1,721,225 research outputs found
A Push-Pull Class-C CMOS VCO
No full textA CMOS oscillator employing differential transistor pairs working in Class-C in push-pull configuration is presented. The oscillator exhibits the same advantages enjoyed by complementary topologies on oscillators based on a single differential pair, while yielding a substantial power consumption reduction thanks to the Class-C operation. The phase-noise performance and the fundamental conditions required to keep the transistors working in Class-C are analyzed in detail. It is shown that, for an optimal performance, both nMOS and pMOS transistors should not be pushed into the deep triode region by the instantaneous resonator voltage, and a simple circuit solution is proposed to accommodate a large oscillation swing. A 0.18-mu m CMOS prototype of the (voltage-controlled) oscillator displays an oscillation frequency from 6.09 to 7.50 GHz. The phase noise at 2-MHz offset is below -120 dBc/Hz with a power dissipation of 2.2 mW, for a state-of-the-art figure-of-merit ranging from 189 to 191 dBc/Hz
COLD DUST AROUND HIGH-REDSHIFT QUASARS
No full textObservations of the continuum emission at 1.25 mm of 10 high-redshift (z > 3) and 10 intermediate-redshift (1.5 < z < 3) quasars, previously observed at 6 cm with the VLA, have been carried out with the SEST 15-m telescope in Chile and the IRAM 30-m antenna in Spain. Four detections at a level better than 3sigma and three marginal ones (2.8sigma and 2sigma) have been achieved. On the basis of these data it is possible to investigate the expected turnover of the AGN submillimetre spectrum, since in the quasar rest frames the 1.25-mm continuum corresponds to emission around 230-500 mum. As far as radio-quiet objects are concerned, the mechanism responsible for part of this emission is probably thermal and can be ascribed to dust grains. We discuss the possibility that this dust belongs to the quasar host galaxies, implying evidence for cold dust in these galaxies. The extinction caused by this dust could account for the higher spectral indices of the UV spectra of the detected objects with respect to the non-detected ones
IS THE UNIVERSE A FRACTAL - TESTS ON QUASAR SAMPLES
No full textWe have used several complete samples of quasars (450 objects in total) to gain information on the large-scale structure of the Universe at redshifts 0.4 < z < 2.2. We have evaluated the conditional density and the average density functions to establish that on scales larger than 12.5 h-1 Mpc the quasar distribution becomes homogeneous. The use of a more conventional and time-honoured technique such as the correlation function is therefore appropriate in the case of the present samples and equivalent to the conditional and average density functions. These latter functions allow us, however, to put slightly stronger constraints on the amplitude of clustering at large scales. Possible observational biases which could lead to incorrect conclusions on quasar clustering are discussed
Quasar clustering: Evidence for an increase with redshift and implications for the nature of active Galactic nuclei
No full textThe evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3 = 15 h(-1) Mpc)= 3/r(3) integral(0)(r)x(2) xi(x)dx = 0.52 +/- 20 at an average redshift of [z = 1.3]. We find a 2 sigma significant increase of the quasar clustering between z = 0.95 and z = 1.8, independent of the quasar absolute magnitude and inconsistent with recent evidence on the evolution of galaxy clustering. If other quasar samples are added (resulting in a total data set of 737 quasars), the increase of the quasar clustering is still favored, although it becomes less significant. With a parameterization of the evolution of the type xi(r, z)=(r/r(0))(-gamma)(1 + z)(-(3-gamma+epsilon)), we find epsilon similar or equal to -2.5. Evolutionary parameters epsilon > 0.0 are excluded at a 0.3% probability level, compared with epsilon similar to 0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Omega = 1 cold dark matter in which (1) the difference between the quasar and the galaxy clustering can be explained as a difference in the effective bias and redshift distributions and (2) the quasars, with a lifetime of t similar to 10(8) yr, sparsely sample halos of mass greater than M-min similar to 10(12)-10(13) h(-1) M.. We also discuss the possibility that the observed change in the quasar clustering is due to an increase in the fraction of early-type galaxies as quasar hosts at high z
On the Bias Noise to Phase Noise Conversion in Harmonic Oscillators Using Groszkowski Theory
No full textWe present a study of tail-bias-current 1/ƒ noise upconversion into 1/ƒ3 phase noise for both Colpitts and differential-pair LC oscillators. We focus on the Groszkowski effect, i.e. the shift in oscillation frequency induced by the higher current harmonics flowing into the LC tank of a harmonic oscillator. Quantitative results match very well numerical simulations run with spectreRF, and show that the Groszkowski effect may indeed be a dominant cause of 1/ƒ3 phase noise generation
Report on the ESO Workshop ''The Impact of Herschel Surveys on ALMA Early Science''
No full textThe ESA Herschel Space Observatory is currently producing new and exciting results, thanks to its unprecedented sensitivity, spectral resolution and wide-area surveying capabilities at far-infrared and submillimetre wavelengths. Many of the new discoveries by Herschel will require high angular resolution follow-up observations with ALMA. The goal of the workshop was to discuss the priorities for ALMA Early Science follow-up of the Herschel photometric and spectroscopic surveys. The possibility, or need for, simultaneous observing programmes with ALMA and Herschel was also discussed
Phase Noise Analysis of the Tuned-Input–Tuned-Output (TITO) Oscillator
No full textThe theoretical phase noise performance of a tuned-input tuned-output (TITO) oscillator is analyzed with a rigorous approach, which yields a compact closed-form phase noise equation that is dependent only on the value of the circuit components and current consumption of the oscillator. A straightforward comparison with the more commonly used differential LC-tank oscillator shows that the latter is in fact superior to the TITO oscillator, at least if the oscillator behavior is not too distant from the ideal behavior considered in the analysis. Phase noise simulations match admirably the theoretical results
- …
