Optimal Dithering Configuration Mitigating Rayleigh-Backscattering-Induced Distortion in Radioastronomic Optical Fiber Systems

In the context of Radioastronomic applications where the Analog Radio-over-Fiber technology is used for the antenna downlink, detrimental nonlinearity effects arise because of the interference between the forward signal generated by the laser and the Rayleigh backscattered one which is re-forwarded by the laser itself toward the photodetector. The adoption of the so called dithering technique, which involves the direct modulation of the laser with a sinusoidal tone and takes advantage of the laser chirping phenomenon, has been proved to reduce such Rayleigh Back Scattering - induced nonlinearities. The frequency and the amplitude of the dithering tone should both be as low as possible, in order to avoid undesired collateral effects on the received spectrum as well as keep at low levels the global energy consumption. Through a comprehensive analysis of dithered Radio over Fiber systems, it is demonstrated that a progressive reduction of the dithering tone frequency affects in a peculiar fashion both the chirping characteristics of the field emitted by the laser and the spectrum pattern of the received signal at the fiber end. Accounting for the concurrent effects caused by such phenomena, optimal operating conditions are identified for the implementation of the dithering tone technique in radioastronomic systems

Development of a New Digital Signal Processing Platform for the Square Kilometre Array

A novel digital hardware platform has been designed for the Low Frequency Aperture Array (LFAA) component of the Square Kilometre Array (SKA). This board, called Analog Digital Unit (ADU), is a 6U board containing sixteen dual-inputs Analog to Digital Converters (ADC) and two Field Programmable Gate Array (FPGA) devices, capable of digitizing and processing 32 RF input signals. We present the main features of the board and the signal processing firmware that has been developed for LFAA. Although the ADU has been conceived mainly for the low frequency band (50-350 MHz), its use has been proved effective also for higher frequencies (375-650 MHz). In this paper we describe also the application of ADU as the digital acquisition and processing system for PHAROS2, a cryogenically cooled 4-8 GHz Phased Array Feed (PAF) demonstrator. The final part is focused on the future developments of the board

Design of cryogenic phased array feed for 4-8 GHz

We describe the design and architecture of PHAROS2, a cryogenically cooled 4-8 GHz Phased Array Feed (PAF) demonstrator with a digital beamformer for radio astronomy application. The instrument will be capable of synthesizing four independent single-polarization beams by combining 24 active elements of an array of Vivaldi antennas. PHAROS2, the upgrade of PHAROS (PHased Arrays for Reflector Observing Systems), features: a) commercial cryogenic LNAs with state-of-the-art performance, b) a “Warm Section” for signal filtering, conditioning and single downconversion to select a ≈275 MHz Intermediate Frequency (IF) bandwidth within the 4-8 GHz Radio Frequency (RF) band, c) an IF signal transportation by analog WDM (Wavelength Division Mutiplexing) fiber-optic link, and d) a FPGA-based Italian Tile Processing Module (iTPM) digital backend.peer-reviewe

The effect of episodes of large capital inflows on domestic credit

This paper analyses the effect of capital inflow surges on the evolution of domestic credit. Using a panel of developed and emerging economies from 1970 to 2007, it is shown that in the two years following the beginning of a capital inflow surge the credit-to-GDP ratio increases by about 2 percentage points. The effect is reversed in the medium-term with the credit-to-GDP ratio decreased by almost 4 percentage points seven years after the initial surge. The paper also finds that the effect is different depending on the type of flows characterising the episode (debt vs. portfolio equity vs. FDI), with large capital inflows that are debt-driven having the largest effect. The results of the paper also suggest that the short-term effect of capital inflow surges on domestic credit depends on countries\u2019 macroeconomic policy stances. In particular, it is found that this effect is lower in countries with higher real exchange rate flexibility and fiscal policy counter-cyclicality

Structural and Cyclical Factors behind Current- Account Balances

Global external imbalances widened persistently over the last several years and have narrowed abruptly over the course of the financial crisis. Understanding the extent to which structural or cyclical factors may have driven these patterns is important to assess the likely evolution of global imbalances going forward, as well as the potential adjustment that can be achieved through changes in policy. This paper assesses the link between structural and cyclical factors and current-account balances using a panel of 94 countries from 1973 to 2008. We find that the medium-term evolution of global external imbalances can be related in large part to structural factors including cross-country differences in demographics, fiscal deficits, oil dependency and intensity, stage of economic development, financial market development, and institutional quality. Part of the narrowing in current-account balances since the financial crisis appears to be related to various cyclical factors including changes in output growth, oil prices, and exchange rates, and may be expected to reverse alongside the economic recovery

MEDIUM-TERM DETERMINANTS OF INTERNATIONAL INVESTMENT POSITIONS: THE ROLE OF STRUCTURAL POLICIES

This paper provides an empirical investigation of the medium-term determinants of international investment positions for a large sample of advanced and emerging economies. In addition to the usually considered drivers offoreign assets and liabilities, the analysis focuses on the role of structural policy indicators. Using cross-section and panel regression techniques the results suggest that structural policy settings are important medium-term drivers of capital flows, having a relatively large impact on gross and net foreign capital positions and on their composition. In particular, the results suggest that certain kinds of structural policy reform could help to narrow global imbalances, and to modify the composition of international capital flows towards more stable and productive sources

Challenges due to rayleigh backscattering in radio over fibre links for the square kilometre array radio-telescope

In the realization of the Low Frequency Aperture Array (LFAA) within the Square Kilometre Array (SKA) project, the radiofrequency (RF) signals coming from sky and celestial sources, and received by each of the 130k+ antennas, are transmitted to the signal processing unit through analogue Radio over Fibre links. Within this low frequency band, the relatively low levels of the power of such RF signals may give rise to unexpected distortion effects. The present works highlights the physical origin of such undesired spurious frequencies generation, which can be also found in other applicative telecommunications scenarios. The details of a possible solution able to keep the above mentioned distortion terms at acceptably low levels are finally illustrated