Increased noise at 12.5 MHz bandwidth spectral line


In recent 12.5 MHz bandwidth spectral line data the noise level is too high by a factor between 2 and 3. The only exception are observations at mode PA, where the cross-hand pairs show the expected noise, but parallel hand data show the increased noise. This occurs at all bands and affects all baselines, whether formed by VLA antennas, EVLA antennas, or either.

When did this start?

We are aware of data taken as recently as December 2006 which do not show this increased noise, so the problem must have started after that. This appears to exonerate the new correlator controller which was deployed in September, 2006.

Current situation

Initial tests appeared to indicate that the problem goes away when the so-called correlator self-test is turned off. We have been unable, however, to confirm the results of those tests: the increased noise remains regardless of whether self-test is on or off. Other bandwidths do not show this behavior; we therefore strongly suggest observers avoid observing at 12.5 MHz bandwidth.

Since the problem is unrelated to the type of antenna (VLA or EVLA), we believe this problem is related to the VLA correlator, and will go away once the WIDAR correlator is deployed.

Modified on Tuesday, 29-Jan-2013 13:57:29 MST by Gustaaf van Moorsel