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Roger,<br>
<br>
For initial engineering work, the beamforming/data recording will
probably happen in the GBT receiver room, at least based on our earlier
discussions, so the question is really how capable that system will be
and whether it could transition to an interim science back end. <br>
<br>
Karl<br>
<br>
Roger Norrod wrote:
<blockquote cite="mid:4B7165FF.9050106@nrao.edu" type="cite">
<pre wrap="">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:
</pre>
<blockquote type="cite">
<pre wrap="">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.
</pre>
</blockquote>
<pre wrap=""><!---->
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</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Karl F. Warnick email: <a class="moz-txt-link-abbreviated" href="mailto:warnick@byu.edu">warnick@byu.edu</a>
Associate Professor Tel: (801) 422-1732
Department of Electrical & Computer Engineering FAX: (801) 422-0201
Brigham Young University
459 Clyde Building
Provo, UT 84602
</pre>
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