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P. E. Dewdney, B. R. Carlson (HIA, National Research Council)
The correlator is a key component of the Expanded Very Large Array (EVLA), enabling greatly increased sensitivity, numbers of spectral channels, high tolerance to man-made radio frequency interference, and observational flexibility.
Interference robustness provided by high spectral resolution and dynamic range enables key observations -- for example, the red-shifted hydrogen line (HI) at decimeter wavelengths will not be in a protected band. At cm wavelengths, the correlator's 16 GHz of instantaneous processing bandwidth will enable sensitive, extinction-free measures of synchrotron and free-free emission in distant galaxies. Many channels and flexibility will be needed for simultaneous observations of recombination lines in the famously obscured Galactic Center. In the same region, the correlator's spectral resolution will permit the direct measurement of magnetic fields in molecular, and possibly ionized gas, through Zeeman splitting of narrow spectral lines. And finally, the reconfigurability of the correlator will enable rapid dynamic scheduling of the VLA, greatly increasing its ability to respond to transient events.
The correlator will process all four polarization products from 40 antennas at full bandwidth. More antennas and/or more spectral channels can be processed at lower bandwidths. 16384 spectral channels can be distributed across all eight 2 GHz IF-bands (typically four left and four right polarizations) or concentrated in narrow sub-bands. Spectral flexibility is provided by sub-bands with individual center frequencies and spectral resolutions. There is also fast dump capability (~10 ms), multiple overlapping sub-arrays, on-the-fly re-configuration within seconds, multiple simultaneous phase centers, digital phased-antenna sums for VLBI, specialized processing for pulsar observations and searches, and independent sky frequencies for each IF-band. Four-bit sampling implies almost no loss of sensitivity due to digitization and confers high spectral dynamic range. An innovative design restricts the generation of harmonics and aliased products of interference.