[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

Also worth a good look for wider bandwidths.  This might be an 
intermediate development platform for implementing digitization in the 
receiver room.  The ROACH board would be severe overkill, but that's OK as 
a prototype.

Rick

On Tue, 9 Feb 2010, G Jones wrote:

> Supposing there is a suitable 250 MHz bandwidth ADC which could be used in a
> quad ADC card for the ROACH, we could capture eight signals with one ROACH.
> We would then need only 5 ROACHs for digitizing the 40 signals. The total
> digital bandwidth would be 8 signals * 8 bits * 500 Msps = 32 Gbit/s per
> ROACH. This would be spread across the four 10 GbE ports on the ROACH.
> Therefore, with 20 CX4 to optical converters, we could transfer all of the
> digital data back to the Jansky lab, where the remaining ROACHs can be
> located. I don't know how many dark fibers are available, so multiple
> optical signals may need to be combined onto one fiber.
> If a narrower bandwidth system is useful to start, we could use the existing
> 125 MHz bandwidth quad ADCs, which would then require only 10 CX4 to fiber
> converters.
>
> This eliminates any concern about analog stability of the fiber links, and 5
> ROACH boards would not take up much space/power.
>
> Glenn
> On Tue, Feb 9, 2010 at 8:31 AM, Rick Fisher <rfisher at nrao.edu> wrote:
>
>> 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.
>>>>
>>>
>>>
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