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K-Band Focal Plane Array Project Engineering Overview

K-Band Focal Plane Array Project Engineering Overview. 2/27/2008. Matt Morgan National Radio Astronomy Observatory. Design Philosophy.  W e are constructing a 7 element prototype, but we are developing a 60-element imaging camera, and an array-receiver development program for the GBT in

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K-Band Focal Plane Array Project Engineering Overview

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  1. K-Band Focal Plane Array ProjectEngineering Overview 2/27/2008 Matt Morgan National Radio Astronomy Observatory

  2. Design Philosophy  We are constructing a 7 element prototype, but we are developing a 60-element imaging camera, and an array-receiver development program for the GBT in general.  As such, we do not avoid tackling design issues that may only be present for arrays larger than 7 elements (weight, heat load, etc.)  the cryogenics, monitor and control, and data-processing software is all being designed for the fully-populated 61-pixel array  There will be no sacrifice in receiver noise temperature relative to our other single-pixel receivers.  The array will be optimized for spectroscopic observation (not continuum).  To minimize cost and development time, existing components will be re-used wherever possible.

  3. Baseline Instrument Specifications *Should in fact be equivalent to EVLA receiver noise temperatures.

  4. System Schematic

  5. Warm vs. Cooled Feedhorns Cooling the feedhorn could in theory reduce the receiver noise by 20%, however this introduces a number of a technical problems for a very large array, including infrared heat loading, and cryostat cavity effects.

  6. Feed Arrangement Pattern on Sky Feedhorn Array refrigerator The front-ends will be fed by a hexagonally close-packed array of room-temperature feedhorns. (Due to thermal and electromagnetic issues, cooling the feedhorns was considered an unjustifiable trade-off).

  7. Feedhorn Array A new compact-taper corrugated feedhorn was designed for this project. It will incorporate a quick-release attachment mechanism for easy insertion and removal from the array.

  8. Electromagnetic Components Existing phase shifter, transition and OMT designs will be used. Thermal isolation will be provided by a thermal gap.

  9. Internal Noise Source for Calibration The noise source will be completely internal to the cryostat, eliminating 60 additional high-frequency thermal transitions.

  10. EVLA-Type K-Band Amplifier {{{picture of amp, plot of performance}}} The existing K-Band amplifier design used by the EVLA will also be used in this instrument.

  11. Bandwidth Limitations

  12. An Upgrade Path to Large-Format Imaging Cameras  The current IF Transmission System and spectrometer are severely limited in the total IF bandwidth that can be processed simultaneously.  These systems are not capable of supporting spectroscopic focal-plane arrays with more than about 10 pixels, at least with bandwidths that are interesting for radio astronomy.  If we want the GBT to have large-format focal plane arrays, than we need to develop both the IF and digital backends as well as the front-ends to use them.  An upgraded IF Transmission System or Spectrometer with larger numbers of channels would not be immediately useful by itself without the front-ends to use them, whereas it appears feasible to develop a small front-end array (<10 pixels) that is scientifically interesting given the existing backend systems.  Such an array could take almost immediate advantage of the upgraded backend systems when they come online.

  13. IF Frequency Plan In order to make use of the existing IF Transmission System and spectrometer, pixel outputs will be multiplexed two at a time onto each fiber channel.

  14. Integrated Downconverter Modules The IF multiplexing scheme requires two different downconverter types to be built.

  15. Multiplexer TO BE DESIGNED

  16. Spectrometer Modes

  17. Future Developments  Larger Focal Plane Arrays will require a new IF Transmission System capable of handling an order of magnitude larger data volume.  digital fiber links are most desirable, but several major technical challenges (RFI, power dissipation, etc.) must be overcome for this to be viable.  A new spectrometer will also be necessary to adequately process the large volume of data produced by a fully-populated array.  Once the backend subsystems are in place, the K-band pixels can be replicated and the array populated with relatively little risk.  Experience gained from this program as well as subsequent upgrades to the backend subsystems will open the door for heterodyne arrays at other frequencies. (W-Band?)

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