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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

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
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

dual rdbe configuration
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

pfb polyphase filterbank fpga personality
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

ddc digital downconverter fpga personality
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

control software for vlba rdbe
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

aside on bandpass shapes
Aside on Bandpass Shapes
  • Bandpass Rationalization
    • Bandpass shapes seen in RDBE data always appear so ugly and variable.

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slide8

IF 1

IF 4

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aside on bandpass shapes1
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.

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slide10

8256 MHz

8288 MHz

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slide11

8320 MHz

8352 MHz

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slide12

8384 MHz

8416 MHz

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slide13

Thank you

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say it ain t so babe
“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

los alamos
Los Alamos

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north liberty
North Liberty

1st Int\'l VLBI Technology Workshop

hancock
Hancock

1st Int\'l VLBI Technology Workshop

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