[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

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.

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.