[Pafgbt] GBT PAF system assumptions
Rick Fisher
rfisher at nrao.edu
Fri Feb 12 11:01:01 EST 2010
Paul,
This is very helpful. I'll watch for other comments, but it appears that
we're not totally out in left field. The one-shot aspect of this doesn't
bother me too much. It's a pretty big project.
Rick
On Fri, 12 Feb 2010, 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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>> >
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