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Wideband VLBI Technology

In addition to the Mark5B VLBI recording system, Haystack Observatory is pursuing a program to develop a new wideband digital VLBI backend. These digital backends (DBE) make use of hardware fabricated by the UC Berkeley Space Science Labs, consisting of a dual 1Gsamp/sec Analog to Digital stage, followed by a digital processing stage that channelizes and formats the data to comply with the VLBI Standard Interface specifications. The heart of the DBE is a poly-phase filter bank (PFB) that splits input bandpasses up to 512MHz wide into 16 channels with exceptionally flat filter responses and sharp spectral cutoffs. The prototype DBE currently under construction will be capable of sampling 2 input analog channels, each 512MHz wide and outputting two streams of VLBI-formatted data, each at 2Gbits/sec, for a total of 4Gbits/sec aggregate rate. The cost of this new system will be ~1/20th of the analog backends currently in use and will lower the bar for new facilities and telescopes to become parts of VLBI arrays. For example, the current sustainable rate at the VLBA (Very Long Baseline Array) is 128Megabits/sec, and the new low cost systems will yield an increase in sensitivity of nearly a factor of 6. The current prototype is contained in a chassis the size of an average PC, but replaces a full sized rack of analog electronics used in current Mark4 Data Acquisition Systems.

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full size image

Simplified block diagram of the Digital Backend for VLBI. Two 512MHz wide analog IF channels are sampled by a dual 1Gb/s ADC, then fed into a FPGA based Poly Phase Filter Bank stage, which channelizes the data stream into 16 channels. These data are then decimated to 2-bits and clocked out on a VLBI Standard Interface.

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Channel filter characteristics from full simulation of proposed PFB design for VLBI

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