[Pafgbt] GBT PAF system assumptions

[Roger Norrod]

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.
>