WIDAR Correlator Options and Potential Craig Walker NRAO Socorro U.S. VLBI Technical Coordination Meeting May 14-15, 2007

WIDAR CORRELATOR 32 Stations for EVLA 8 GHz X 2 polarizations –8 IFs of 2 GHz each –Sampled at 4 Gsamp/s (wideband) or 2 GHz (narrow band) –2 (8) bit samples for wideband (narrow band) –96 Gbps per station input –Digitally filtered to 128 MHz subbands channels per baseline –Far more with recirculation Designed and built at DRAO in Canada –Paid for by Canada Smaller version being built for eMERLIN

WIDAR FOR VLBI WIDAR designed for VLBI –Option for VSI input –Can handle large delays and fringe rates 4 station cards per EVLA antenna –Each handles a polarization pair of IFs –Each station card could take data from a different station

Old Wiring Scheme In the original design, each 16 GHz EVLA input station could handle: –4 stations at 1 GHz (4 Gbps with 2 bit samples) –2 stations at 4 GHz (16 Gbps) There are 5 extra stations available for VLBI –Stations come in groups of 8 (reason for 32 instead of 27) –20 station, 1 GHz VLBI correlator –10 station, 4 GHz VLBI correlator Operation at VLA. Was very attractive when EVLA2 (NMA) proposed –Would have merged VLA/NMA/VLBA –But that proposal was rejected

New Wiring Scheme New wiring scheme saves on hardware costs –Proposed by DRAO, approved in principle. Review needed. –Stations now come in groups of 4 (could be 28, but going for 32) Multiple VLBI station inputs per EVLA station no longer works. Can’t get all baselines. Options: –Add stations at EVLA, but perhaps with fewer input IFs –Build a mini-WIDAR for the VLBA 16 stations, 4GHz (16 Gbps for 2 bit samples) –Cost in range of $250k - $500k depending on details –A few man years of software effort required above what is already being done for EVLA –Cost is less than savings from new wiring scheme, but not clear if Canada can pay for it.

WIDAR vs Software Correlator WIDAR: –Probably not available until late in EVLA project –Greater capability for the cost when turned on –Capabilities set at construction –Requires 2-4 FTE years of software development for VLBI –Biggest advantage would come with > 4Gbps eVLBI Software correlator (next talk): –Small version available soon (now) –Continuous upgrades natural –Highly flexible –Eventually will be more capable than a long-lived hardware correlator –Costs less than a 30 day disk supply, independent of bandwidth We have not explored FPGA or other options Should we “jump off the cliff” and let DRAO stop thinking about VLBI in the WIDAR?