[Pafgbt] GBT PAF system assumptions

[G Jones]

Regarding point 5, below, there is an existing Quad ADC card for the ROACH,
a pair of which would allow a ROACH to capture eight 125 MHz inputs.
Unfortunately, the analog bandwidth is not great, so digitizing at RF is not
practical with these ADCs. I haven't looked at ADC options in a while, but
perhaps there is a more suitable quad ADC that could digitize ~250 MHz at
RF.

Glenn

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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