Etude comparative de diverses structures de filtres numériques : application aux signaux à très large bande et au corrélateur ALMA

The ALMA (Acatama Large Millimeter Array) project is a large interferometric array for radioastronomy at millimeter and submillimiter wavelengths. The astronomical signal detection is performed by digital electronic systems implementing signal pprocessing algorithms (filtering system, correlation). The main characteristics of the digitized signal are: broad input nband (2 GHz) with rapid spectral variations, random nature, few quantization levels. Massive parallel architectures are required by the broad band signal for signal processing, especially for the digital filtering system. The latter is at the heart of the correlation versatility and performances because it offers high spectral resolutions for different observing modes. The principal goal of this study is to optimize the power consumption of the FPGAs used for digital filtering, and to review the various possible structures leading to broad band filtering. A multistage structure based on a CIC filter with a demultiplexed architecture has been selected by the ALMA project.Le projet ALMA (Atacama Large Millimeter Array) vise à construire un grand réseau interférométrique pour la radioastronomie en onde millimétrique et sub-millimétrique. La détection du signal astronomique est réalisée par des systèmes d'électronique numérique qui intègrent des algorithmes de traitement du signal (filtrage, corrélation). Les caractéristiques principales du signal numérisé sont : très large bande (2GHz) mais présentant des variations spectrales fines, de nature aléatoire, avec quantification sur peu de niveaux. La largeur de bande requiert des architectures massivement parallèles pour le traitement du signal notamment pour le sous-système de filtrage numérique. Celui-ci est au coeur de la souplesse et des performances du corrélateur, offrant de hautes résolutions spectrales dans divers modes d'observation. Le but principal de cette étude est d'optimiser la consommation des FPGAs utilisés pour le filtrage numérique, et de recenser les diverses structures possibles amenant au filtrage de larges bandes. Une structure multi-étages basée sur l'utilisation d'un filtre CIC à entrée démultiplexée a été retenue par le projet ALMA

Comparative study of several digital filtering architectures. Application to very large band signals and to the ALMA Project

Le projet ALMA (Atacama Large Millimeter Array) vise à construire un grand réseau interférométrique pour la radioastronomie en onde millimétrique et sub-millimétrique. La détection du signal astronomique est réalisée par des systèmes d'électronique numérique qui intègrent des algorithmes de traitement du signal (filtrage, corrélation). Les caractéristiques principales du signal numérisé sont : très large bande (2GHz) mais présentant des variations spectrales fines, de nature aléatoire, avec quantification sur peu de niveaux. La largeur de bande requiert des architectures massivement parallèles pour le traitement du signal notamment pour le sous-système de filtrage numérique. Celui-ci est au coeur de la souplesse et des performances du corrélateur, offrant de hautes résolutions spectrales dans divers modes d'observation. Le but principal de cette étude est d'optimiser la consommation des FPGAs utilisés pour le filtrage numérique, et de recenser les diverses structures possibles amenant au filtrage de larges bandes. Une structure multi-étages basée sur l'utilisation d'un filtre CIC à entrée démultiplexée a été retenue par le projet ALMA.The ALMA (Acatama Large Millimeter Array) project is a large interferometric array for radioastronomy at millimeter and submillimiter wavelengths. The astronomical signal detection is performed by digital electronic systems implementing signal pprocessing algorithms (filtering system, correlation). The main characteristics of the digitized signal are: broad input nband (2 GHz) with rapid spectral variations, random nature, few quantization levels. Massive parallel architectures are required by the broad band signal for signal processing, especially for the digital filtering system. The latter is at the heart of the correlation versatility and performances because it offers high spectral resolutions for different observing modes. The principal goal of this study is to optimize the power consumption of the FPGAs used for digital filtering, and to review the various possible structures leading to broad band filtering. A multistage structure based on a CIC filter with a demultiplexed architecture has been selected by the ALMA project

Modulation directe d'un synthétiseur de fréquence à division fractionnaire en boucle fermée

L'étude de la modulation directe d'un synthétiseur de fréquence à division fractionnaire en boucle fermée a été menée dans le but de faciliter la réalisation d'un tel modulateur de fréquence. L'étude concerne la partie synthétiseur de fréquence à division fractionnaire, et en particulier le convertisseur Sigma-Delta qui permet de réaliser la division fractionnaire, ainsi que la partie numérique rajoutée pour transformer le synthétiseur de fréquence en modulateur de fréquence. Cette étude a été appliquée a la modulation GMSK du standard GSM par la réalisation d'un modulateur direct en technologie BiCMOS SiGe.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

A 4 GHz digital receiver using the Uniboard platform

The Uniboard is a general purpose board, developed as a part of the Radionet FP7 program, that hosts 8 Altera StratixIV FPGAs interconnected by high speed links. It can be used standalone or as a part of a more complex system. The Digital receiver application uses a single Uniboard to implement a flexible packetization of a wideband signal in the frequency domain. It accepts a 4 GHz (8 GS/s) input bandwidth and provides up to 64 output bands. The bandwidth and position of each output band can be independently adjusted. The input signal is first analyzed by a polyphase filterbank, that splits the input band into 32 sub-bands with a bandwidth of 190 MHz and a spacing of 128 MHz. The overlap among adjacent bands allows the positioning of the output bands without dead regions. This filterbank is followed by an array of digitally defined downconverters, each one composed of a mixer/LO and a variable decimation filter. The filter band can be adjusted in binary steps from 1 to 128 MHz. Using tap recirculation, the filter shape remains constant over this whole range, with about 60 dB of stopband rejection and 90% usable passband. The output bands are packetized according to the VDIF VLBI standard, over eight 10G Ethernet links. Further processing can be done either on board, or in a cluster of conventional PCs. In addition, high speed ADC are in-house developed (ASIC 65nm CMOS STmicroelectronics) to feed the Uniboard card with 8GS/s, 4GHz BW, 3bits samples

The new 3-satge, low dissipation digital filter of the ALMA correlator

ALMA Memo 579 (2008)The main goal of this study is to reduce the power dissipation of the 2-stage digital filter used in the ALMA Correlator system. This has been achieved by optimizing the number of FPGA logic elements used for the filter implementation. We have investigated the implementation of various structures based on the Cascaded Integrator Comb (CIC) filter in order to replace the present first filter stage, a 32-time demultiplexed input decimation filter. We conclude that a CIC filter cascaded with a quarter-band filter significantly improves the overall power dissipation and thus the FPGA thermal behaviour and reliability. This new design results in a significant improvement (nearly 25%) in the dissipation of each one of the ALMA filter cards

A Multi-Rate Filter for Broadband Radiotelescopes

International audienceWe have investigated the implementation of various structures based on the Cascaded Integrator Comb (CIC) filter to replace the decimation filter in the first stage, 32-time demultiplexed input digital filter of the large Correlator System of the ALMA (Atacama Large Millimeter Array) interferometer project. The main goal of this study is to reduce the power dissipation in the 2-stage, 32-time demultiplexed ALMA digital filter implemeneted in large FPGA's by optimizing the used number of logic elements available in these chips. Different modified CIC filter implementations are presented and compared in terms of complexity and power performances. We conclude that a CIC filter with a Quarter-band filter significantly improve the overall power dissipation and thus the ALMA filter efficiency