The VLA receivers have been upgraded gradually since the early 1980s. Initially, better low-noise amplifiers were used in existing receivers. More recent systems have used the VLBA design, in which the receiver is attached directly to the feed and the polarizer is cooled in the cryogenic dewar. This design reduces the noise contribution from the polarizer and eliminates long, ambient temperature waveguide runs that added to the system temperature.
The ``VLBA-style'' receivers are now used at 1.4, 8.4, and 40-50 GHz. These receivers will likely remain, with the only changes being new, modern amplifiers, and improved, wider-band polarizers and feed horns. The greatest improvement in system temperature can be made in the 5, 15, and 23 GHz bands using the VLBA-style receivers and modern HFET amplifiers. Completely new receivers will be built for these bands, and should reduce the system temperatures by up to a factor of three. The new receivers will also provide much wider bandwidth capability (needed for continuum sensitivity) and will tune over a wider frequency range (to include spectral lines, methanol, whose astrophysical significance was unknown when the VLA was built, and permit observing of redshifted molecular lines from distant galaxies). Current plans call for bandwidth ratios of order 1.5 to 2.0 in all bands.
The improvements at the 23 GHz and 40-50 GHz bands are already funded, and will be completed late in 2001.