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VLBA Implementation of the ROACH Digital Backend

VLBA Implementation of the ROACH Digital Backend. Jonathan Romney on behalf of the VLBA Upgrade development team NRAO / Socorro First International VLBI Technology Workshop Haystack Observatory 2012 October 23. Context. RDBE: Central Element of VLBA Sensitivity Upgrade

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VLBA Implementation of the ROACH Digital Backend

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  1. VLBA Implementation of the ROACH Digital Backend • Jonathan Romney • on behalf of the VLBA Upgrade development team • NRAO / Socorro • First International VLBI Technology Workshop • Haystack Observatory • 2012 October 23

  2. Context • RDBE: Central Element of VLBA Sensitivity Upgrade • Original goal (April 2007): “4 Gbps by 2011”. • Almost made the date • Routine scientific observations started 2012 / 2 / 19. • BUT only at 2 Gb/s • Instrumentation was designed for 4 Gb/s. • Expansion 2 => 4 Gbps “only requires additional funding”. • Developed in Various Collaborations • NRAO with KAT and CASPER to develop Roach (1) board. • NRAO with Haystack on firmware for Xilinx Virtex-5 FPGA. • First test 2010 / 7 / 8: “What could have gone wrong?” (AEN). • Operational for wideband continuum observations: 2012 / 2 / 19. 1st Int'l VLBI Technology Workshop

  3. Dual-RDBE Configuration • Planned from the beginning ... • to support VLBA's four-IF modes and 4-Gb/s data rate. • Additional, more recent and essential requirement • Essential for 8-channel spectroscopic observations with DDC. • Currently using only single RDBE • Installation of second set recently completed. • Software-based network switches bought from XCube • Sequence two packet streams to record on one Mark 5C unit. 1st Int'l VLBI Technology Workshop

  4. PFB (polyphasefilterbank) FPGA Personality • Developed by Haystack • Has become our workhorse for ongoing scientific programs • Used in 108 observations since 2012 / 2 / 19, through 2012 / 9 / 30, a total of 900 array-hours, about 17% of full time, 33% of obstime. • Fixed configuration • 16 x 32-MHz sub-bands. • Selectable among total of 30 at 32-MHz steps in both IF inputs. • Samples in second Nyquist zone, at 1024-MHz sample clock. • Requantizedafter filtering to 2-bit samples for recording. • Significantly inconvenient in VLBA application • Sub-bands fixed in IF frequency mismatched to VLBA's (current) inflexible LO generator (500*n ± 100 MHz). • Still using PFBG_1.4; have not upgraded (yet) to 1.4.1 • Fixing fourth-channel phase flip not worth down time to validate. 1st Int'l VLBI Technology Workshop

  5. DDC (digital downconverter) FPGA Personality • Developed by NRAO — Still in commissioning • Basic specifications • 1, 2, or 4 sub-bands, selectable between two IF inputs. • Bandwidths 128 / 64 / 32 / ... / 1 MHz. • DiFXspectral zoom may obviate need for narrowest bandwidths. • Sideband selectable relative to band edge. • Sub-bands may not cross IF zone boundaries at 640 and 896 MHz. • Fine band-edge tuning possible, in ~59-mHz steps. • But prefer 15.625 kHz for integral-Hz operation ... • Or 250 kHz for compatibility with 10-kHz “legacy” systems. • Requantized after filtering to 2-bit samples, more possible eventually. • Additional features • Advanced requantizer level-setting algorithm. • Multi-thread VDIF output formatter, required for use with switch. 1st Int'l VLBI Technology Workshop

  6. Control Software forVLBA RDBE • Extension of EVLA Control System • Facilitated rapid implementation. • Provided well-understood monitoring/logging system. • Integrated system; also supports Mark5C (via DRS) and 4x4 IF matrix switch. • BUT Not Portable • Use requires availability of many parts of EVLA system, including monitor database. • Thus, NRAO not participating in compatibility testing this week. 1st Int'l VLBI Technology Workshop

  7. Aside on Bandpass Shapes • Bandpass Rationalization • Bandpass shapes seen in RDBE data always appear so ugly and variable. 1st Int'l VLBI Technology Workshop

  8. IF 1 IF 4 1st Int'l VLBI Technology Workshop

  9. Aside on Bandpass Shapes • Bandpass Rationalization • Bandpass shapes seen in RDBEdata always appear so ugly and variable. • We expect digital filters to have a deterministic shape. • Usual hand-waving explanation: “The shape of the underlying instrumental bands, at RF and IF, is showing through.” • Sounds reasonable, but is it true? • An early test with the PFB personality confirmed this concept. • Eight contiguous dual-polar sub-bands were set at three different 32-MHz offsets. • Each of six RF ranges appeared in three different sub-bands, i.e., AIPS “IF”s. 1st Int'l VLBI Technology Workshop

  10. 8256 MHz 8288 MHz 1st Int'l VLBI Technology Workshop

  11. 8320 MHz 8352 MHz 1st Int'l VLBI Technology Workshop

  12. 8384 MHz 8416 MHz 1st Int'l VLBI Technology Workshop

  13. Thank you 1st Int'l VLBI Technology Workshop

  14. “Say It Ain’t So, Babe” • Disheartening recommendation by NSF Portfolio Review • that NSF “divest” the VLBA • We don’t know yet exactly how NSF will interpret “divest”. • We continue to seek new funding partners and to reduce operational costs. • We are investigating whether e-VLBI could be more cost effective than disk transport. • The overall capabilities of the VLBA are likely to be reduced. • Some VLBA antennas may be closed. 1st Int'l VLBI Technology Workshop

  15. Los Alamos 1st Int'l VLBI Technology Workshop

  16. North Liberty 1st Int'l VLBI Technology Workshop

  17. Hancock 1st Int'l VLBI Technology Workshop

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