[evlatests] Antennas 13, 14, 16 and 18 at X-band

[Jim Jackson]

The main 128MHz oscillator in the L350 is phase locked to the 5MHz signal 
directly from the maser. Due to this odd combination of frequencies, the 
internal PLL in this oscillator contains several digital dividers. We do 
not know the exact makeup of these dividers (though we can guess) and do 
not have access to them for synchronization.  If the 5MHz from the maser is 
interrupted, it will cause this oscillator to drop out of lock. When the 
5MHz is restored and the oscillator  re-locks, the ambiguity in these 
dividers almost certainly will cause a phase jump that will affect all EVLA 
antennas with respect to the VLA.

This brings up another thing people need to note - The EVLA LO system is 
currently connected by a single cable directly to a 5MHz output on the back 
of the maser. If the VLA is switched from the maser to a backup reference 
in the VLA master rack, the EVLA system does not automatically follow it.

Jim


At 08:47 AM 3/28/2006, Barry Clark wrote:
> >     No drops and phase oddities were seen, except that all four EVLA
> > antennas jumped 150 degrees in phase
> > (w.r.t the VLA antenna phases) during an 8-minute gap in my observations
> > which occurred around 19:35 IAT.  I didn't get a log from the operator,
> > but received a message about this time saying something was changed.
> > Barry was at the site of the time, perhaps he can provide some insight
> > here.  Otherwise, phase behavior was normal.
>
>They installed an extra battery to ensure that the maser would stay
>powered during today's UPS maintenance.  In the process of doing so,
>they managed to upset the VLA ten second tic.  I'm not quite sure
>why getting it started again makes a phase jump, but I guess I could
>believe it would.
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