[mmaimcal]Re: Phase Repeatability of the Attenuators

[Bryan Butler]

some comments on larry's comments...

On 2002.08.07 11:45 Larry D'Addario wrote:
> Next, I wonder where the 20d accuracy spec comes from.  We have been
> working with the far tighter goal set forth in the Project Book
> (section 7.0.6) of visibility calibration to 0.1 radian at 950 GHz.
> Allocating 1/sqrt2 of this to each antenna, and allocating this among
> antenna structure, electronics, and atmosphere, we are left with only
> 2.4d (6.9 fsec) of systematic phase error for one antenna's
> electronics, and 2.9d (8.4 fsec) for the "atmosphere."  The latter is
> intended to be achieved using all available correction techniques,
> including fast switching, and it must include the two items you
> mention: thermal noise in the calibrator observation and the
> difference in atmospheric delay between calibrator and target.
> Perhaps the goal is grossly unrealistic, but it is what ALMA has
> officially adopted.  You have suggested a much looser goal, but you
> have allocated a far smaller fraction of the total to electronics;
> nevertheless, your result for electronics (6.4d) is looser than the
> official goal (2.4d).  The designers would be happy to accept such a
> change!

this should not be taken as a project endorsement of such a loosening
of spec.  mark was just calculating 'back of the envelope', i think,
and probably should be using the tighter spec.


> Third, I want to point out that fast phase switching (with a frequency
> change) provides *no* information about the instrumental phase at the
> target's observing frequency, so it is not a complete phase
> calibration scheme.  An additional calibration observation must be
> made with exactly the same instrumental setup as was used on the
> target source.  Presumably this can be done less frequently, and with
> longer integrating time.  But its error must also be part of the phase
> calibration error budget.  Any instrumental phase variation over this
> longer time interval is part of that error.

in fact, this can be done once, and only once, per observation, right?
you would probably want to do it twice, to be safe, but otherwise
there is no reason that the phase offset between the two bands cannot
be just measured once.  i.e., it doesn't have to be measured every
10 mins (or 30 mins, or 60 mins, or whatever), but rather just once.
or am i missing something?


> You make a good point about the scaling from 90 GHz to 950 GHz.  The
> official goal is written only for 950 GHz, on the assumption that
> performance will be somewhat better at lower frequencies, but with no
> requirement on how much better.  You point out that it ought to be
> *proportionally* better if it is used to calibrate higher frequency
> measurements.  While the errors in some elements of the 1st LO system
> will scale in this way, other parts of the system will scale
> differently and some will have systematic phase errors that are
> independent of the observing frequency.  You are correct to assume
> that variable attenuators are in the latter category.  The overall
> situation is rather complicated.  In some areas, it could imply a
> tightening of requirements relative to our current specs, in spite of
> the fact that you are suggesting a loosening of the overall
> high-frequency accuracy goal.  Variable attenuators are one example
> of this, but there are many others.

yes, it's a very complicated system.  as far as i know, we have no
explicit treatment of where *all* of the gain (amplitude and phase)
errors come from in the system.  is that right?  do we even have a
design that is final enough that such a treatment could be attempted?


	-bryan