[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

Karl, Brian,

Are you thinking of implementing a modest bandwith beamformer on a single 
ROACH board for starters?

Rick

On Tue, 9 Feb 2010, Karl Warnick wrote:

> Rick,
> 
> We are having an iADC64 board built right now, which has 64 RF inputs and
> a sample rate of 50 or 64 Msamples/sec per channel. It plugs into one
> ROACH board. Would this work?
> 
> Karl
> 
> Rick Fisher 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.
>
> 
>
>  _______________________________________________
> Pafgbt mailing list
> Pafgbt at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/pafgbt
>
> 
> 
>
>  -- 
> Karl F. Warnick                                  email:  warnick at byu.edu
> Associate Professor                              Tel:    (801) 422-1732
> Department of Electrical & Computer Engineering  FAX:    (801) 422-0201
> Brigham Young University
> 459 Clyde Building
> Provo, UT 84602
> 
> 
> 
> 
> 
> 
>