At 03:06 PM 8/5/2008, you wrote:
On Tue, Aug 5, 2008 at 2:45 PM, Chris R. Anderson <[EMAIL PROTECTED]> wrote:
> Brian -- Thanks for the suggestions on the FPGA. We've been using the
> Altera parts up here for several years now, so our guys are definitely
> familiar with the development process. Personally, I'm leaning towards the
> EP3C16 to help keep the costs down. We're probably going to use the AD9862
> ADC/DAC (which is basically the same thing used on the USRP), which retails
> for $57.75 on Digikey.
No problem, and sounds good. It definitely is pretty darn cheap for
the resources you get.
As for the ADC/DAC, do you plan on trying to do the full 64MSPS
processing? How much bandwidth are you looking to process?
What I'd like to do is run a 64 MHz clock into one of the PLLs on the
FPGA, and let the FPGA control the clock speed of the ADC/DAC. That
way you have some reasonable control over the sampling rate/bandwidth
you want to work with. Ideally, I'd like to be able to process as
much bandwidth as possible, but I suspect the limitation will be the
interface to the Beagleboard. Most of the commercial radios seem to
max out at around 20 MHz, if we could get even 1/4 of that (5 MHz), I
think we would be doing pretty good.
With trying to keep costs down, using the same AFE as the USRP seems
like it isn't that good of an idea mainly due to the fact that it is
built on old technology and costs so much.
I guess I should clarify that we'd like to use the same daughterboard
connector as the USRP. I agree that its older technology and may not
be the best solution, but it does give us a working AFE to start
with. Our guys get lots of digital design experience, but absolutely
zero RF design experience--which I learned the hard way last
year. Using the USRP AFE means one less thing we have to worry about
debugging.
The upside to this approach is that we're not strictly limited to
using USRP AFE's: any board that matches the pinout could be
substituted instead. The limitation is that the ADC has to be
separated from the AFE. I'm not dead-set on going down this route,
however. If we could find a good RF designer that wanted to create a
custom AFE for the board, it would certainly be worth entertaining the idea.
Philip, do you have any thoughts?
Chris
MAXIM makes a nice full duplex AFE that has a couple of slow-speed ADC
and DACs as well for VCTCXO's, AGC, etc. It can be found here:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5163
10-bit resolution for the ADCs and DACs, and only a 45MSPS part, but
for $16 in small quantities (less than $10 in higher) it should still
be enough for plenty of bandwidth.
Brian
-------------------------------------
Chris R. Anderson, Ph.D.
[EMAIL PROTECTED]
410-293-6185
Assistant Professor
US Naval Academy
Electrical Engineering Department
105 Maryland Avenue
Annapolis MD 21402
