[Pafgbt] GBT PAF system assumptions

[Paul Demorest]

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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