[mmaimcal] ImCal Meeting WEDS 22 Feb

[Darrel Emerson]

To:  ImCal group

Here's a little more on the background to the question on
frequency changes, which is on the agenda for Wednesday's meeting.  It's
the draft abstract of a protomemo on the subject.

================================

"ALMA LO Timing Considerations

Abstract:
For a connected-element interferometer, in order to retain coherence the total 
signal propagation time, from the source to each antenna and then to the 
correlation process, must to be equalized.  However, for a fixed frequency of 
operation, there is no need to equalize the local oscillator propagation time 
from the central technical building to each antenna, provided the phase of all 
local oscillators at the receiver can be kept constant, and can be calibrated.
    At Chajnantor, there may be a difference of 10 km or more in the lengths of 
fiber optic cables, carrying local oscillator and timing signals, to different 
antennas. The consequence of this differential delay of ~50 microseconds, if 
uncorrected, is that very small changes of frequency will result in significant 
differential phase changes in some antenna pairs. If the delays are known, this 
can be allowed for in software, either in real time or in post observation data 
reduction.  The precision to which the delays are known sets the limit on how 
great a local oscillator frequency change can be tolerated without independent 
calibration.  Equivalently, the scientific requirement for magnitude of local 
oscillator frequency changes, without carrying out an independent phase 
calibration, sets an additional engineering requirement on stability, and
sets an operational requirement on the measurement precision of the LO 
propagation delay within, and to, each antenna."

================================

            Cheers,
                    Darrel.


Al Wootten wrote:
> Folks,
> 
> Recall that collisions of our usual time with a PSI meeting and with the
> ASAC telecon have prompted us to move our meeting to tomorrow, which some of us
> will remember as Washington's REAL birthday.
> 
> Date: 22 February 2006
> Time: 4:00 pm EST (6:00 pm Santiago; 2:00 pm Socorro, 2:00 pm Tucson)
> Phone: Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor). No Video planned.
> Past minutes, etc on MMA Imaging and Calibration Division Page
> 
> News
> The AOS TB Shell Arises The AOS TB Shell Arises: 15 Feb
> 
>     * Science Requirements -- The revised document is located in the Science area.
>           o Subarrays:
>     * Characterization -- Move to Ops?
>           o 19 applications for 2nd ALMA postdoc. Status report.
>           o Science IPT Q3 Q4 2005 Quarterly Report
> 
> Topics
> 
>     * SRR Topics
>           o Date, venue of followup
>           o AI #18 from your report of the July 2005 SRR says:18 OPEN Sci, SE -> FE DS 2006-02-17 70 "Check need for requirement on -3dB freqs.
>           o QUESTION: How big a frequency change of ANY of the local oscillators will we ever want, where we want to refer the interferometric phase of the new frequency to that of the original frequency? An equivalent question is how big a frequency change will we ever want, without having to recalibrate the phase?
>                 + Should our spec be "we want the phase to be reproducible within 0.5 degrees (??) for any frequency change of any LO, but we want the phase change to be computable without further calibration, to the same tolerance, for any frequency change up to [X] GHz."
> 
> Please think about the Question up for discussion.  Peter, Dick and Clint may
> join the discussion.
> 
> Clear skies,
> Al
> _______________________________________________
> mmaimcal mailing list
> mmaimcal at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/mmaimcal