[Pafgbt] GBT PAF system assumptions

[Paul Demorest]

Rick,

Just ran a few rough numbers, and it turns out a 1400 MHz PAF pulsar 
survery is actually pretty comparable to the 350 MHz GBT (single-beam) 
survey currently being run by Scott and co.  The FoV are almost identical. 
Due to lower sky and rcvr temps, the PAF has better SEFD by a factor of 
~2-20 (direction dependent), and would have ~2x the BW.  This is mostly 
offset by pulsars being typically about 10x fainter at 1400 vs 350 MHz. 
But the PAF definitely wins in the galactic plane.  The PAF survey would 
also be sensitive to MSPs out to much higher DM.

So the main motivation for a L-band PAF psr survey would be to find 
pulsars (especially higher-DM MSPs) in the galactic plane.  We'll need to 
compare these parameters to past/current work at Parkes to see how much 
telescope time would be needed to beat what has already been done.

Another possible consideration is that this is basically a one-shot 
project.. it's a pretty large project, but still, once the deep galactic 
plane survey is done, I don't think there is much other use for the feed 
pulsar-wise.

-Paul

Field of view
   350: (36')^2 * 1 beam  = 1300 arcmin^2
   PAF: (9')^2 * 19 beams = 1500 arcmin^2

T_rcvr
   350: 50 K
   PAF: 28 K

T_sky
   350: 0 to ~1000 K
   PAF: 0 to ~10 K
   Direction dependent, highest in gal. plane

Pulsar flux
   S_350 / S_1400 = (350/1400)^-1.7 ~ 10

BW
   350: 100 MHz
   PAF: 250 MHZ (?)

Max. DM for MSPs
   350: ~50-100 (~0.5 ms smearing at DM=100)
   PAF: ~500-1000 (dependent on Nchan)

On Fri, 12 Feb 2010, Rick Fisher wrote:

> 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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>>
>>
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