There are a variety of digital backends available at the GMRT. The principle backend used for interferometric observations is a 32 MHz wide FX correlator. The FX correlator produces a maximum of 256 spectral channels for each of two polarizations for each baseline. The integration time can be as short as 128 ms, although in practice 2 sec is generally the shortest integration time that is used. The FX correlator itself consists of two 16 MHz wide blocks, which are run in parallel to provide a total instantaneous observing bandwidth of 32 MHz. For spectral line observations, where fine resolution may be necessary, the total bandwidth can be selected to be less than 32 MHz. The available bandwidths range from 32 MHz to 64 kHz in steps of 2. The maximum number of spectral channels however remains fixed at 256, regardless of the total observing bandwidth. The GMRT correlator can measure all four Stokes parameters, however this mode has not yet been enabled. In the full polar mode, the maximum number of spectral channels available is 128. Dual frequency observations are also possible at 233 and 610 MHz, however in this case, only one polarization can be measured at each frequency. The array can be split into sub-arrays, each of which can have its own frequency settings and target source. The correlator is controlled using a distributed control system, and the data acquisition is also distributed. The correlator output, i.e. the raw visibilities are recorded in a GMRT specific format, called the ``LTA'' format. Programmes are available for the inspection, display and calibration of LTA files, as well as for the conversion of LTA files to FITS.
The first block of the GMRT pulsar receiver is the GMRT Array Combiner (GAC) which can combine the signals from the user-selected antennas (up to a maximum of 30) for both incoherent and coherent array operations. The input signals to the GAC are the outputs of the Fourier Transform stage of the GMRT correlator, consisting of 256 spectral channels across the bandwidth being used, for each of the two polarization from each antenna. The GAC gives independent outputs for the incoherent and coherent array summed signals, for each of two polarizations. For nominal, full bandwidth mode of operation, the sampling interval at the output of the GAC is sec.
Different back-end systems are attached to the GAC for processing the incoherent and coherent array outputs. The incoherent array DSP processor takes the corresponding GAC output signals and can integrate the data to a desired sampling rate (in powers of 2 times 16 microsec). It gives the option of acquiring either one of the polarizations or the sum of both. It can also collapse adjacent frequency channels, giving a slower net data rate at the cost of reduced spectral resolution. The data is recorded on the disk of the main computer system.
The coherent array DSP processor takes the dual polarization, coherent (voltage sum) output of the GAC and can produce an output which gives 4 terms - the intensities for each polarization and the real and imaginary parts of the cross product - from which the complete Stokes parameters can be reconstructed. This hardware can be programmed to give a sub-set of the total intensity terms for each polarization or the sum of these two. The minimum sampling interval for this data is 32 microsec, as two adjacent time samples are added in the hardware. Further preintegration (in powers of 2) can be programmed for this receiver. The final data is recorded on the disk of the main computer system.
There is another independent full polarimetric back-end system that is attached to the GAC. This receiver produces the final Stokes parameters, I,Q,U & V. However, due to a limitation of the final output data rate from this system, it it can not dump full spectral resolution data at fast sampling rates. Hence, for pulsar mode observations the user needs to opt for online dedispersion or gating or folding before recording the data (there is also a online spectral averaging facility for non-pulsar mode observations).
In addition, there is a search preprocessor back-end attached to the incoherent array output of the GAC. This unit gives 1-bit data, after subtracting the running mean, for each of the 256 spectral channels. Either one of the polarizations or the sum of both can be obtained.
Most sub-systems of the pulsar receiver can be configured and controlled with an easy to use graphical user interface that runs on the main computer system. For pulsar observations, since it is advisable to switch off the automatic level controllers at the IF and baseband systems, the power levels from each antenna are individually adjusted to ensure proper operating levels at the input to the correlator. The format for the binary output data is peculiar to the GMRT pulsar receiver. Simple programs to read the data files and display the raw data - including facilities for dedispersion and folding - are available at the observatory and can be used for first order data quality checks, both for the incoherent mode and coherent mode systems.