[Pafgbt] GBT PAF system assumptions

[G Jones]

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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