[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

I was thinking that we might get early HI science by putting a narrow band 
beamformer in the receiver room, but this may not make sense.  It's been 
pointed out that we'd need at least 10 ROACH boards just to accommodte 38 
ADCs.  I'd be prepared to abandon the idea of any beamformer in the 
receiver room, but maybe there's a counter-argument.  Eliminating an 
interm solution may very well shorten the time to implement a wider 
bandwith beamformer.

Rick

On Tue, 9 Feb 2010, Roger Norrod wrote:

> I wonder about the wisdom of #5.  It sounds like many months of specialized 
> effort to get a limited system in the Receiver Room, and it could be a 
> serious diversion from where we need to concentrate work. The analog links 
> may be considered a diversion too, but at least there's a chance they become 
> part of a long-range solution.  If we could manage to get some people to 
> really concentrate for a few months on the analog/digital link comparisons 
> (#7), and leave #5 as a fall-back position, I think it would be good.
>
> Roger
>
>
> Rick Fisher wrote:
>
>>
>>  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.
>>
>>  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?]
>>
>>  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.
>> 
>
>