[evlatests] 10-second tuning failures at X-band

[Rick Perley]

     The calibrator survey run, taken last October, has revealed the 
continued presence of short-lived tuning failures.  The characteristics 
are quite simple:

     1) No fringes at the beginning of a scan, lasting exactly 10 
seconds.  After this interval, full-valued fringes abruptly occur. 
Hence, this is not a pointing problem.

     2) Roughly 20% of scans are affected, nearly always a single 
antenna, occasionally two.  This seems consistent with a random 
process.  There was no clear tendency for any particular antenna to fail 
more often.

     3) The four IFs failed independently, with equally likelihood -- 
failures are always on a single IF.

     4) So far as I can determine, this problem only occurs at X-band.  
(I have checked P, L, S, and C, so far).

     5) The on-line flagging system (unsurprisingly) does not flag these 
failures.

     These X-band observations followed X-band referenced pointing, 
which utilized a different tuning and correlator configuration.

     There was also a single incidence (out of 182 scans) where the 
*last* 10 seconds abruptly lost fringe power.