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[<< Home](/home#1-alpaca-introduction-and-instrument-description)
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[<< Section 1.1](./1.1)
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## 1.2 Introduction to PAF Instruments and Digital Beamformers
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Traditional horn feeds provide one pixel on the sky and have a fixed dish illumination pattern and beam shape. Cluster feeds increase survey speed by allowing for multiple simultaneous pixels. At mm-wave frequencies and above, individual sensors can be packed tightly enough to provide a continuous field of view on the sky. These receivers are commonly bolometer arrays and are referred to as focal plane arrays.
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At microwave frequencies, horn feeds are electrically large and cannot be spaced closely enough to achieve a continuous field on the sky. In order to implement a true radio camera, with multiple overlapping pixels and a continuous field of view, beams must be synthesized using many electrically small antenna elements in a dense phased array. This is a phased array feed (PAF). Some of the PAF development projects from recent years in various stages of progress around the world are shown below.
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<div align="center">
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<img src="../uploads/d7c226d819df63055d92f75758508d40/paf_projects.png" width="800"/>
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</div>
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Figure a: Phased array feed development projects around the world. Astronomical PAF development began with an effort at NRAO in the 1990s shown in the center panel. The AO19 receiver at bottom left was a prototype instrument with the array developed by Cornell and digital back end by BYU. It was tested on the Arecibo telescope as a precursor to ALPACA.
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BYU and Cornell have a lengthy history over two decades of research on phased array feeds as they developed the technology which is at the core of ALPACA. In addition to the references in the figure above to AO19 and the BYU/NRAO PAF experiment on the Green Bank 20 meter Telescope, the following figure presents recent work directly related to ALPACA.
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<div align="center">
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<img src="../uploads/010febb08d89b9a88df38743e01dd3ca/FLAG_and_AO19_images.png" width="650"/>
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</div>
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Figure b: Top left: The BYU/NRAO/GBO FLAG 19 element PAF and front end box. Top 2nd: Detail of the FLAG cryogenically cooled array element package. Top 3rd: FLAG beamformer XB-Engine GPU servers. Top right: FLAG FPGA frequency channelizer F-Engine. Bottom left: The Cornell/BYU AO19 Cryogenic PAF mounted on the Arecibo Telescope as test prototype for ALPACA technology. Bottom right: AO19 cryostat on transport cart at AO.
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The primary advantage of a PAF is a wide, continuous field of view with overlapping pixels on the sky. Other benefits include (digital) electronic control of beam steering and shapes, the ability to optimize the dish illumination pattern, and interference mitigation by placing nulls on unwanted RFI sources.
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For single-pixel feeds, cluster feeds, and widely spaced focal plane arrays (FPA), the signal from each feed horn or FPA element is processed independently. PAFs require that signals from multiple antenna feed elements (typically densely packed) be combined with amplitude and phase adjusted to produce in combination a high-quality dish illumination pattern for each beam. Thus, a PAF requires a digital beamforming back end, similar to the digital back end for a sparse, synthesis imaging array like the Very Large Array (VLA). Signal processing operations in the back end include frequency channelization (F-engine), correlator (X-engine), and beamformer (B-engine). Correlation is required to generate beam weighting coefficients using a bright sky source. This is referred to as array calibration. Beamforming and frequency channelization are used to form operational images used for astronomical observations.
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Key measures of the performance of a PAF system include:
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- Operating frequency: PAFs have been demonstrated at frequencies from hundreds of MHz to 90 GHz. ALPACA operates at L band.
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- Bandwidth: Analog front end bandwidth for PAF systems ranges from several hundred MHz with resonant antenna elements like dipoles to several GHz with wideband elements such as Vivaldi or chequerboard arrays. If the front end bandwidth is wider than the instantaneous bandwidth processed by the digital back end, the receiver for some PAF systems can be tuned within the analog front end bandwidth.
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- Aperture efficiency, system noise temperature, $`T_{\rm sys}`$, and sensitivity: Each formed beam has an aperture efficiency and equivalent system noise temperature that varies depending on the beam steering angle. Aperture efficiency and noise temperature combine to determine the SNR achieved by each beam for a given source intensity, often quantified by the sensitivity ($`G/T`$ or $`A/T`$) in antenna theoretic terms.
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- Field of view: The angular extent of beams formed on the sky depends on the size of the phased array and the number of elements. The larger the array and the more elements in the array, the wider the sky field of view of the PAF on a dish antenna. The field of view is often defined to be the angular extent on the sky over which the sensitivity of formed beams is no less than a threshold (often 1 dB) below the peak sensitivity near the reflector boresight direction. A PAF can form beams beyond the nominal field of view, but widely steered beams provide a lower SNR than beams within the 1 dB field of view and are less useful astronomically.
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- Number of beams: In principle, any number of overlapping beams can be electronically synthesized in back end processing. If a beam overlap angular distance or power level is specified, the field of view can be expressed as a "fully sampled" or "Nyquist sampled" number of beams. Due to limitations in computational power, the digital back end may limit the number of beams to some fixed value. Ideally, the number of digitally formed beams equals or exceeds the number of Nyquist beams for the PAF field of view. As the bandwidth, number of PAF array elements, and the number of formed beams increase, the required computational power and cost of the digital back end scales accordingly.
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- Survey speed and survey efficiency: The instantaneous bandwidth, field of view, and sensitivity combine to determine the time required to detect a source of a given intensity over a given area of sky. This is quantified by the instrument survey speed. Survey speed is proportional to the product of bandwidth, field of view, and squared sensitivity. When survey speed is normalized to some comparison instrument survey speed, we refer to the relative value as survey efficiency.
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- Digital back end specifications: PAF observation modes (imaging, mapping, wide field spectral line surveys, pulsar and fast radio burst searches) require a range of capabilities in the digital signal processing software. Key parameters include number of formed beams, spectral resolution, or the width in Hz of frequency bins of output spectra, and integration dump time in a correlator or spectrometer.
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For more detailed background on phased arrays for astronomy, please see this article:
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[High-Sensitivity_Phased_Array_Receivers_for_Radio_Astronomy.pdf](../uploads/908e581b95604c9b0fff176b3b210f4c/High-Sensitivity_Phased_Array_Receivers_for_Radio_Astronomy.pdf)
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And the following article presents more detail on the Cornell and BYU work on the AO19 PAF for Arecibo, which was used as a technology development prototype for the present 40 element ALPACA instrument:
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[A Fully Cryogenic Phased Array Camera for Radio Astronomy](../uploads/f9a1e6ad9610d5abc8c877081589d5a3/German_Cortes_2015.pdf)
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[Section 1.3 >>](./1.3) |
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\ No newline at end of file |