Developments of Digital Backend & Software Correlator Systems at NICT Kashima.
Kazuhiro Takefuji, Hiroshi Takeuchi, Moritaka Kimura, Tetsuro Kondo, and Yasuhiro Koyama; NICT, Japan.
In Japan, National Institute of Information and Communications Technology (NICT) has been developing VLBI observation systems and data processing systems since 70fs. The K5 VLBI system is designed with the commodity products such as personal computers, hard disks, and network components. This strategy has been quite successful to develop highly flexible and high performance observation systems and data processing systems for VLBI. As the data acquisition terminals of the K5 system, we have developed two independent series of systems, the K5/VSSP ( K5/VSSP32 ) and the K5/VSI systems. The concept of the K5/VSSP and K5/VSSP32 systems is to develop AD sampling units interfaced to the commodity PC systems by using the PCI expansion bus (VSSP) and USB2.0 interface (VSSP32). Each AD sampling unit can support four analog inputs. Four units are used together to record 16 channels of signals from baseband converter units in the case of usual geodetic VLBI observations. On the other hand, K5/VSI series are realized by high speed AD sampler units and a commodity Linux PC system to record data with the VSI-H (VLBI Standard Interface - Hardware specifications). VSI-H was proposed to define standard interface for the high speed data transfer between data input modules, data transfer modules, and data output modules to improve the compatibility between next generation VLBI observing systems and the correlator systems. Three high speed AD sampler units, ADS1000, ADS2000, and ADS3000, have been developed to support various sampling modes. ADS1000 can sample one baseband channel at the sampling rate of 1024Msps. ADS2000 can sample 16 baseband channels at the sampling rate of 64Msps suitable for observations using the bandwidth synthesis method. ADS3000 can sample wide range of baseband frequency band up to 1024MHz with the sampling rate of 2048Msps. Currently, ADS3000+ is under development to support 4Gbps*1ch and 2Gbps*2ch sampling modes by using faster AD sampler chip. ADS3000 and ADS3000+ are equipped with FPGA chips to realize digital baseband converter (DBBC) with user-selectable bandwidth of 2 ~ 16 MHz. We will present details of the digital backend system mainly focusing the newly designed ADS3000+ system, and high speed software correlators (GICO3[CPU] and GICO4[GPU: now developing]) which can process various data formats recorded by K5, VERA, and Mark-V systems.