[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

Paul,

Could someone do an analysis for the optimum pulsar frequency for an array 
feed?  In the meantime, is it possible to say whether there is strong 
interest in a PAF on the GBT for pulsar work near 1400 MHz?  I've been 
assuming there is, but the original science case is nearly 10 years old.

Rick

On Mon, 8 Feb 2010, Paul Demorest wrote:

> On Mon, 8 Feb 2010, Rick Fisher wrote:
>
>>  To begin getting a handle on the constraints and options for a GBT PAF
>>  system design, let me list some assumptions that we might adopt.  Feel
>>  free to argue with any of these and suggest alternatives.  The immediate
>>  objective is to get the options on the table.
>>
>>  Rick
>>
>>  1. Because data rate, storage, and management will always be primary
>>  limiting factors in PAF science output, system temperature and aperture
>>  and beam efficiency of each beam will be top priorities.  The goal for
>>  system temperature divided by aperture efficiency is 28 K for all beams
>>  that are formed and processed.  The first array on the GBT may not meet
>>  this goal, but front-end development will continue at least until this
>>  goal is achieved.  The first array on the GBT will be cryogenic.
>>
>>  2. The first array on the GBT will be have 19 dual-polarized elements.
>>  It will be optimized for the HI line at 1.42 GHz and cover at least
>>  1.3-1.5 GHz.  This frequency range is bounded by the radar at 1.292
>>  GHz and the satellite band at 1.52-1.57 GHz.  Another array will need
>>  to be built to cover the 1.7-2.3 GHz band preferred by pulsar
>>  observers when sufficient beamforming bandwidth becomes available.
>
> 1.7-2.3 GHz might not be the best pulsar band for this feed.  I think it 
> mainly became the standard for the GBT because the (3-level) spigot was so 
> sensitive to RFI at the lower L-band freqs.  With higher dynamic range 
> instruments it might make sense to move down in freq where pulsars are 
> stronger and survey speed goes up.  someone will have to do a real analysis 
> of all these factors though...
>
> -Paul
>
>>
>>  3. Ultimately we want to digitize the signal from each array element
>>  in the front-end box for greatest phase and amplitude stability and
>>  lower cable weight of optical fibers.  However, the first array will
>>  use 38 coaxial cables to carry the element signals into the GBT
>>  receiver room.  These cables should have sufficiently low loss and
>>  outer shield leakage to carry signals frequencies up to 2.3 GHz so
>>  that they can transfer either IF or RF signals to the receiver room.
>>
>>  4. A phase and amplitude monitor signal will be distributed in the
>>  front-end box and injected into the signal path of each element after
>>  the cryogenic LNA.  (A signal transmitted to the array from an antenna
>>  somewhere in the dish is subject to multi-path distortions that make
>>  it an unreliable primary calibrator, at least until its reliability
>>  can be validated against the directly injected calibrator.  Calibrator
>>  injection immediately ahead of the LNA would degrade noise
>>  performance.  Experience with single-beam GBT receivers indicates that
>>  the LNAs are stable enough to be left outside of the phase and
>>  amplitude monitor loop.)
>>
>>  5. The long-range plans are to locate the beamformer electronics in
>>  the Jansky laboratory.  This offers the greatest room for growth and
>>  minimizes the problems of space, weight, and EMI in the GBT receiver
>>  room.  However, the first beamformer with modest bandwidth will be
>>  located in the GBT receiver room so that its implementation is not
>>  dependent on transmitting its input signals to the Jansky lab.  [Can
>>  fewer ROACH boards accommodate 38 lower speed ADCs?]
>>
>>  6. A 250-MHz bandwidth beamformer that uses 20 ROACH boards and 20 iADC
>>  boards plus ethernet switch and associated electronics and power
>>  supplies is too big and noisy for the GBT receiver room.  This should
>>  be planned for installation in the Jansky lab.
>>
>>  7. We'll vigorously develop digitizers and digital fiber links that
>>  allow signals from the array elements to be transmitted to the Jansky
>>  lab on digital fiber links, but we don't want this to be on the critical
>>  path to implementing a wider bandwidth beamformer.  An alternative
>>  solution will be to install commercial 0.9-2.2 GHz analog fiber modems
>>  to transmit RF signals directly to the lab.  The feasibility of such a
>>  solution depends on it being stable enough to be tracked with the
>>  phase and amplitude monitoring system.  Two modem pairs are in hand,
>>  and tests of them on fibers between the GBT and the lab will begin
>>  soon.  Each modem pair costs about $2K, and a set to handle 38 signal
>>  paths will cost about $80K so we need to be certain that it will offer
>>  significant scientific pay-off before taking this option.  Note that
>>  the modems in hand do not work below 900 MHz so they would not transmit
>>  low-frequency IF signals from the BYU receiver modules currently under
>>  construction.  Analog modems that work at lower frequencies are
>>  available, but they may be more expensive.
>>
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>
>