[asac][almanews] ALMA Memos 422, 426, and 427 Released

[Stacy Oliver]

ALMA MEMO #422

The Dual-Load Calibration device revisited

S.Guilloteau (IRAM/ESO)

2002/05/23

Keywords: Receiver, Calibration, Atmosphere

Following ALMA memo 371, I re-investigate the dual load calibration scheme
which has been proposed for ALMA. This memo improves on the requirements
derived in memo 371 by using a better prediction for the atmospheric
fluctuations at short time scales. It is shown that because of atmospheric
properties and of the antenna geometry, a dual-load system located in the
ALMA antenna subreflectors needs to switch at a rate in excess of 20--30 Hz
to be efficient in the submillimeter regime. Moreover, integrations in
excess of 10 seconds are required to get sufficient calibration accuracy at
these frequencies. Because of the exceedingly high rate, and since the
system has not been demonstrated to allow an absolute calibration, it is
proposed to abandon this development.

View a pdf version of ALMA Memo #422.
http://www.alma.nrao.edu/memos/html-memos/alma422/memo422.pdf

Download a postscript version of ALMA Memo #422.
http://www.alma.nrao.edu/memos/html-memos/alma422/memo422.ps

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ALMA MEMO #426

4-Gsample/s, 2-bit SiGe Digitizers for the ALMA Project. Paper II

David Deschans 1,2, Jean-Baptiste Begueret 1, Yann Deval 1, Pascal Fouillat
1, Alain Baudry 2, Guy Montignac 2

1 Laboratoire IXL, Universite de Bordeaux, 351 Cours de la liberation, 33405
Talence, France
2 Observatoire de Bordeaux, BP 89, 33270 Floirac, France

2002/05/16

Keywords: Analog-to-digital converter (ADC), SiGe technology, high speed
bandpass ADC

We report on the design details and first dynamic tests of high speed
analog-to-digital converters (ADC) with characteristics approaching those
desired for the ALMA project. A conventional flash ADC architecture has been
adopted with monolithic ADCs implemented in BiCMOS 0.35 um and 0.25 um SiGe
(Silicon-Germanium) processes. We concentrate here on our 2-bit, 0.35 um
designs and test results, while details on our 3-bit designs and 0.25 um
SiGe technology will be given in another paper. The main features of these
2-bit ADCs are 4 GHz clock rate, 3 quantization levels, and an input
bandwidth from 2 GHz up to 4 GHz. The two chips tested in this work
dissipate 650 and 975 mW under 2.5 V supply; the die areas are 5.4 mm2 and
6.5 mm2, respectively.

View a pdf version of ALMA Memo #426.
http://www.alma.nrao.edu/memos/html-memos/alma426/memo426.pdf

Download a postscript version of ALMA Memo #426.
http://www.alma.nrao.edu/memos/html-memos/alma426/memo426.ps

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ALMA MEMO #427

Antenna Position Calibration

Melvyn Wright Radio, Astronomy laboratory, University of California,
Berkeley

2002/05/16

Keywords: Antenna Position, Calibration

This memo summarizes the antenna position calibration used at Hat Creek, and
discusses its application to the CARMA and ALMA telescopes. Observations of
quasars over wide range of HA and DEC provide data from which the antenna
positions are determined. If the antenna positions are in error by many
wavelengths, 2pi phase ambiguities may make this process difficult,
especially when the atmospheric phase coherence is poor on long baselines.
In this case the antenna positions can be first determined from the phase
difference between the sidebands in a double sideband system, or across a
wideband single sideband. In addition to the antenna positions, there may be
axis offsets on the antennas which can be fitted from the quasar data or
measured mechanically. These offsets should be stable and not change when
the antennas are moved.

View a pdf version of ALMA Memo #427.
http://www.alma.nrao.edu/memos/html-memos/alma427/memo427.pdf

Download a postscript version of ALMA Memo #427.
http://www.alma.nrao.edu/memos/html-memos/alma427/memo427.ps

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