During the Zpectrometer's first "shared risk" observations this past winter, a baseline ripple was detected that had an amplitude of a few mK and a width of about 300 MHz. After extensive tests on the telescope, it was determined that this baseline ripple originated with the Ka-band receiver. This receiver was removed from the GBT to the lab in April to determine the cause of the ripple and to mitigate the generation of the baselines if possible. The first downconversion mixers were found to be the source of the baseline ripple, which arose from a high thermal sensitivity of these components. The mixers' thermal mass was increased by attaching metal blocks to the mixer bodies and was thermally isolated from the rest of the receiver by removing thermal shunts and encasing in insulating material. Additionally, the thermal insulation of the entire receiver was increased.
Based on laboratory tests, significant improvements have been made in the baseline performance, but definitive new observing results on the GBT were not available at press time. As results become available, information will be posted on the Ka-band receiver web page at http://wikio.nrao.edu/bin/view/GB/Projects/SpectralBaselineObsLogs090402.