[Pafgbt] GBT PAF system assumptions

[Wes Grammer]

Regarding the analog fiber modem alternative below, do you have a feel 
for what the signal amplitude and phase stability requirements are  for 
this case? I'd guess if you ran all 38 fibers in a single fiber cable, 
any differential phase or amplitude change would be minimized. But I'd 
assume you'd still want high common-mode amplitude stability as well, right?

With a large number of elements, it would seem the cost of the analog 
modems would start to add up quick. Would it be more cost-effective to 
transmit multiple IF channels riding an RF carrier into a single modem pair?

-Wes


Rick Fisher wrote:
> 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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