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

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop2 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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop3 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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop4 PFB (polyphase filterbank) 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 obs time. 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. Requantized after 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 Fixing fourth-channel phase flip not worth down time to validate.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop5 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. DiFX spectral 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 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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop6 Control Software for VLBA 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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop7 Aside on Bandpass Shapes Bandpass Rationalization Bandpass shapes seen in RDBE data always appear so ugly and variable.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop8 IF 1IF 4

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop9 Aside on Bandpass Shapes Bandpass Rationalization Bandpass shapes seen in RDBE data 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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop MHz8288 MHz

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop MHz8352 MHz

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop MHz8416 MHz

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop13 Thank you

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop14 “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.

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop15 Los Alamos

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop16 North Liberty

2012 / 10 / 23 – jdr1st Int'l VLBI Technology Workshop17 Hancock