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[<< Home](/home#1-alpaca-introduction-and-instrument-description)
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[<< Section 1.2](./1.2)
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## 1.3 High-level ALPACA System Description
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The ALPACA instrument will consist of a fully cryogenic 69-element phased array feed (PAF) and real-time
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digital beamformer back end capable of producing 40 simultaneous dual polarized
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beams with approximately 305 MHz of instantaneous bandwidth at 1.4 GHz. Key performance targets for ALPACA are a system noise temperature of 25 K, aperture efficiency 70%, and field of view +/- 20 amin from the reflector boresight.
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ALPACA consists of the following major subsystems:
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- **Analog Front End:** Phased array elements, cryogenics, and low noise amplifiers
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- **Signal Transport:** Gain stages, signal interconnects, variable attenuation, filters, phase calibration tone injection, and RF over fiber links
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- **Digital Back End:** Course filterbank, data packet distribution, fine filterbank, beamformer, correlator, disk array for data storage
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The analog front end includes the PAF array and and first stage LNAs housed in the cryostat are mounted to a front end box (FEB) which contains support electronics. The FEB/Cryostat will be mounted on the GBT at prime focus in the Stirling mount, and will utilize its rotational axis actuator for parallactic tracking during long exposure observations. The cryo compressor will be mounted on the GBT alidade with cooling Helium lines running the 600 feet to prime focus.
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Signal transport is via 144 (138 + spares) analog RF over optical fiber (RFoF) links from prime focus to the new Data Center building under construction at GBO, which will house the digital back end. Custom boards for signal conditioning, full operational bandwidth bandpass filtering, and the RFoF transmitter are housed in the FEB at prime focus, while mating receiver cards will connect into the RF system on a chip (RFSoC) boards in the Data Center equipment racks.
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The digital back end receiver is housed in 6 equipment racks in the Data Center. It consists of 12 RFSoC boards, 25 dual CPU rack mount numerical processor servers, 50 graphical processor cards in those servers, a 60 port 100 Gigabit network switch, a 32 port 100 Gigabit InfiniBand network switch, a 3 rack mount mass storage servers for a BeeGFS distributed storage system, and associated peripherals and power conditioning. The RFSoCs include on-chip 2 Gsps direct sampling ADCs to digitize the L-band RF inputs, and FPGAs with custom firmware to frequency channelize these signals (the F-Engine). The 100 Gigibit switch distributes channelized data to the 50 GPUs where the XB-engine performs array beamforming, array correlator, and integrating beamformed spectrometer operations. Data products are routed to the distributed file storage subsystem for archiving.
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Supporting elements of ALPACA include software interfaces to GBO monitor and control systems and software for loading and processing stored PAF data for image formation.
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A block diagram of the full ALPACA system is shown below:
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<div align="center">
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<img src="../img/alpaca-system-diagram.png" width="700">
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</div>
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[Section 2 >>](/2) |
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\ No newline at end of file |
