A Wide-Band VLBI Digital Backend System Alan Whitney Shep Doeleman Brian Fanous Hans Hinteregger Alan Rogers MIT Haystack Observatory 10 Jan 2006 IVS General Meeting Concepion, Chile

Current VLBI backend system

Drawbacks of current analog backend Fixed IF-frequency input range –Many modern RF-IF systems employ IF frequency ranges that extend as high as GHz –Often requires custom electronics at each site to accommodate –High cost Each BBC module costs ~$10-25K; formatter cost is $10-45K Non-uniformity of channel bandpasses –Analog-filter variations of up to ~2.5% can produce phase errors of several degrees Lack of affordable expandability Physical bulk –Current system requires nearly a full rack of equipment –Fragile and expensive to ship

Advantages of DBE Uniformity and repeatability of channel bandpasses is guaranteed Unaffected by environmental factors (temperature, line voltage, etc) Low cost –Expected to be <10% of the cost of the BBC’s that are replaced Economical expandability Easy transportability –Entire unit will fit in a small chassis

DBE development at Haystack DBE design is based on Polyphase Filter Bank (PFB) concept DBE being developed collaboratively between Haystack and UC Berkeley Space Sciences Laboratory in support of an NSF- sponsored program for ultra-wideband VLBI Hardware is based on a flexible FPGA-based signal-processing board from UC Berkeley (“iBOB”) VSI-H connector is mounted directly on board for interfacing to Mark 5B FPGA design is being done using high-level design tools developed at Berkeley Prototype DBE system is now under test at Haystack

Example of PFB technique for VLBI

PFB channel-filter characteristics

DBE implementation on iBOB prototype

Prototype iBOB with two sampler boards attached

DBE lab setup at Haystack

Current status and plans Operation of the DBE hardware at 1024MHz clock speed has been achieved Design is developing rapidly with collaborative effort between Haystack and SSL A 1024/2048 MHz sample-clock board with very low jitter and <1 psec/degC delay drift is being developed to provide a sample clock to the iBOB board Expect fully working DBE prototype by 1 Feb 06 Plan to use in 3-station mm-wavelength VLBI experiment in Mar-Apr 06 with Mark 5B+; goal is 4 Gbps/station PFB concept is easily adaptable to geodetic VLBI by selecting specific output channels over a very broad frequency range; this will be pursued as part of the VLBI2010 development effort Cost of complete DBE (samplers and iBOB) is <$5K

Flexible “Front-End Converter” for DBE (proposed development) Problem: Different systems need different IF frequency ranges –Even Mark 4 and VLBA systems use different IF ranges –Many modern RF/IF systems, particularly those at mm wavelengths, employ IF frequency ranges as high as GHz –Often requires custom electronics at each such VLBI site Proposal is to couple a flexible dual-polarization IF frequency converter to the DBE for easy adaptability to almost any existing RF/IF system

Concept for DBE “front-end conversion” system + + Front-end conversion DBEData system

Block diagram of flexible IF converter

Conclusions DBE offers many advantages over old analog systems DBE development for VLBI is moving rapidly and will be tested in real experiments within next few months Proposed flexible “front-end conversion” system will add much flexibility for connection DBE is one of the necessary technologies that must be applied in order to realize the goals of VLBI2010

Example of PFB technique for VLBI