[Pafgbt] GBT PAF system assumptions
Rick Fisher
rfisher at nrao.edu
Tue Feb 9 16:48:52 EST 2010
I risk suggesting too many options, but a narrowband beamformer is
worth keeping in the back of our minds. My concern is that we might end
up building a beamformer that's much wider bandwidth than the spectral
line people need but not wide enough to interest the pulsar folks. We'll
need to iterate on this one a bit.
A replacement for the stream to disk acquisition system sounds attractive
for development purposes.
Rick
On Tue, 9 Feb 2010, Karl Warnick wrote:
> I'll let Brian comment, but I think our near-term goal is a prototype FXB
> engine using several ROACH boards with standard two channel iADC boards
> that we would eventually scale to a 40+ channel broadband back end.
>
> The narrowband iADC64 board is a side project that could be a possible
> replacement for our stream to disk acquisition system. If it made sense,
> we could certainly consider implementing a beamformer on the 64 channel
> system.
>
> Karl
>
> Rick Fisher wrote:
>
> Karl, Brian,
>
> Are you thinking of implementing a modest bandwith beamformer on a single
> ROACH board for starters?
>
> Rick
>
> On Tue, 9 Feb 2010, Karl Warnick wrote:
>
>
>
> Rick,
>
> We are having an iADC64 board built right now, which has 64 RF inputs and
> a sample rate of 50 or 64 Msamples/sec per channel. It plugs into one
> ROACH board. Would this work?
>
> Karl
>
> Rick Fisher wrote:
>
> I was thinking that we might get early HI science by putting a narrow band
> beamformer in the receiver room, but this may not make sense. It's been
> pointed out that we'd need at least 10 ROACH boards just to accommodte 38
> ADCs. I'd be prepared to abandon the idea of any beamformer in the
> receiver room, but maybe there's a counter-argument. Eliminating an
> interm solution may very well shorten the time to implement a wider
> bandwith beamformer.
>
> Rick
>
> On Tue, 9 Feb 2010, Roger Norrod wrote:
>
>
>
> 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.
>
>
>
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> --
> Karl F. Warnick email: warnick at byu.edu
> Associate Professor Tel: (801) 422-1732
> Department of Electrical & Computer Engineering FAX: (801) 422-0201
> Brigham Young University
> 459 Clyde Building
> Provo, UT 84602
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> --
> Karl F. Warnick email: warnick at byu.edu
> Associate Professor Tel: (801) 422-1732
> Department of Electrical & Computer Engineering FAX: (801) 422-0201
> Brigham Young University
> 459 Clyde Building
> Provo, UT 84602
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