[asac]FW: [almanews] ALMA Memos 424 and 425 Released

[Stacy Oliver]

ALMA Memo # 424

Development of 4-Gsps 2-bit ADC with GaAs ICs for radio astronomy

Mahoko Okiura(The University of Tokyo / NRO), Satoru Iguchi(NAO), Sachiko K.
Okumura(NRO), Munetake Momose(Ibaraki University), Kinfya Matsumoto(Kyushyu
Toukai University), and Noriyuki Kawaguchi(NAO)

17/05/2002

Keywords: Analog to Digital converter

We developed two sets of the prototype of 4-Gsps 2-bit Analog to Digital
converter (ADC) with GaAs IC for the realization of 4 GHz sampling in an ADC
system of Atacama Large Millimeter / submillimeter Array (ALMA). This
consists of three sets of 12-Gbps Decision Circuits and 12-Gbps
Demulitiplexers, and has a capability of sampling in a bandwidth of 6 GHz.
Toward the implementation of GaAs IC in the ADC system, we measured the
sampling jitter of the ADC using the Ultra-Wide Band Correlator (UWBC)
developed for the Nobeyama Millimeter Array (NMA). The measured Allan
standard deviation of phase, corresponding to the stability of sampling
timing is 2.3x10^-15 at 10 seconds, and the Allan standard deviation due to
the Flicker-frequency noise is 0.8x10^-15. It is shown that the coherence
loss successfully becomes 0.95 at coherence time of 86400 seconds (24
hours). The Alan standard deviation of sampling timing and its long-time
stability are acceptable for the application of radio astronomical ADC
system.

View a pdf version of ALMA Memo 424.
http://www.alma.nrao.edu/memos/html-memos/alma424/memo424.pdf

Download a postscript version of ALMA Memo 424.
http://www.alma.nrao.edu/memos/html-memos/alma424/memo424.ps

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ALMA Memo # 425

The Bøifot Orthomode Junction

Edward J. Wollack (NASA/GSFC),Wes Grammer (NRAO),Jeff Kingsley (NRAO)

22/05/2002

Keywords: Orthomode Transducer, Orthomode Junction, Polarization Diplexer,
Waveguide Junction

Wideband dual-polarization performance is desired for low-noise receivers
and radiometers at centimeter and millimeter wavelengths. The use of a
waveguide orthomode transducer (OMT) can increase spectral coverage and
sensitivity  while reducing exit aperture size, optical spill, and
instrumental polarization offsets. For these reasons, an orthomode junction
is favored over a traditional quasi-optical wire grid for focal plane
imaging arrays from a systems perspective. The fabrication and performance
of wideband symmetric Bøifot OMT junctions at K-, Ka-, Q-, and W-bands are
described. Typical WR10.0 units have an insertion loss of <0.2 dB, return
loss ~20dB, and >40dB isolation over a >75-to-110 GHz band. The OMT operates
with reduced ohmic losses at cryogenic temperatures.

View a pdf version of ALMA Memo 425.
http://www.alma.nrao.edu/memos/html-memos/alma425/memo425.pdf

Download a postscript version of ALMA Memo 425.
http://www.alma.nrao.edu/memos/html-memos/alma425/memo425.ps







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