Operational status of the EVN MkIV Data Processor

The EVN MkIV data processor at JIVE has been operational since September 1999. Development is of course a continual process. The most recent milestone has been the operational status of recirculation, a means of time-sharing correlator resources for observations using less than the maximum 32 MHz sampling rate (Nyquist-sampled 16 MHz BBC filters); this provides improved spectral resolution, with narrow-band experiments having 9 or more stations being the greatest beneficiaries. A great deal of work has been devoted to making real-time e-VLBI an operational capability; and the number of regular and Target-of-Opportunity e-VLBI observations has increased dramatically since 2006/7.

The correlator can input data recorded at rates from 32 to 1024 Mbps per station, up to a maximum of 16 stations at a single pass. The total correlator capacity is a quarter million complex lags, and the entire correlator can be read-out in 0.25 second. There are thus two limitations to the correlator capacity, the first is the total number of lags available, the second is the read-out capacity.

The first restriction stems from the physical size of the correlator -- there are only a finite number of lags, and hence frequency points, available. This is a function of how many stations, subbands, and polarizations are correlated. If you correlate only 1 SB and 1 polarization, then you can get 512 spectral points with 9-16 stations or 2048 spectral points with 8 or fewer stations. If you correlate >1 SB and/or >1 polarization, then you must reduce the above numbers by the product (N_sb * N_pol). In the case that indidivual subband bandwidths are less than 16 MHz, then recirculation can be used to increase the number of spectral points by a factor up to 16/BW_sb (where the SB bandwidth is in MHz), with a maximum factor of 8 (the combination of recirculation + oversampling is currently unavailable). A maximum of 2048 spectral points applies per baseline/subband/polarization in any case. If the number of spectral points is still not sufficient, it may be possible to run multiple correlator passes (e.g., separate line/continuum passes, where the line pass comprises only 1 subband at the highest possible spectral resolution) if the PC approves the increased correlator load, so make sure to request this at proposal time.

The other restriction is on output capacity. The entire correlator can be dumped at 0.25 second intervals, setting the minimum integration time. For some configurations that use less than half the correlator, integration times of 0.125 second seem possible. If recirculation is used, the minimum integration time would increase by the recirculation factor used. The growth rate of resulting FITS files given the maximum correlator output rate is on the order of 7-12 GB per hour of observation.

Correlation of experiments with more than 16 telescopes is possible in multiple passes. Such data would be merged off-line into a single set of FITS files.

Status of the JIVE Software Correlator, SFXC

Development of the software correlator SFXC at JIVE began as part of the FP6 EXPReS joint-research activity FABRIC, and has continued under the related project SCARIe funded by the Dutch national science agency (NWO).

Correlation and output capacities of SFXC are conceptually simpler than for the EVN MkIV Data Processor because it lacks the specific hardware limitations of the station units and correlator-board architecture. The anticipated advantages of SFXC would lie principally in those areas beyond the operating capabilities of the MkIV Data Processor: correlating data at total rates above 1Gbps in a single pass, handling sampling rates higher than 32Mbps in a subband, achieving more than 2048 frequency points per baseline/subband/polarization, using integration times under 1/4 sec, and applying gating/binning to pulsar observations.

People requiring detailed information on the capabilities of the EVN MkIV data processor at JIVE are advised to contact Bob Campbell before submission of their proposal.

A summary of the correlator capabilities (present and planned) can be found in the Operational & Development Status Sheet.