[evlatests] Numerology and a historical perspective

[Barry Clark]

Pursuant to the email message below, we have an annoyance that the
antenna heartbeat frequency is 19.2 Hz, and the Widar correlator
heartbeat frequency is 100 Hz, so that these time references only line
up every 1.25 seconds.  I am convinced that this will become an
increasing annoyance as we commission Widar.  Rather than writing
software to handle the case of misalignments, I think we need to take
care of this with a small design change, and that it should be done
at this time.  We should change the heartbeat signal to the L302s
to 20 Hz.  I would like to see this done on all new L305s.  Existing
ones can change when convenient - living with a mixed system will be
less disruptive than continuing the 19.6/100 Hz asynchronisity into the
future.

It is probably least confusing if the MIBs in the LO rack are also
changed to 20 Hz.  The drive for the calibration noise sources must
remain at 9.6 Hz for compatibility with the VLA baseband system while
the VLA correlator is in use, and it is probably better to leave the
frontend rack MIBs at 19.2 Hz.

It is my belief that the software in the L302 MIBs (and other LO rack 
MIBs) will handle a 20 Hz heartbeat transparently, although this needs
to be checked.

For current VLA operation, scripts generated by obs2script synchronize
things with 10 second ticks, which are coincident times of the 20 Hz and
19.2 Hz cycles, so this also should work transparently.

I think that probably the best way to proceed with the Executor software
is to change it for the 20 Hz rate, and to accept the fact that the
antennas with the 19.6 Hz heartbeat may (or may not) have phase jumps
when used with scripts which are not synchronized with 10 seconds.
Alternately, we could put a parameter in the database saying which type
L305 we have, and execute code appropriately.

Not quite the right email list for requesting a design change, but all
the relevant people listen to this one, I think.

Barry Clark wrote:
> The heartbeat frequency at the antennas is 19.2 Hz.  This was chosen,
> rather than 20 Hz, to avoid a simple ratio relationship with the (then)
> ubiquitous 60 Hz.  Heartbeats are synchronized such that a heartbeat
> pulse occurs on an even TAI 10s.
> 
> The L302 synthesizers are driven by a 128 MHz signal.  To be synchronized
> reliably, they must be commanded to sync at integer number of cycles of
> this rail.  This occurs on every third cycle of the 19.2 Hz, every 
> 0.15625 seconds.  The Executor software selects appropriate heartbeat
> pulses to synchronize the L302s.
> 
> The VLA correlator is commanded every 10 seconds, with commands taking 
> effect on the even TAI 10 seconds.  EVLA scripts are in UTC; the .obs
> files from which they are (usually) derived are in LST.  When making 
> scripts for the EVLA Executor, obs2script caters to that peculiarity
> of the correlator, forcing source changes to occur on the TAI 10s
> (with a few interesting exceptions).
> 
> Widar has its own numerology.  Its heartbeat is 100 Hz, anchored to UTC.
> When we run hand edited scripts that are not synchronized to 10s TAI,
> we see phase jumps, almost certainly due to improper truncating when 
> these various periods do not line up.
> 
> Possible fixes:
> 
> 1.  Require observations to be synchronized to the shortest common multiple
> of the intervals.  This is 1.25 seconds.  Problem:  this is too long for 
> OTF mosaicing.
> 
> 2.  Change the antenna heartbeat; make the 19.2 Hz change to 20 Hz on 
> command.  Problem:  the L302s are sensitive to the line lengths of the 
> heartbeat signal.  Problem:  getting the L302s to work reliably again 
> after changing to 20 Hz might be traumatic.
> 
> 3.  Software fix in Executor; the commands to L302s could be synchronized
> differently from those to Widar.  Problem:  something will be doing the
> wrong thing for a few milliseconds at source changes.
> 
> 4.  Software fix in Widar.  The information to correct the phases is 
> available in the message to Widar.  Problem:  as above,  something will 
> be doing the wrong thing for a few milliseconds at source changes.
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