On Wed, Aug 6, 2008 at 8:47 AM, Philip Balister <[EMAIL PROTECTED]> wrote:
> I suspect that sampling at 64 MHz is the best approach. The existing USRP
> daughterboards all make this assumption. We can do sample rate conversions
> in the FPGA. I'd like to make as much BW available to the FPGA as possible
> and let users decide how to process it.
Understandable with regards to the bandwidth and wanting to get as
much into the FPGA as possible, but the daughterboards know nothing
about the sample frequency. Looking at the dboard connector, there is
no LO or reference clock going up to that header (from what I saw) -
just some GPIO digital lines along with some analog lines.
> I am familiar with the existing part, and we can see how the USRP interfaces
> with it. Using it reduces risk in my eyes. Looking into data converter
> alternatives would be a good exercise for your students, but as we know,
> there are lots of choices :) Also the transmit path signal processing is
> mostly done in the AD part, saving space in the FPGA for RX processing.
I am not positive it reduces the risk substantially. I agree it does
reduce risk, but remember the USRP layout is not released - just the
schematics. You'd get the same reference layout design from Analog
Devices as you would for any other converter.
I am not 100% positive, but I am reasonably the FPGA does all the
required interpolation using the interpolating CIC filter before
sending samples to the converter. The part itself might do another 2x
interpolation to 128MSPS, but a 3x cost increase for not a heck of a
lot to pay for that feature.
The converter does have a lot of other neat features - but as far as
GNU Radio is concerned, I don't think they are used.
> Re-using the existing daughterboards is pretty key here. Without the
> daughterboard support, you may as well hang a dev board off the expansion
> connector and cobble together some RF from mini-circuits parts .
I agree. The daughterboard support is essential. By AFE I mean't the
{dual ADC, dual DAC, low-speed ADC's, low-speed DAC's} in one package
- analogous to the AD9862 in the USRP.
I think it's a good idea to go through some of these lower cost
devices to see if any of their features mesh better with the actual
subset of features being used on the current USRP, the future USRP2
and within GNU Radio.
Brian
