TECHNICAL DETAILS OF THE DIGITAL BACKENDS :
HARDWARE CORRELATOR :
- Processes signals from 30 Dishes.
- Each Dish generates 4 streams of 16MHz BANDWIDTH , pertaining to the "Upper" and "Lower" sidebands of the "Right" and "Left" circular POLARIZATIONS. Hence handles 120 data streams.
- Processes a maximum of 32MHz BANDWIDTH.
- Compensates a maximum of 128microsec. delay with an effective step of 1nanosec.
- Provides a maximum of 256 SPECTRAL channels.
- Provides a nominal integration time of 64ms with 128ms as maximum integration time & as small as 4ms are possible if the number of SPECTRAL channels is less than 256.
PULSAR RECEIVER :
- GMRT Array Combiner (GAC), built by the Raman Research Institute, Banglore. It combines the signal from a maximum of 30 antennas for both incoherent and coherent array operation. In both cases, the signals have 256 spectral channels, which are generated by the FFT units ( which are part of the FX correlator used at GMRT ).For all data output by the GAC, the sampling rate is 16 micro second.
- The POLARIMETER is designed primarily for use with the G.M.R.T. radio telescope and perform the above mentioned operations over dual, orthogonal-polarization data samples of 512 FREQUENCY channels covering a maximum BANDWIDTH of 32MHz.
- Incoherent Array (IA) Mode :In this mode, the power signals from different antennas are combined to give the final signal for pulsar processing. For addition of signals from N antennas, this mode will give a √N(squarerootof 'N') improvement in sensitivity over a single antenna while the array will have a beam width equal to that of a single antenna. This will be useful primarily for large scale pulsar search observations.
- Coherent or Phased Array (PA) Mode :In this mode, the voltage signals are added to get the final signal for two orthogonal polarisations. It will require proper phasing of the array to exactly compensate the delays and phases of signals from different antennas. This will give a sensitivity of N over a single antenna while the beam width will be much narrower than that of a single antenna. This mode will be used primarily for studies of individual pulsars, including polarimeters.
- The PHASED ARRAY bin computes the basic SELF and CROSS terms between the VOLTAGE signals of the two POLARIZATIONS, from which the FULL STOKES parameters can be constructed. The highest TIME RESOLUTION achievable is 128 microsec for the INCOHERENT ARRAY mode and 512 microsec for the full Stokes PHASED ARRAY mode. These back-ends are available only for one sideband ( 16 MHz ) BANDWIDTH and are connected to the upper sideband(USB).
NEW HARDWARE CORRELATOR :
- iADC with max 1Gs/s capability, operating at 800MS/s ,atmel/E2V 84AD001BCT, analog input channels 2 , 1 clock and 1 sync inputs ,output LVDS through Z-DOK connector compatible to CASPER H/Ws iBOB & ROACH BOARDS.
- iBOB : VIRTEX 2 PRO FPGA.
- ROACH: VIRTEX 5 SX 95.
- Processing Bandwidth is 400MHz.
- FFT 1 Kpt real.
SOFTWARE CORRELATOR :
GMRT SOFTWARE CORRELATOR BACKEND (GSB) :
Two Operating modes of S/W back-end :
- Real-time data acquisition + writing to disks off-line read-back of recorded data and
- computation Real-time data acquisition + computation.
Basic Methodology :
- Run synchronous sampling on all 8 ADC boards (32 antennas) – 16/32 MHz BW.
- Transfer data from ADC board to CPU unit (via interrupt driven DMA)in large blocks (32 MB block size --> 8 MB per antenna).
- For recording mode, synchronous write to disk locally at each node.
- For correlations, distribute data from all antennas (using time division multiplexing) to all nodes -- each node handles 1/8 time slice from each block.
- Carry out FFT, fringe stop, MAC and other required operations at each node.
- Record integrated visibilities results to local disk on each node, or send them to collector nodes.
Required Specifications :
- Input data rate :32/64 Mega samples/sec per antenna --> 32 MB/sec (for 8-bit 16 MHz / 4-bit 32MHz).
- Computational Requirements : FFT ops per node ~ 4.3 GFLOPS , MAC ops per node ~ 8 GFLOPS.
- Networking requirement : 32 MB to be transferred to and from each node in ~ 251 msec --> ~ 230 MB/sec bidirectional data transfer speed for each node.
Node configuration :
- Dual core, dual processor Intel Xeon CPUs
- 2 GB RAM, 1 TB SATA RAID storage
- Dual Gigabit Ethernet ports
- 8-bit, 4 Channel, 100 MSPS, PCI-X compliant ADC card
- Data transfer from ADC card to memory using DMA @ 145 MB/s