50 research outputs found
Room Temperature, Quantum-Limited THz Heterodyne Detection? Not Yet
In their article, Wang et al. [1] report a new scheme for THz heterodyne detection using a laser-driven LTG-GaAs photomixer [2, 3] and make the impressive claim of achieving near quantum-limited sensitivity at room temperature. Unfortunately, their experimental methodology is incorrect, and furthermore the paper provides no information on the mixer conversion loss, an important quantity that could readily have been measured and reported as a consistency check. The paper thus offers no reliable experimental evidence that substantiates the claimed sensitivities. To the contrary, the very high value reported for their photomixer impedance strongly suggests that the conversion loss is quite poor and that the actual sensitivity is far worse than claimed
The Cosmic Background Imager
Design and performance details are given for the Cosmic Background Imager
(CBI), an interferometer array that is measuring the power spectrum of
fluctuations in the cosmic microwave background radiation (CMBR) for multipoles
in the range 400 < l < 3500. The CBI is located at an altitude of 5000 m in the
Atacama Desert in northern Chile. It is a planar synthesis array with 13 0.9-m
diameter antennas on a 6-m diameter tracking platform. Each antenna has a
cooled, low-noise receiver operating in the 26-36 GHz band. Signals are
cross-correlated in an analog filterbank correlator with ten 1 GHz bands. This
allows spectral index measurements which can be used to distinguish CMBR
signals from diffuse galactic foregrounds. A 1.2 kHz 180-deg phase switching
scheme is used to reject cross-talk and low-frequency pick-up in the signal
processing system. The CBI has a 3-axis mount which allows the tracking
platform to be rotated about the optical axis, providing improved (u,v)
coverage and a powerful discriminant against false signals generated in the
receiving electronics. Rotating the tracking platform also permits polarization
measurements when some of the antennas are configured for the orthogonal
polarization.Comment: 14 pages. Accepted for publication in PASP. See also
http://www.astro.caltech.edu/~tjp/CBI
Design, Implementation and Testing of the MAP Radiometers
The Microwave Anisotropy Probe (MAP) satellite, launched June 30, 2001, will
produce full sky maps of the cosmic microwave background radiation in 5
frequency bands spanning 20 - 106 GHz. MAP contains 20 differential radiometers
built with High Electron Mobility Transistor (HEMT) amplifiers with passively
cooled input stages. The design and test techniques used to evaluate and
minimize systematic errors and the pre-launch performance of the radiometers
for all five bands are presented.Comment: Updated with comments; 24 pages with 10 low-resolution figures;
version with better figures is at
http://lambda.gsfc.nasa.gov/product/map/map_bibliography.htm
Limits on Arcminute Scale Cosmic Microwave Background Anisotropy with the BIMA Array
We have used the Berkeley-Illinois-Maryland-Association (BIMA) millimeter
array outfitted with sensitive cm-wave receivers to search for Cosmic Microwave
Background (CMB) anisotropies on arcminute scales. The interferometer was
placed in a compact configuration which produces high brightness sensitivity,
while providing discrimination against point sources. Operating at a frequency
of 28.5 GHz, the FWHM primary beam of the instrument is 6.6 arcminutes. We have
made sensitive images of seven fields, five of which where chosen specifically
to have low IR dust contrast and be free of bright radio sources. Additional
observations with the Owens Valley Radio Observatory (OVRO) millimeter array
were used to assist in the location and removal of radio point sources.
Applying a Bayesian analysis to the raw visibility data, we place limits on CMB
anisotropy flat-band power Q_flat = 5.6 (+3.0 -5.6) uK and Q_flat < 14.1 uK at
68% and 95% confidence. The sensitivity of this experiment to flat band power
peaks at a multipole of l = 5470, which corresponds to an angular scale of
approximately 2 arcminutes. The most likely value of Q_flat is similar to the
level of the expected secondary anisotropies.Comment: 15 pages, 5 figures, LaTex, aas2pp4.sty, ApJ submitte
Design, development and verification of the 30 and 44 GHz front-end modules for the Planck Low Frequency Instrument
We give a description of the design, construction and testing of the 30 and
44 GHz Front End Modules (FEMs) for the Low Frequency Instrument (LFI) of the
Planck mission to be launched in 2009. The scientific requirements of the
mission determine the performance parameters to be met by the FEMs, including
their linear polarization characteristics.
The FEM design is that of a differential pseudo-correlation radiometer in
which the signal from the sky is compared with a 4-K blackbody load. The Low
Noise Amplifier (LNA) at the heart of the FEM is based on indium phosphide High
Electron Mobility Transistors (HEMTs). The radiometer incorporates a novel
phase-switch design which gives excellent amplitude and phase match across the
band.
The noise temperature requirements are met within the measurement errors at
the two frequencies. For the most sensitive LNAs, the noise temperature at the
band centre is 3 and 5 times the quantum limit at 30 and 44 GHz respectively.
For some of the FEMs, the noise temperature is still falling as the ambient
temperature is reduced to 20 K. Stability tests of the FEMs, including a
measurement of the 1/f knee frequency, also meet mission requirements.
The 30 and 44 GHz FEMs have met or bettered the mission requirements in all
critical aspects. The most sensitive LNAs have reached new limits of noise
temperature for HEMTs at their band centres. The FEMs have well-defined linear
polarization characteristcs.Comment: 39 pages, 33 figures (33 EPS files), 12 tables. Planck LFI technical
papers published by JINST:
http://www.iop.org/EJ/journal/-page=extra.proc5/1748-022
Implications of the Cosmic Background Imager Polarization Data
We present new measurements of the power spectra of the E-mode of CMB
polarization, the temperature T, the cross-correlation of E and T, and upper
limits on the B-mode from 2.5 years of dedicated Cosmic Background Imager (CBI)
observations. Both raw maps and optimal signal images in the uv-plane and real
space show strong detections of the E-mode (11.7 sigma for the EE power
spectrum overall) and no detection of the B-mode. The power spectra are used to
constrain parameters of the flat tilted adiabatic Lambda-CDM models: those
determined from EE and TE bandpowers agree with those from TT, a powerful
consistency check. There is little tolerance for shifting polarization peaks
from the TT-forecast locations, as measured by the angular sound crossing scale
theta = 100 ell_s = 1.03 +/- 0.02 from EE and TE cf. 1.044 +/- 0.005 with the
TT data included. The scope for extra out-of-phase peaks from subdominant
isocurvature modes is also curtailed. The EE and TE measurements of CBI, DASI
and BOOMERANG are mutually consistent, and, taken together rather than singly,
give enhanced leverage for these tests.Comment: 15 pages, 9 figures, submitted to ApJ -- Accepted version. The
fine-bin spectrum, covariance matrix, and window functions are now available
on the web (suitable for use in COSMOMC) at:
http://www.astro.caltech.edu/~tjp/CBI/data2006/index.html The pipeline in the
previous version inadvertently omitted one antenna, so the new spectrum
contains ~15% more data. We emphasize that previous results were in no way
biased, and that the (small) changes to the spectrum solely reflect the
inclusion of the additional data. Numbers and figures in the paper have been
updated correspondingly. All maps now have color bar