The VLA's intermediate-frequency transmission system and correlator were designed with a maximum total bandwidth of 200 MHz. This limits the sensitivity, particularly at frequencies above 2 GHz, where wider bandwidths are permitted by the antennas, the feeds, and the level of interference. Fiber optic signal transmission, combined with a new correlator, can increase the maximum total bandwidth to 16 GHz, enabling a huge improvement in the VLA's capability in many astrophysical arenas. Similarly, the VLA correlator is based on a custom ECL circuit that was the state of the art in the 1970's. The correlator limits the number of frequency channels and frequency resolution in ways that are particularly onerous for spectral studies at high frequencies, and for wide-field surveys at low frequencies.
Modern designs allow much greater spectral resolution, wider bandwidths, and increased flexibility.
We could also independently and simultaneously observe multiple spectral transitions in one or two frequency bands. An expanded correlator would let us exploit the sensitivity of broad-band systems to image wide fields of view in the continuum, by using bandwidth synthesis to image nearby galaxies and to inventory their stellar and interstellar emissions with less bias toward the brightest or most compact features. The VLA antennas can be used up to roughly 50 GHz, and we have experience with holography and reference pointing to optimize their high-frequency performance. These techniques justify outfitting the VLA more fully for frequencies above 20 GHz, where we can explore thermal processes around galactic stars, image the structures of protoplanetary disks, detect young supernovae in nearby galaxies, and study CO and other molecules in galaxies at high redshifts. The VLA's maximum baseline of 35 km sets an angular resolution limit at any frequency.