Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. VLBI digital backend is a wideband data acquisition system with dedicated real time data processing algorithms implemented and installed in each VLBI station. The VLBI institutes in American. Europe, Japan and China have start to develop VLBI digital backends in 2000s. The first VLBI digital backend named Chinese VLBI Data Acquisition System (CDAS) developed by Shanghai Astronomical Observatory (SHAO) was finished in 2009. It has four intermediate frequency (IF) input channels with 512 MHz bandwidth for each. After processing, the output data rate is up to about 2048 Mbps. CDAS has begun to serve for Chinese Lunar Project in Chinese VLBI Network (CVN) stations in 2010. Since 2013, another type of VLBI digital backend has been designed in order to meet the requirement of VLBI Global Observation System (VGOS), it was named CDAS2. Comparing with CDAS, CDAS2 not only doubles the number of input channels but also makes the 10-Gigabit Ethernet (10GE) interface available. Due to the different algorithms implemented, the output data rate of CDAS2 is up to about 16384 Mbps. It will be installed in Sheshan VGOS station this year. Furthermore. CDAS3 has been put on the schedule. It will double the bandwidth of each IF channel and the maximum data rate of output, comparing with CDAS2. It is prepared for the next stage of VGOS, which need 1024 MHz bandwidth input for each channel and 32768 Mbps maximum data rate output. In this article, we will introduce these digital backends in detail including the hardware and algorithm. And also comparisons will be made.
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