[evlatests] [Fwd: Re: Numerology and a historical perspective]
Jim Jackson
jjackson at nrao.edu
Thu Feb 26 17:51:51 EST 2009
Thanks Barry. We'll get Leon and Matt working on this. If we can add
the ability to switch at the same time we add the new DTS timecode,
this should be fairly painless. We'll set one bit to switch between
9.6/10 Hz to the noise diodes and 19.2/20Hz to MIBs and
synthesizers. I think the new timecode to the DTS is independent of
these clocks.
Cheers,
Jim
At 03:41 PM 2/26/2009, Barry Clark wrote:
>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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> >> 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.
> >>>> _______________________________________________
> >>>> evlatests mailing list
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> >>>> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
> >>>>
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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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