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A New Multibeam Spectrometer for the Green Bank Telescope

A New Multibeam Spectrometer for the Green Bank Telescope. A work in progress…. John Ford (NRAO), Anish Roshi (NRAO), Mark Wagner (UCB), Dan Werthimer (UCB), Suraj Gowda (UCB), Glenn Jones (Caltech), Paul Demorest (NRAO),Terry Filiba (UCB)……. Molecules and Comets.

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A New Multibeam Spectrometer for the Green Bank Telescope

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  1. A New Multibeam Spectrometer for the Green Bank Telescope A work in progress… John Ford (NRAO), AnishRoshi (NRAO), Mark Wagner (UCB), Dan Werthimer (UCB), SurajGowda (UCB), Glenn Jones (Caltech), Paul Demorest (NRAO),Terry Filiba (UCB)……

  2. Molecules and Comets Molecular emission maps of comet Hale-Bopp made in HCN (left) and HCO+ (right) with the 14 m telescope of the FCRAO, whose angular resolution is about the size of the cross in the right panel. Overlaid dots show the footprint of a 16-pixel GBT array in these lines. Figure courtesy A.J. Lovell. Eight new biologically significant molecules were discovered in the interstellar medium in a two-year period using the GBT.

  3. Pulsars at the Galactic Center ?? 100’s of pulsars predicted in the central pc none undiscovered - Macquart, Frail, Ransom, Bower.. Map gravitational field (timing), ISM at GC, black hole spin? Extreme scattering smears out the pulse High Frequency Observation to minimize scattering High Bandwidth Needed at High Frequency (low flux) 800 MHz  8 GHz

  4. Why a New Spectrometer?

  5. Specification Development -- Inputs • Spectrometer IF Inputs • 16 IF inputs • Matched to the GBT Focal Plane Array pilot programs of 16 IFs per instrument • Usable with the existing GBT IF system • 150 to 1500 MHz input bandwidth • Allows coverage of extragalactic line observations up to ~3000 km/s at 90 GHz • 150 MHz is the low frequency cutoff in GBT IF system • 1500 MHz is the Nyquist frequency for 3 GHz sampling rate • Isolation • GBT low frequency feeds have polarization isolation of more than 35 dB, so we specify more than 35 dB isolation, and stability of one hour to allow for calibration

  6. Specification Development -- Outputs • Support for frequency-switched and position-switched observations, and pulsed cal signals. • Up to 6 switching states will be available • Controlled by the spectrometer or by an external switching master • Minimum switching state width of 25 milliseconds • Full-Stokes available if required for polarization studies • Modest pulsar capabilities • ~ 4096 channels, dumped at a maximum rate of 500 microseconds • Full-Stokes • Maximum disk output of 100 MB/sec per IF pair, full Stokes

  7. Specification Development – Processing • High-resolution modes • OH Maser Observations • Require about 100 km/s range, and about 0.01 km/s resolution • 1 MHz bandwidth • 32,768 channels • Extragalactic H2O Maser Observations • Require about 5000 km/s range and about 0.3 km/s resolution • 370 MHz of bandwidth • 16,384 channels • Wide-bandwidth modes • Extragalactic spectral line observations of, e.g. 12CO • Require about 3000 km/s range and about 5 km/s resolution • 900 MHz bandwidth, 1024 channels

  8. Specification Development -- Processing • Multiple Spectral Windows • For many observations, several narrow lines are separated across the bandpass by large expanse of uninteresting or RFI infested spectrum. • Recombination line transitions in the 342 MHz band • Multiple NH3 transitions at ~22 GHz • Requires ~8 sub-bands to cover these • Bandwidth • Recombination lines require about 500 km/s range and about 0.5 km/s • Requires about 1 MHz and 2048 channels • NH3 requires about 100 km/s range and 0.04 km/s resolution • Requires about 14 MHz and 4096 channels

  9. Summary of Required Observing Modes

  10. Overall Block Diagram

  11. Roach Motel (Roach Nest) (KAT)

  12. Polyphase Filter Bank

  13. Current CASPER ADC Boards ADC2x1000-8 (dual 1GSa/sec, single 2Gsps, 8 bit) ADC1x3000-8 (3GSa/sec, 8 bit) ADC (6Gsps interleaved) 64ADCx64-12 (64x 64MSa/sec, 12 bit) ADC4x250-8 (quad 250MSa/sec, 8 bit) katADC (dual 1.5GSa/sec, 8 bit, with gain, atten, synth) ADC2x550-12 (dual 550 Msps, 12 bit) ADC2x400-14 (dual 400 Msps, 14 bit) ADC1x5000-8 (1x5Gsps,2x2.5Gsps,4x1.25G sps – Taiwan) ADC1x1000-12 (optically isolated 12 bit 1Gsps – JPL)

  14. Upcoming Hardware • Roach II (Virtex 6 – South Africa team) • Rhino (Spartan 6, ARM CPU, FMC connect) • Roach III ? (Virtex 7) • 20 to 26 Gsps ADC board

  15. Preliminary Design Work • Concentrating on the hard parts • 3 GS/s sampling and PFB/FFT calculations • Heterogeneous Computing Approach • Divide processing into front/back ends • Use FPGAs to fully process bandwidths greater than 200 MHz • Use FPGA front-ends to pre-process, split and packetize data, then GPUs to provide fine channelization on narrower chunks • Software Design • Adapting concepts and code from the Green Bank Ultimate Pulsar Processing Instrument (GUPPI) • Adding support for the K-band FPA processing pipeline • Integrating with the GBT Monitor and Control System for user ease.

  16. Preliminary Design Work • Developing support hardware • IF system interface • Anti-alias low-pass filters • Noise/test signal injection • Amplification • Analog to Digital Converter sampling clock generation/distribution • 1 Pulse Per Second distribution • Packaging

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