[asac] [almanews] ALMA Memos 360, 361, and 362 Released

[Carolyn White]

ALMA Memo #360

Design of Sideband Separation SIS Mixer for 3 mm Band

Vessen Vassilev and Victor Belitsky

As a part of Onsala development of a single sideband mixer for ALMA band 7
(275-370 GHz), we present design of a prototype sideband separation mixer
for 85-115 GHz.; The mixer employs a quadrature scheme with two
identical DSB SIS mixers pumped by a local oscillator with 90 deg phase
difference.; The mixer uses a new device, a double-probe coupler,
which splits the input RF signal and provides transition from a waveguide
to a microstrip line, allowing the integration of all mixer components on
the same compact substrate and thus ensure a high degree of similarity in
the SIS junction performance and the geometry of all the mixer elements
including integrated tuning circuitry.

We present the design of all the mixer components, detailed simulation
results using High Frequency Structure Simulator and measurements of the
double probe coupler.

View a pdf version of ALMA Memo 360 at URL:
     http://www.alma.nrao.edu/memos/html-memos/alma360/memo360.pdf

View a html version of ALMA Memo 360 at URL:
          http://www.alma.nrao.edu/memos/html-memos/alma360/memo360.html

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ALMA Memo #361

PHASE CROSS-CORRELATION OF A 11.2 GHZ INTERFEROMETER
AND 183 GHZ WATER LINE RADIOMETERS AT CHAJNANTOR

Guillermo Delgado

The phase variation from a 300-m baseline 11.2 GHz interferometer was
cross-correlated with the phase variation estimated from the PWV
measurements using two radiometers operating near the water vapour line at
183 GHz at the ALMA site of Llano de Chajnantor in Northern Chile at an
altitude of 5,000 m. Care has been taken to have the radiometers observing
the same path of atmosphere as the interferometers, with both beams
matched as close as possible.

The result indicates that the cross-correlation varies during the day and
thus the phase correction possible to achieve using the radiometric method
at Chajnantor.

The results are discussed and comparisons are done with other
variables, specially the height of the turbulence layer. The later is
determined using two different methods: the first one is direct
measurements from radiosonde data, and the second method involves the
calculation of the time lag between the turbulence structures seen by the
two interferometers deployed at Chajnantor. A relation is proposed between
the height of the turbulence layer and the success of the
cross-correlation, with a better cross-correlation when the turbulence
layer is higher than about 300-400 m.

View a pdf version of ALMA Memo 361 at URL:
     http://www.alma.nrao.edu/memos/html-memos/alma361/memo361.pdf

Download a ps version of ALMA Memo 361 at URL:
          http://www.alma.nrao.edu/memos/html-memos/alma361/memo361.ps

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ALMA Memo No. 362

ALMA Receiver Optics Design

J. W. Lamb (Caltech), A. Baryshev (SRON), M. C. Carter (IRAM),
L. R. D’Addario (NRAO), B. N. Ellison (RAL), W. Grammer (NRAO),
B. Lazareff (IRAM), Y. Sekimoto (NRO) , C. Y. Tham (U. Cambridge)

A detailed design for the optical configuration of the ALMA receivers is
presented.  Individual frequency bands are implemented as self-contained
cartridges holding two orthogonally polarized channels. The cartridges are
arranged on concentric circles round the center of a 970 mm diameter dewar
located on the telescope axis. The beams from them illuminate the
secondary mirror through windows on the top of the dewar, either directly
or via reflective optics. By having all the beams separate in the focal
plane, all bands view the sky simultaneously and selection of the
observing band simply requires re-pointing the antenna.

Where possible all the optical elements are integral with the
cartridge. For the lowest frequency bands, the optics are too large to go
on the cartridge and are located on the top of the dewar. There are no
optical elements inside the dewar that are not attached to a
cartridge. Since some of the cartridges are far off the telescope axis,
mirrors are used to bring the beam closer to the center to reduce
aberrations, polarization distortion, and vignetting by the hole in the
primary. Provision is made for a mirror to bring the beam of the water
vapor radiometer for atmospheric phase correction to the center of the
focal plane so it is close to all observing beams.

Several measures are taken to ensure low optical losses: the number of
elements is minimized; reflective optics are used where possible; large
beam clearances are maintained; and accurate fabrication and alignment
tolerances specified. A major driver was to generate minimal
cross-polarization, and this was realized by minimizing angles of
incidence on offset reflectors, and balancing cross-polarization between
consecutive mirrors.

Detailed calculations of the performance, including losses, noise, and
polarization have been carried out and are tabulated. There are also
estimates of the cryogenic loading. The principal uncertainties are the
optimum designs for the vacuum windows and infrared filters.

View a pdf version of ALMA Memo 362 at URL:
     http://www.alma.nrao.edu/memos/html-memos/alma362/memo362.pdf
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