ALMA/US Imaging and Calibration Group

Agenda for meeting Tuesday, 20 Mar 2001 at 2:45 pm EST.

Date: 20 Mar 2001

Time: 2:45 pm EST (12:45 pm Socorro, 12:45 pm Tucson)

Phone: (804)972-7268 (CV SoundStation Premier Conference phone 2nd floor).

Past ImCal minutes, etc on MMA Imaging and Calibration Division Page

ALMA - Tom Leher

The loveliest girl in Vienna
Was Alma, the smartest as well.
Once you picked her up on your antenna,
You'd never be free of her spell.

Her lovers were many and varied
From the day she began her - beguine.
There were three famous ones whom she married,
And God knows how many between.

Alma, tell us,
All modern women are jealous,
Which of your magical wands
Got you euros and dollars and yens?

The rest.

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Agenda

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News -- Wootten

Calibration meeting redux.

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Receiver PDR -- Wootten

The Recommendations are available.

Tuning Range Question

The AEC asks the ASAC to comment on the following issue: If it is not feasible to achieve adequate receiver performance over the full RF bandwidth for the initial receivers what is the tuning range that *necessarily* must be covered. This applies to all four initial ALMA bands, not just band 7.

There are three important points here: (1) The firm plan is to achieve the full RF bandwidth in the Project Book for all receiver bands, and do so by the end of construction; (2) But the first few (~10?, 20?) frequency cartridges delivered for each band may have more limited performance. It is for these cartridges that it would be helpful to have the ASAC comment on the more restricted frequency range. (3) If it is necessary that the initial cartridges have somewhat limited performance, those cartridges will be retrofited with cartridges that do achieve the specs as soon as the retrofits can be made available.

Details: Dear Wolfgang and John,

During yesterday's telecon, the subject of the frequency coverage in band 7 was discussed.

As we understand it, it appears that in at least one combination, i.e. single-ended mixers with a 4-8GHz IF, it would not be possible to provide enough LO power on a band wide enough to cover the RF range 275-370GHz.

If we understand well, John Webber stated that he felt it might be possible to provide adequate LO power for balanced mixers over a LO range equal to RF-2*8GHz.

Maybe he could supply enough LO power for single-ended mixers over a range equal to RF-2*12GHz (to be confirmed).

>From the mixer side, this means either balanced mixers that _may_ have a 4-8GHz IF, or single-ended mixers with a 8-12 (or 4-12) GHz IF. Of course, the combination balanced mixers, plus IF extending to 12GHz would be even better. However, either of these (balanced, higher IF) increases the technical difficulty of mixer design and prototyping, in a context where the technical ambition conflicts with the schedule.

>From the LO side, and if we remember well, John Webber stated that the projected performance was at the limit of the simulation results, i.e., it will be hard to reach.

This being said, we may, within the FE subsystem, discuss the way to meet the specs with least technical risk, but we know that is a difficult goal to reach. So, we propose to re-examine the scientific rationale behind the specs (a point raised by the PDR reviewers).

As a first step, we have performed a simple exercise, trying to estimate the relative priority in various parts of band 7. Using the data in: http://physics.nist.gov/cgi-bin/micro/table5/start.pl we plotted the line intensities versus frequency, overlaying the atmospheric transmission curve for 1mm PWV. Two plots are attached, with the second one having an expanded vertical scale. As far as we can see, there are only rather weak lines (most intensities are recorded in Orion) between the 275GHz lower edge of band 7 and at least 290GHz, and, to a lesser degree, up to 300GHz. The absence of lines on our plot between 363 and 370 GHz is probably due to lack of coverage of the observations, so we won't discuss it.

Given that the coverage of the specified RF band is at best difficult, and may be uncertain, should not the technical efforts be targeted at regions of maximum scientific interest. In other words, should we not propose to the Science group to define a "first priority" coverage of band 7, not excluding full coverage as development progress might allow. Please note that even the more modest option of dropping 275-290 GHz from first priority yields a reduction of LO range (all other things equal) of 15 GHz, almost twice the reduction provided by raising the upper edge of the IF band from 8 to 12GHz (2*4=8GHz).

We provide the enclosed arguments and data so that you may use them as you see fit to approach the ASAC and maybe obtain a reasonable compromise between requirements and technical difficulty. You might also consider to distribute the present message to the JRDG for consideration at the next telecon.

With our best regards,

S.Claude & B.Lazareff

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ASAC Report