Introduction

Chapters 8 and 9 covered the basics of correlator design and implementation. Recall that there are two popular types of correlators, viz. the FX and XF types. The FX design has a number of advantages including (a) low cost, (b) digital fringe stopping and fractional delay compensation and (c) minimal closure errors. The GMRT correlator is an FX correlator. The integrated circuit (IC) used for performing the FFT and the correlation is an application specific IC (ASIC) designed by the NRAO for the VLBA correlator. This chapter provides an overview of the GMRT correlator and discusses its various modes of operation. The material is meant as a guide the correlator users (i.e. astronomers). For details of hardware implementation see Tatke (1997).

The main considerations while designing the GMRT correlator were the following:

1. Astronomical requirement : Briefly, the correlator should have the capability to make continuum radio maps of all the stokes parameters as well as spectral line radio maps.
2. Radio Frequency interference : As a low frequency telescope the GMRT is highly susceptible to man made interference. To observe weak celestial sources in the presence of strong radio frequency interference (RFI), the dynamic range of the receiver system and the correlator should be large. If the RFI spectrum is narrow band, it may also be possible to edit it out from the data if the visibility spectrum is measured with sufficiently high resolution.
3. Cost : The overall cost of the correlator system should be kept at a minimum.

The last two requirements favor an FX configuration. Since the FX correlator inherently measures the visibility spectrum, any narrow band RFI can be edited out. To improve the dynamic range 4-bit sampling is used.

Figure 25.1: Schematic showing the four baseband outputs from each GMRT antenna. Each antenna is dual polarized and each polarization signal (which is of maximum bandwidth 32 MHz) is split into two sidebands, each of maximum bandwidth 16 MHz. At all frequencies of operation, except L band, right (R) and left (L) circular polarizations are measured. At L band two orthogonal linear polarizations are measured. The two sidebands are called the Upper Side Band (U) and Lower Side Band (L) respectively. So $R_U$ is the upper side band of the right circular polarization, and so on.

Recall that the GMRT has 30 antennas and that each antenna provides signals in two orthogonal^25.1 polarizations. The maximum operating bandwidth at all frequency bands is 32 MHz, which is provided as two 16 MHz wide baseband signals (corresponding to the two sidebands) for each polarization (see Fig. 25.1). From the basic block diagram of an FX correlator (see Fig 9.4 in Chapter 9) it is evident that the GMRT correlator should have $120~ (=30\times 4)$ ADCs, integral delay compensation units, number controlled oscillators, FFTs and fractional delay compensation units.

The total number of multiplier units required for the GMRT can be calculated as follows. The total number of cross products for a $n$ element array is $n \times (n-1)/2$. If the self products are also computed then the total number of products is $n (n-1)/2 + n = n (n+1)/2$. In an FX correlator these products have to be measured for each spectral channel. Since the GMRT correlator provides 256 spectral channels, the total number of multiplier units required is $n \times (n+1)/2 \times 256$. Further since, as discussed above, there are four baseband signals for each antenna, the number of multiplier units required goes up by a factor of 4. To measure all the four Stokes parameters the cross products between different polarizations need to be measured (see chapter 15), this causes the required number of multiplier units to increase by another factor of 2. Thus for $n = 30$ the total number of multipliers required is 9,52,320. However, to lower the cost and to simplify the hardware design the number of multiplier units in the GMRT correlator is only 2,38,080. To minimize the impact of this reduction in multipliers, the GMRT correlator has a a highly configurable design. Depending on the astronomical requirement the correlator can be configured to minimize the loss of information, for example in may spectral line observations it is not neccessary to measure all four stokes parameters. The following sections give an overview of the GMRT correlator and also discuss these different correlator configurations.

Footnotes

... orthogonal^25.1

All the frequency bands of GMRT except the L band are circularly polarized. At L band two linearly polarized signals are provided.