[Pafgbt] GBT PAF system assumptions

[Rick Fisher]

The receiver being built at BYU converts to near baseband so that may not 
be an immediate problem for digitizing in the receiver room.

Rick

On Mon, 8 Feb 2010, G Jones wrote:

> Regarding point 5, below, there is an existing Quad ADC card for the ROACH,
> a pair of which would allow a ROACH to capture eight 125 MHz inputs.
> Unfortunately, the analog bandwidth is not great, so digitizing at RF is not
> practical with these ADCs. I haven't looked at ADC options in a while, but
> perhaps there is a more suitable quad ADC that could digitize ~250 MHz at
> RF.
>
> Glenn
>
> 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?]
>>
>> 6. A 250-MHz bandwidth beamformer that uses 20 ROACH boards and 20 iADC
>> boards plus ethernet switch and associated electronics and power
>> supplies is too big and noisy for the GBT receiver room.  This should
>> be planned for installation in the Jansky lab.
>>
>> 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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>