[asac] [almanews] ALMA Memos 349, 350, and 351 Released

[Carolyn White]

ALMA Memo #349

Fibre-Optic Link Design of the Atacama IF Data Transfer System

Roshene McCool
February 2001

This report examines the designs for two possible data links for IF
signal
transfer from the ALMA antennas to the data correlator. The first design
is 
for the case where the correlator is located on the array site. The
second 
design is for the case where the correlator is located at or near San
Pedro, 
perhaps some 70km away, by fibre, from the array site.

The main focus of the report is the technical feasibility or otherwise
of 
the options. The report also provides some of the information from which 
the better option, from a systems point of view, can be selected. No
estimates 
of costs are provided. Proper costings will be provided in a forthcoming 
companion report together with recommendations of actual components.
This 
part of the whole is being released so that the ALMA community can
assess 
the technical arguments which justify a necessary but modest increase in 
costs for IF data transfer.

View a PDF version of ALMA Memo #349
     http://www.alma.nrao.edu/memos/html-memos/alma349/memo349.pdf

Download a postscript version of ALMA Memo #349
     http://www.alma.nrao.edu/memos/html-memos/alma349/memo349.ps

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ALMA Memo #350

Feasibility Study of the Enhanced Correlator for 3-way ALMA I. 

Sachiko K. Okumura, Yoshihiro Chikada, Munetake Momose, and Satoru
Iguchi

February 2001

Here we report the feasibility study of the enhanced correlator
proposed by Japan for 3-way ALMA. First we briefly review the scientific
significance of the enhanced correlator, which does NOT depend on the 
correlator architecture. Then we describe the proposal of a
high-performance 
FX correlator system for an enhanced correlator of 3-way ALMA. This FX 
correlator system always realizes both super-high spectral-resolution (
< 0.1km/s 
at 40GHz ) and wideband ( > 700km/s at 850GHz ) observations
simultaneously 
up to 850GHz for each 2GHz baseband of the ALMA IF system. This FX
system 
consists of 1024 x 1024 - FFT parts, 4-bit cross-correlation parts, and 
control parts. Re-quantization and flexible frequency-channel smoothing
is 
newly installed. Re-quantization reduces the lines of connection between
F 
and X parts compared with the previous FX system. Flexible
frequency-channel 
smoothing makes the output frequency channels from 524288(=512 x 1024)
to 
8192 per baseband and eliminates the fear that the large amount of
frequency 
channels might increase the costs of post-detection computing and
archiving. 
Realization of this correlator system will allow us to make breakthrough
in both
sub-millimeter line and continuum observations with 3-way ALMA. We
present the 
detailed specifications, block diagrams, estimated hardware size and
power 
consumption of the high-performance FX correlator system. Preliminary
plan for 
its implementation to the 3-way ALMA is also commented. 

View a PDF version of ALMA Memo #350
     http://www.alma.nrao.edu/memos/html-memos/alma350/memo350.pdf
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ALMA Memo #351

275-370 GHz DSB and SSB Waveguide Mixers Employing a Tuned 
Nb/Al-AlOx/Nb SIS Tunnel Junction 

A. Navarrini and B. Lazareff 
February 2001

We discuss the design and optimisation of two full height waveguide SIS
mixers 
for astronomical applications both covering the 275-370 GHz frequency
band: a
Double Side Band (DSB) and a Single Side Band (SSB) mixer. The expected
SSB 
receiver noise temperature referred to the mixer input are in the range
23-35 K
for the DSB and 25-38 K for SSB mixer. A 30 % operating bandwidth can be
achieved 
in the DSB case by using an "end-loaded" tuning stub to tune out the
junction 
capacitance of 75 fF (junction size 1 µm2) followed by two quarter-wave
transformer 
sections. A similar operating bandwidth is obtained in the SSB mixer by
using a 
parallel tuning inductor with a radial microstrip stub. Single junctions
are mounted 
on a 80 mm thick quartz which stretches only part way across the
waveguide. In the 
SSB mixer the image rejection is obtained using a mechanically rugged
noncontacting
backshort with a circular cross-section which can be moved inside a
circular 
waveguide. Both mixers have a 4 GHz IF passband and a central IF
frequency of 6 GHz.
 
A stability criterion for intrinsically DSB and SSB mixers under typical
operating 
conditions has been derived. We have shown that when an inductive series
matching
structure is used to compensate the junction capacitance, the SSB mixer
cannot be 
operated over a wide frequency range in a stable way. An inductive
parallel
matching structure allows us to fulfil the necessary conditions of
stability. Receiver 
performance has been optimised for both mixers in order to guarantee a
low mixer
noise temperature while maintaining adequate gain and stable operations
over the 
whole frequency band of interest. 

View a PDF version of ALMA Memo #351
     http://www.alma.nrao.edu/memos/html-memos/alma351/memo351.pdf
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