[mmaimcal]Re: Phase Repeatability of the Attenuators

[Larry D'Addario]

Mark,

I have several comments on your notes.

First, I hope you realize that far more than an attenuator setting
changes when we switch bands.  The requirement you suggest for the
electronics applies to the whole system, not just an attenuator.  In
particular, both of the tunable LOs (1st and 2nd) need to be reset,
and various switch positions need to be changed.  The LO that's doing
phase tracking ("fringe rotation"), probably the 1st LO, needs to be
programmed very differently at each sky frequency.  All of these need
to be highly repeatable.  We describe this by saying that if the
observing frequency changes from f1 to f2 and then back to f1, the
instrumental complex gain should be the same as it would have been had
we stayed at f1 the whole time.

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!

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.

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.

Regards,
Larry