[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

Do you know the number of beams and Tsys of the competition?

Rick

On Fri, 12 Feb 2010, Scott Ransom wrote:

> Hi All,
>
> I was going to writeup almost exactly what Paul already has.  He is right
> on the money.
>
> The one thing missing is that both Effelsberg (definitely) and the new
> Sardinia telescope (likely) will be doing multi-beam L-band searches of
> the Galactic Plane (probably to +/- 3 deg lat or similar).  So we wouldn't
> have to just beat the Parkes Multibeam survey (or match it in the North),
> we'll have to significantly beat Effelsberg and/or Sardinia as well.
>
> Scott
>
>
> On Friday 12 February 2010 10:54:41 am Paul Demorest wrote:
>> 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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