Minutes for meeting Mon, 30 August 1999 at 4pm EDT.
Date: 30 August 1999
Time: 4:00 pm EDT (2:00 pm Socorro, 1:00 pm Tucson)
Phone: (804)296-7082 (CV SoundStation Premier Conference phone).
Past minutes, etc on MMA Imaging and Calibration Division Page
Minutes
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I think we need to extend the work Tamara began on simulating ALMA images to (1) reproduce it, and (2) include single antenna data to determine the effect that has on the sidelobes.
Stefan proposed mixing the compact and 420m arrays into an all-purpose 32 element compact plus 32 element extended configuration. Let's discuss this a bit.
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A short review of the URSI meeting session.
183 GHz on the site--there is a new memo (No. 271).
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MMA PHASE STABILITY SPECIFICATION -- NOTES
Goals:
1. Greater than 90% interferometric coherence at 950 GHz (77 fsec
rms), after all calibrations and corrections, on all time scales
from 1s to 1e4 sec.
2. Absolute visibility calibration to 0.1 radian at 950 GHz (16.8
fsec).
Assuming that phase errors are independent among antennas, allocate
half of the *squared* error budget to each. Of this, allocate half
to the atmosphere, one-third to electronics, and the balance to the
antenna structure. This gives:
Atmosphere Electronics Structure Total/antenna
systematic (avg) 8.4 6.9 4.8 11.9 f sec
random (rms) 38.5 31.4 22.2 54.5 fsec
These allocations are somewhat different than those assumed by Woody
et al. (MMA Memo 144). There, it was planned to achieve 90%
coherence only 50% of the time at 300 GHz; here we use 950 GHz, and
do not specify the distribution. Of the error sources, only the
atmosphere is non-stationary, and it is uncertain whether the above
goals for it can really be achieved; if not, then the goals for the
other components can be relaxed somewhat.
Relevant time scales:
. shortest correlator integrating time ~.001 sec
. astronomical calibration cycle time ~10 (typ fast sw.)
. u-v cell crossing time (config dependent) ~100
. source mapping time ~1e4 (3h)
Requirements for support of fast switching for phase cal:
Calibrator is observed at a different frequency and/or mode (e.g.,
bandwidth and frequency resolution), maybe even a different
receiving band.
. Unambiguous phase synthesis in all LOs, with repeatable phase
after switching frequency.
. Stable phase in all electronics *common* to calibrator and target
(e.g., LO reference) for > cal cycle time [typ 10s]. This
allows correction of phase *changes* on switching time scale, but
not absolute calibration.
. Stable phase in electronics not common to calibrator and target
for much longer, since absolute cal must be done with same setup
on calibrator and target.
Additive noise:
Noise added to a nominally sinusoidal LO signal has a phase error
component given by $d\phi = 1/\sqrt(SNR)$, where SNR is the ratio of
the signal power to the total noise power.
31.4 fsec at 950 GHz -> d\phi=.187 rad -> SNR = 14.5 dB
MULTIPLIER CASE:
If multiplied from a 13 GHz ref:
N=73 SNR(ref) = 51.8 dB
Cleanup loop BW=1MHz -> reference noise may be -114.8 dBc/Hz (2sided )
v. easy for opt link
Source at 26.4 GHz -> noise outside loop BW:
N=36 SNR(src) = 45.6 dB
f^-2 spectrum -> S(1MHz) =-108.6 dBc/Hz
feasible for YTO
PHOTONIC REFERENCE CASE:
Multiply from 105.5 GHz ref:
N=9 SNR(ref) = 33.6 dB
No cleanup loop, 25 GHz BW mixer -> ref noise -137.0 dBc/Hz
feasible for opt link
but PLL will provide cleanup, so very easy for opt link
PHOTONIC DIRECT CASE:
No multiplication, transmit 950 GHz SNR = 14.5 dB
No cleanup loop, mixer BW 100 GHz:
additive noise may be -127.5 dBc/Hz
easy for opt link
To avoid adding receiver noise:
assume LO is 1uW; 10K extra noise -> -158.6 dBc/Hz
with LO rejection of 10dB -> -148.6 dBc/Hz
marginal for opt link
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I will update everyone on the status of this event.
Kate has posted these to the WWW.
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A new NRAO dispay debuts at the ALMA meeting. Four panels will cover the MMA areas of Star Formation, Solar System, Cosmology and Galaxy Evolution. We need to come up with important points and images for these. Suggestions: Star Formation
* Determination of the Dynamics of Star-Forming Cloud Cores, and the Origin of Binary Stars
* The Structure of Protoplanetary Disks and the Formation of Planets.
Of somewhat lesser importance (they require more explanation) I would put the origins of outflows from young stars, and the role of magnetic fields.
For star formation the above two items suggest images which show protoplanetary disks and/or binary star formation.
Cosmology
If Steve Myers is at the VLA, you might talk to him as this is his field. He is joining NRAO 1 September at the AOC.
Planetary Science goals:
* Study of Primitive Solar System Material -- what molecules seeded the early planets
* The Solar System Weather Channel -- dynamic images of volcanoes erupting on Io to hurricanes on Mars.
Galaxy Evolution:
* How has the Star Formation Rate Evolved since the Big Bang?
* How has gas content influenced the structure of galaxies?
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The data rate figure I've been carrying in the project book is 1MB/s average, 10MB/s max sustained. This comes from the Scott et. al. memo (#164) which in turn apparently came from a Tucson science meeting (40 antennas). Mark Holdaway should know about this number. Rupen in a later memo comes up with a science case for 100's of MB/s (OTF synthesis surveys). Note that the NRAO correlator design is capable of dumping data from the LTA at a rate of 1GB/s.
What is the science-driven rate?
Here are the spectral channels/bandwidth values for the prototype LTA. The
minimum integration time is 16 ms for cross-corretations & 1 ms for
auto-correlations
(i.e., there is no time/spectral channel trade off as in earlier designs)
Mode Spectral Baseband Baseband
Channels Bandwidth Bandwidth
not oversampled oversampled
0 512 2 GHz 1 GHz
1 1024 1 GHz 500 MHz
2 2048 500 MHz 250 MHz
3 4096 250 MHz 125 MHz
4 8192 125 MHz 62.5 MHz
5 16384 62.5 MHz na --------
ALMA block diagrams
The ALMA telescope block diagrams have been further updated, including
the inserion of the two pages that were previously "not yet drawn."
The files are all in:
heineken:~ldaddari/public_html
which is accessible via
http://www.tuc.nrao.edu/~ldaddari/[file]
Here is a list of the relevant files:
mma-top.txt ; description and discussion of key features.
loPlan.txt ; frequency plan for first LOs.
mma-top.dsn ; OrCAD Capture binary file will all schematic pages.
mma-top.pdf ; PDF file with 10 pages, 11x17 in, includes all sheets.
mma-top1.pdf ; PDF file with 3 pages, 11x17 in, overall block diagrams.
mma-lodetails.pdf ; PDF file with 7 pages, 8.5x11 in, LO details only.
mma-pllblock.pdf ; PDF file with 1 page, 8.5x11 in, PLL block only.
The first of the PDF files contains all the pages, but not in their
natural sizes. The others contain only some of the pages, but perhaps
in more convenient sizes. The sheets containing the overall block
diagrams for central and antenna portions (3 pages) are drawn in D
size, they can be reasonably viewed at B size;, but not smaller. (My
PDF writer's max size is B, 11x17.) The B size sheets can be
reasonably viewed at A size.
Here is the structure of the sheets, with the natural size of each and
the page number in the PDF files. Note that the PDF conversion is not
perfect, so some details may be not completely clear, but I hope it is
all readable. Viewing in or printing from Capture is better.
Size ------PDF file page number------
mma-top mma-top1 mma-lodetails
Central Electronics
Generic D 2 1
Antenna Electronics
Conventional or
photonic-reference D 5 2
Photonic-direct D 6 3
LO Details
Conventional -- central B 8 5
Conventional -- antenna B 1 1
Photon.-ref -- central B 9 6
Photon.-ref -- antenna B 4 3
Photon.-dir -- central B 7 4
Photon.-dir -- antenna B 3 2
PLL block A 10 7
...
-Larry
COMMENTS AND NOTES ON BLOCK DIAGRAM
LRD 1999-08-22
Since distribution of the draft block diagram for ALMA around Aug 8, I
have received a few comments that I'll summarize here. All of them
have to do with the local oscillators. So far, there has been little
discussion of the signal transmission or digital signal processing.
My notes on the comments are given in square brackets [...].
1. Durga Bagri pointed out that the secondary LO reference at 125 MHz
is multiplied nearly to 10 GHz when synthesizing the second LOs.
This is true for all three options. In the conventional option,
the same reference is multiplied nearly to 13 GHz for the first LO
synthesis. Therefore, the phase stability of the 125 MHz
reference is nearly as important as that of the high frequency
reference. Its distribution cable should also be
length-corrected.
[The draft diagrams show the 125 MHz reference included on the
same optical carrier as the high frequency reference in the
conventional (C) and photonic-reference (PR) options. In those
cases, it thus participates in the two-way length monitoring and
correcting. For the photonic-direct (PD) case, the 125 MHz is
sent on a separate carrier; this may have to be re-considered.]
2. John Battle pointed out that it may be difficult to obtain the
17-33 GHz YTO shown as the fundamental source in the C and PR
cases, but that one covering 20-33 GHz is available. [If the
nearly-octave range indeed proves too difficult, the required
range can be reduced by implementing Driver B and Driver C each
with a tripler rather than with two doublers. The feasibility of
those triplers would need to be investigated.]
3. Various people at the ALMA meetings in Toronto (17-Aug-1999)
questioned whether the tuning resolution is adequate. In all
cases the overall resolution is 250 MHz or less, but for case C
the first LO tuning has larger steps at the higher frequency
bands. Worst is Band 10, where it becomes 2 GHz per step. The
only scenario so far suggested where this might be a significant
limitation is this: we wish to observe two lines simultaneously,
in opposite sidebands, with the (presumed) double-sideband
receiver. By careful selection of the first LO frequency, the
lines can be kept separated in the IF; but with only 2 GHz tuning
resolution they cannot. [This situation depends on an
unfortuitous combination of line frequencies, widths, and
strengths. Whether it is of practical importance is not yet
known.] There may be other situations where finer resolution is
required. The maximum acceptable step size is presently
uncertain.
[It is possible to observe two lines in opposite sidebands even
though they overlap at IF. Since we have separate 2nd LOs on each
baseband converter, we supply the same IF to all converters of
one polarization, but we operate half of the LOs so as to suppress
the upper sideband and the other half to suppress the lower.]
4. Richard Hills (and others) suggested that finer tuning resolution
in case C might be obtained by making the high frequency reference
variable frequency. I objected that departing from fixed
references makes it difficult to guarantee unambiguous phase, due
to the cycle ambiguity of the length correctors during a frequency
switch. Besides, a rather wide range (~20%) would be needed to
produce continuous coverage in the simplest scheme (no
second-stage synthesizer), so that the reference might encroach on
the IF range. [A careful detailed design might produce solutions
to these problems.]
[However, a variable frequency reference is intrinsic to cases PR
and PD. In PD, the frequency varies during a single observation
because of fringe tracking; in PR, it changes only when the setup
is changed. To ensure unambiguous phase in these cases, the
problem of cycle ambiguity in the length correctors will have to
be overcome, or an alternate scheme will have to be devised. This
requires more detailed study.]
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DECISION: Configurations--where are we?
DECISION: Is a nutating secondary necessary?
DECISION: What is the effect of 1/f noise in the HEMT amplifiers of SIS receivers upon our ability to combine total power and interferometric images into a faithful representation of the sky?
MEETINGS: MAC meeting 15 September 1999 at noon. Next ImCal meeting 13 September 1999. ------
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If your travel isn't on here you have not sent me a travel authorization.
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A. Wootten:
J. Mangum:
M. Yun:
B. Butler:
S. Radford: