21 research outputs found
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
Planck pre-launch status: calibration of the Low Frequency Instrument flight model radiometers
The Low Frequency Instrument (LFI) on-board the ESA Planck satellite carries
eleven radiometer subsystems, called Radiometer Chain Assemblies (RCAs), each
composed of a pair of pseudo-correlation receivers. We describe the on-ground
calibration campaign performed to qualify the flight model RCAs and to measure
their pre-launch performances. Each RCA was calibrated in a dedicated
flight-like cryogenic environment with the radiometer front-end cooled to 20K
and the back-end at 300K, and with an external input load cooled to 4K. A
matched load simulating a blackbody at different temperatures was placed in
front of the sky horn to derive basic radiometer properties such as noise
temperature, gain, and noise performance, e.g. 1/f noise. The spectral response
of each detector was measured as was their susceptibility to thermal variation.
All eleven LFI RCAs were calibrated. Instrumental parameters measured in these
tests, such as noise temperature, bandwidth, radiometer isolation, and
linearity, provide essential inputs to the Planck-LFI data analysis.Comment: 15 pages, 18 figures. Accepted for publication in Astronomy and
Astrophysic
An adaptive drift-diffusion model of interval timing dynamics
Animals readily learn the timing between salient events. They can even adapt their timed responding to rapidly changing intervals, sometimes as quickly as a single trial. Recently, drift-diffusion models—widely used to model response times in decision making—have been extended with new learning rules that allow them to accommodate steady-state interval timing, including scalar timing and timescale invariance. These time-adaptive drift-diffusion models (TDDMs) work by accumulating evidence of elapsing time through their drift rate, thereby encoding the to-be-timed interval. One outstanding challenge for these models lies in the dynamics of interval timing—when the to-be-timed intervals are non-stationary. On these schedules, animals often fail to exhibit strict timescale invariance, as expected by the TDDMs and most other timing models. Here, we introduce a simple extension to these TDDMs, where the response threshold is a linear function of the observed event rate. This new model compares favorably against the basic TDDMs and the multiple-time-scale (MTS) habituation model when evaluated against three published datasets on timing dynamics in pigeons. Our results suggest that the threshold for triggering responding in interval timing changes as a function of recent intervals
The differential role of the dorsal hippocampus in initiating and terminating timed responses: A lesion study using the switch-timing task
Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solution
A kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl-
3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed
an absence of buffer catalysis using H2PO4
-/HPO4
2-, H3BO3/H2BO3
-, and HCO3
-/CO3
2-
regulators and a complex dependency of the rate constant on the pH. We discovered the existence
of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the
N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the
dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the
formation of the monoanion of the N-nitrosoguanidine, with values of pKa I ) 11.5. The reaction
rate for the process through the monoanion, k2, decreases as the acidity increases. The application
of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate
when there exists a possibility of stabilization of the negative charge by resonance. This behavior
is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the
elimination reaction. When dealing with the case of the elimination through the neutral form we
observe that the reaction rate increases together with the capacity of stabilization of the positive
charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we
used a maximum value of k3 ) 1010 s-1 to estimate the value of pKa
II for the formation of the
dianion of the N-nitrosoguanidine, obtaining values of pKa
II < 24