 MB/s storage rate on a PC.