[evlatests] Hysteresis in Tuning failures

[Rick Perley]

    An extraordinary phenomenon has been noted. 

    In the Cygnus A observations taken last Friday, I noted that antenna 
9, in IFs A and C, consistently failed to tune to 2500.0 MHz (in OSRO 
mode 1 with 128 MHz subband width) -- but *only* for the scans on Cygnus 
A.  Observations of the nearby calibrator J2007+4029 always fringed.  
How can the LOs 'know' the antenna is on Cygnus A, rather than the 
calibrator?

    It seems the answer depends on what was observed on the previous 
scan.  In all cases, the observation of J2007+4029 at S-band was 
preceded by an observation (on that same source) at X-band (at 8100.0 
MHz).  But for all observations of Cygnus A at S-band, the preceding 
observation was at C-band (at 4500.0 MHz). 

    It would be very good to understand and eliminate this phenomenon.