No subject
Karl Menten
kmenten at mpifr-bonn.mpg.de
Wed Jun 28 10:16:46 EDT 2000
Dear ASAC members,
I have finished a draft of the ASAC response to the ALMA Receiver Subsystem
Top-level Requirements and Specifications document, which you find
attached to this message. Please email me final comments by Thursday,
June 29.
Cheers,
Karl
P.S. The document can also be found on:
http://www.mpifr-bonn.mpg.de/staff/kmenten/jrdg-response.txt
-----------------------------------------------------------------------------
Dr. Karl M. Menten (kmenten at mpifr-bonn.mpg.de)
Max-Planck-Institut fuer Radioastronomie
Auf dem Huegel 69, D-53121 Bonn, Germany
Tel.: +49 (0)228-525297 * Fax: +49 (0)228-525435
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MEMORANDUM
Date: 28-June-2000
To: Wolfgang Wild & John Payne, ALMA JRDG
From: ALMA Scientific Advisory Committee (ASAC)
Topic: Comments on ALMA Receiver Subsystem Top-level Requirements and
Specifications (specifically Draft Version 1.3, 19-May-2000,
http://www.cv.nrao.edu/~awootten/mmaimcal/receiverspecs.html)
------------------------------------------------------------------------
This document summarizes the ASAC's commnents on the ALMA Receiver Subsystem
Top-level Requirements and Specifications document.
------
* Section 2.2 REFERENCE DOCUMENTS
ASAC: Add CDR and PDR reports to be found at
http://www.alma.nrao.edu/administration/index.html
---
* Section 3.1 FREQUENCY COVERAGE (in particular Table 1):
Issue: First sentence ("The ALMA receiver subsystem will cover all
the available atmospheric frequency windows between 30 GHz and
950 GHz.")
ASAC: Since the definition of "atmospheric frequency windows" is, e.g.
opacity dependent, we recommend to change this to
"The ALMA receiver subsystem will cover frequencies between
30 GHz and 950 GHz as given in Table 1."
Issue: lower frequency range of Band 3 (now 86 GHz)
ASAC: The ASAC actually wrote
(http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node3.html):
"We strongly urge that the JRDG study the possibility of extending
the lower frequency range of Band 3 to include the SiO maser
transition near 86 GHz. If this is possible, Band 2 would drop to
third priority."
Since the VLBA 3 mm receivers will go down to 84 GHz, perhaps
some attention should be paid to the actual number of the lower
limit. We recommend that 84 GHz is adopted.
---
* Section 3.2 POLARIZATION
ASAC: The ASAC report at
http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node12.html
needs to be made more specific. Larry D'Addario and Steve Myers
have put work into this, and their draft recommendations are
located at:
http://www.aoc.nrao.edu/~smyers/alma/polspecs-imcal.txt
We invite comments on this document.
The polarisation purity requirement mostly influences the optics
and horn/orthomode transducer. One would assume that the 1% goal
in calibration requires a cross-polarisation better than 20 dB,
and that all antennas should also be co-aligned in polarisation
to the same level. Polarisation experts should comment on this!
Note that it may be difficult to reach this level with an
orthomode transducer.
* Section 3.3 OPTICAL COUPLING TO THE TELESCOPE
ASAC: The ASAC urges that coupling efficieny specs are defined and
that a measurement scheme for various efficiencies are
discussed.
---
* Section 3.4 RECEIVER NOISE PERFORMANCE
Issue: Receiver noise requirements
ASAC: These should be stated in a more specific way. Our recommendation:
The noise temperature measured at the dewar input window to the
cartridge shall not exceed a value of TrxSSB for SSB response
and 0.5*TrxSSB for DSB response. TrxSSB will in general be a
function of the frequency nu and is given by the following
formula
TrxSSB= A * (h*nu/k) + 4 K
where h and k are the usual physical constants. The frequency
dependent quantity A has the following specification and goal
values:
Bands 1-6 (below 275 GHz) Spec: A = 6 / 10 Goal: A = 3 / 5
Bands 7-8 (275-500 GHz) Spec: A = 8 / 12 Goal: A = 4 / 8
Band 9 (602-720 GHz) Spec: A = 10 / 15 Goal: A = 6 / 9
Band 10 (787-950 GHz) Spec: A = 10 / 15 Goal: A = 8 / 12
For both, the specification and the goal values, two numbers
are given. The first one of these refers to the value that A must
not exceed over the 80% range of the nominal bandwidth that has
the best performance, whereas the second value may not be
exceeded at any frequency within the nominal bandwidth.
Furthermore, the receiver temperatures should be measured in a
reference plane outside the dewar, which involves the
following technical issue. Ideally, one would like to measure
at the secondary focus, including all receiver related optics,
i.e. with the full final cryostat and optics. However, since
receiver cartridges will be developed and tested separately, this
cannot be performed on the development site. We propose that the
receiver group equips the test cryostats for the cartridge testing
in such a way as to provide a comparable receiver temperature
reference plane.
---
* Sections 3.5 SIDEBANDS, 3.6 IF BANDWIDTHS, and 3.7 SIMULTANEOUS OPERATION
OF BANDS
ASAC: Specs made here are rather vague.
The goal is: SSB 2 x SB 2 x Pol = 4 x 8 GHz (sideband separating)
The Munich PDR
(http://www.cv.nrao.edu/~awootten/mmaimcal/ALMA-DesRevRec4.html)
said:
"In addition to the baseline IF bands of 8 GHz Upper and Lower
Sideband, receiver designers are free to select any of the
following alternatives: 8 GHz Single-Side-Band, Upper or Lower,
8 GHz Double-Side-Band or 4 GHz Upper and Lower Sideband. In all
cases, dual polarization for a total of 16 GHz IF band width.
Sideband separation in DSB-mode will be possible for integration
times in multiples of 1 sec. Depending on the choice, and
maintaining the currently proposed LO coverage, this might lead
to some loss of frequency coverage. The impact of this should be
evaluated by the Science Group."
This is only a recommendation, but it might be a starting point
for more stringent specs.
---
* Section 3.6 IF BANDWIDTH
Issue: 4 GHz IF bandwidth as a fall back position
ASAC: The 8 GHz IF bandwidth is a very strong science requirement and
every effort should be made to keep the IF bandwidth at 8 GHz at
the lower frequencies, where continuum sensitivity and broad
bandwidth for high-z CO line searches are most crucial. At higher
frequencies, this could perhaps be somewhat relaxed, but only as
a last resort.
Officially declaring 4 GHz IF bandwidth as a fall back position
even for only the initial bands is a significant deviation from
the original specs. Any specific proposal for a design with less
than 8 GHz needs more discussion.
What would be the cost of upgrading receivers with 4 GHz IF
bandwidth to 8 GHz bandwidth?
As a basis for discussions of the tradeoffs between bandwidth
and receiver temperature, Neal Evans has made the following
proposal, which will be discussed by the ASAC. We include it
here to indicate in which direction these discussions may lead.
"I don't like the idea of relaxing the bandwidth spec so easily,
but I recognize that there are tradeoffs. It would be better to
formulate a figure of merit that balances T_rx with BW. If one
sacrifices too much on T_rx to meet a spec on BW, one may quickly
lose more than one gains. If we adopt Al's formulation, with
T_rx = A hnu/k + 4 K and IF bandwidth symbolized with BW, the
figure of merit for line observations is (I leave
out the constant 4 K for clarity, but it could be worked in)
F_line = T_rx/A
For continuum, it is
F_cont = (T_rx/sqrt(BW))/(A/sqrt(8GHz))
Note that this formulation relates to what you can achieve in
a fixed integration time, which is a more realistic approach in
my view than asking what integration time is required for a given
sensitivity level, which would lead to squaring these figures.
The overall figure of merit depends on your evaluation of the
overall importance of line and continuum observations. Call the
overall figure of merit F_total. Then
F_total = X F_line + Y F_cont ; X + Y = 1
The ASAC could vote on the values of X and Y. My vote would be
X= 0.6, Y = 0.4. The reason for this choice is not that I think
line observations are more important than continuum, but because
they are harder-- the continuum sensitivity of ALMA will be very
good, but for some line problems, the sensitivity will be more
marginal.
One could make X and Y a function of band, but I think there won't
be large differences. The idea is that one would relax the BW
spec, replacing it with the spec that F_total be less than or
equal to unity."
---
* Section 3.7 SIMULTANEOUS OPERATION OF BANDS
Issue: It is stated tha "The water-vapor monitoring radiometer shall
operate simultaneously with bands 2 to 10, but not with band 1
which can operate without the water-vapor monitoring radiometer.
ASAC: The ASAC actually said:
"The ASAC does note that the simultaneous operation at 183 GHz
and Band 1 receivers is not a scientific requirement, so it is
straightforward to locate these systems in the same Dewar if that
makes sense."
In the best of all possible worlds, the WVR would work with band 1
as well as the other bands--this will be necessary under a wide
range of conditions at Chajnantor for which ALMA may be operable
at band 1 with WVR but ALMA would be shut down otherwise.
However, if the cost of operating both simultaneously is very
high, one possible sacrifice might be simultaneous operation of
WVR and band 1. This is very different from designing a receiver
system which will not accommodate simultaneous operation of both
from the outset.
---
* Section 3.8 RECEIVER STABILITY
Issue: It is stated that "A preliminary suggestion for a gain
fluctuation limit is: 1e-4 rms over a 1 sec interval."
ASAC: This was indeed suggested in the ASAC report. A memo from Wright
(http://www.alma.nrao.edu/memos/html-memos/abstracts/abs289.html)
suggests 1e-4 over 0.1s, which begins to define the spectrum of
stability.
---
* 4.3 SELECTION OF A NEW OBSERVING BAND
Issue: The statement "Selecting and tuning a new band shall require no
more than 15 min." caused quite some confusion.
ASAC: This issue was clarified by Wolfgang Wild, who states that
"What we meant with the 15 min is not the tuning time, it means
that you need to switch on a particular band 15 min before using
it. This is to reach thermal equilibrium. In practice, this time
could probably be shorter, but nobody has measured it so far.
Switching from on band to another would be done in 1.5 sec (if
the band is switched on). Since we cannot have all bands switched
on at all times due to cryogenic limitations we introduced this
spec to minimize the impact on scheduling."
We recommend that this explanation should be incorporated into
the document.
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