[evlatests] [Fwd: Re: Numerology and a historical perspective]
Barry Clark
bclark at nrao.edu
Thu Feb 26 17:41:32 EST 2009
It is probably too messy to try to do anything about this before
the VLA correlator is turned off. Although it would be nice to
have a switch that we could throw at that time, it is _believed_
that the software consequences are pretty localized, and that a
mixed system would be acceptable, so that you could just cycle
things through, starting at that time. And yes, we should have
a 10 Hz noise diode at that time as well.
Jim Jackson wrote:
> Any more thoughts on this? Leon just asked me about it. I must admit, I
> had totally forgot this.
>
> Our original thought about doing this had been to add a command point to
> the L305 MIB that selected between two timing standards. From all of
> this discussion, I guess those would be 1) all timing based on 19.2 Hz
> or 2) all timing based on 20.0 Hz. I think this would be a fairly
> straight forward change to the L305. Would the noise diode switching
> want to be 10Hz, 20 Hz, or something else in this scheme? Note: if we
> implement a dual scheme during the transistion, you are effectively
> resyncing the antenna every time you switch between the two timing
> standards. Currently that affects delays, though if we wait until the
> DTS module update, that problem should go away.
>
> On that note, we only want to make Xilinx firmware changes to the L305
> module once - it requires opening up every module. That means adding the
> new timecode for the updated DTS modules at the same time. This would
> have to wait to be done at the same time we install the updated DTS
> modules with the 3-bit digitizers. Waiting for this upgrade also takes
> care of the changing delay issue. I really don't want to spend the time
> and effort to make an extra upgrade to the L305 before the 3-bit
> digitizer rollout.
>
> Also, it does not seem practical to me to use mixed timing modes - ie
> some outputs at 19.2 and some at 20.
>
> Jim
>
> At 02:31 PM 1/9/2009, Barry Clark wrote:
>> Ray is right - I had totally spaced out. We need the 19.6 Hz
>> to run the phase switching. Is it possible to install a switch
>> to change the frequency?
>>
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>> From: Ray Ferraro <rferraro at nrao.edu>
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>> To: Barry Clark <bclark at nrao.edu>
>> Subject: Re: [evlatests] Numerology and a historical perspective
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>> Barry,
>> I would just like to point out that to be compatible with the
>> VLA correlator phase switching at the antenna must occur at the 19.2
>> Hz rate. I'm not sure where this is done, but I would guess the L302 DDS.
>> Ray
>> Barry Clark wrote:
>>> 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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>>
>> --
>> Ray Ferraro
>> National Radio Astronomy Observatory
>> Array Operations Center
>> P.O. Box O
>> 1003 Lopezville Road
>> Socorro, NM 87801-0387
>> 575-835-7196
>> fax: 575-835-7027
>>
>>
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