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Re: [Discuss-gnuradio] An interesting application for my new USRP... som

From: Larry Doolittle
Subject: Re: [Discuss-gnuradio] An interesting application for my new USRP... some input requested
Date: Tue, 24 May 2005 14:38:44 -0700
User-agent: Mutt/1.5.9i

John -

> Actually, over these time intervals GPS wouldn't be as accurate as my 
> local references

I guess you have better local references than I do.  :-(

> I snipped your earlier comment about the data stream, but are you 
> suggesting just capturing a single wide data stream instead of four 
> discrete ones?  I was thinking that going with the four narrow channels 
> would reduce the data transfer requirements, but if that's not an issue, 
> what the heck...

The USB doesn't have quite enough bandwidth to get the full 65 MS/s
of 12-bit data -- that would be about 97.5 MByte/sec, and nobody
seems to do better than about 33 MByte/sec.  The latter seems to
me like a CY7C68013 limitation (at least with the existing code).

I just wanted to talk you out of using multiple receivers and analog
filtering.  The digital filtering concept works just the same as if
you built it out of discrete mixers and filters, but with better control
and (maybe) less noise.  And yes, you end up with eight channels of
low frequency data coming across the link: I and Q around each of your
four carriers.  I think the FPGA code is already set up to do that,
but you'll have to research the details.

     - Larry

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