[asac] [almanews] ALMA Memos Released

[Carolyn White]

ALMA Memos Can be Found via:

http://www.alma.nrao.edu/memos/index.html


-----------------------------------------------------------
ALMA Memo # 507


ALMA First LO Reference : Elimination of Large Phase Fluctuations Due to
Lightwave Polarization Effects 



B. Shillue and S. AlBanna
2004-12-15


Abstract: The ALMA 1st LO reference is sent from the central Array
Operations Site Technical Building to each of 64 antennas over optical
fiber. Two high-coherence and phase-locked lightwaves are transmitted
that are separated by a variable frequency ranging from 27-142 GHz. The
round-trip stabilized fiber optic distribution system has been
previously described in this memo series [1-3,13]. In August of 2003, a
first generation version of the line length correction system was tested
on a prototype ALMA antenna. These were the first systematic
measurements of the system on the moving structure. During these
measurements we noticed an undesirable and unexpected phase fluctuation
which was correlated with the antenna azimuth and elevation position.
Those tests are described in an internal test report [4]. Further tests
and meetings took place in an effort to resolve this issue [5-8]. In
addition, an ALMA memo was written describing a theoretical treatment of
the measured phase fluctuation [9], supporting the measurement results
which showed that the phase fluctuation was due to the absolute
polarization change (caused by the fiber movement) of the two
lightwaves, and that the phase fluctuation magnitude was inversely
proportional to the degree of polarization alignment of the two
lightwaves. To put it another way, if the state-of-polarizations (SOPs)
of the two lightwaves were different at the receiver end, then any
movement of the fiber would cause a phase change. The purpose of this
memo is to summarize the main points from the references listed above,
and additionally to describe more recent measurements that utilize
improvements to the 1st LO reference baseline design. This is mainly an
experimental report, there is a related theoretical study as well [14]. 

-------------------------------------------------------


ALMA Memo # 515


Calculation of Integration Times for WVR 



Alison Stirling, Mark Holdaway, Richard Hills, John Richer 
2005-03-02


Abstract:In this memo we address the issue of how to apply water vapour
radiometer estimates of atmospheric phase to visibility data. We
consider the impact of smoothing the radiometer data over a period of
time to reduce the noise in the w.v.r. estimate, and applying a
multiplicative factor to decrease the impact of the w.v.r. estimate when
the phase fluctuation amplitude is small compared with the radiometer
noise.

We find that when fast-switching is taken into account, for fully
three-dimensional turbulence, and r.m.s. path length fluctuations of
order 75 9m, the optimal smoothing timescale is 11 seconds. This
timescale decreases to around 3 seconds as the thickness of the
turbulence layer becomes small compared with the baseline length. These
values are found to decrease as the r.m.s. fluctuations increase. A
multiplicative factor is required to modify the w.v.r. correction term
for fluctuations less than ~ 50 9m, where the noise in the radiometer
becomes comparable to fluctuations at the site.

----------------------------------------------------------


ALMA Memo # 516


Evaluation of a Commercially Available High Efficiency Photomixer 



A. Ueda, Y. Sekimoto, M. Ishiguro, S. Asayama, M. Yamada, T. Noguchi 
2005-02-18


Abstract: Application of photomixer technologies to radio astronomy has
been developed under the collaboration between NAOJ and NTT
Laboratories1-7. We have evaluated high efficiency photomixers for
W-band (75 - 110 GHz) and D-band (110-170GHz) using UTC-PD8 photodiodes
developed by NTT Photonics Laboratories and fabricated by NTT
Electronics Co. (NEL)9-10. Responsivity of the photodiode is
approximately 0.4 A/W. The photomixer output power is measured for
photocurrents ranging from 1mA to 10mA. The output power with an optical
input power of 0.7mW (0.28mA photocurrent) is estimated to be 1 W. This
result indicates that the photomixers meet the requirement for the ALMA
photonic reference system without using the optical amplifier at each
antenna. We also measured output powers for 75GHz - 350GHz and noise
performances for 130GHz - 165GHz. 


_______________________________________________
Almanews mailing list
Almanews at listmgr.cv.nrao.edu
http://listmgr.cv.nrao.edu/mailman/listinfo/almanews