[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

Wes,

I'm assuming that we'll need to stabilize the relative phases and 
amplitudes of the fiber links with a phase monitors signal injected into 
the front-end.  Slow phase drifts could be tracked, but phase jumps would 
be a problem.

Yes, the modem cost does add up.  I assume that one would need N RF 
carriers to put N signals on one fiber modem pair.  The modem bandwidth 
would need to be N or 2N times the bandwidth of one channel.  Is there a 
more clever way than N mixers and N LO's on each end?

Rick

On Tue, 9 Feb 2010, Wes Grammer wrote:

> 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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>