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Re: [Discuss-gnuradio] frequency tuning word quantization

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] frequency tuning word quantization
Date: Sun, 5 Mar 2006 12:03:33 -0800
User-agent: Mutt/1.5.9i

On Sun, Mar 05, 2006 at 08:12:37AM +0100, Matteo Campanella wrote:
> I digged into the usrp c code, and I have found the following code, that 
> basically says we could use 32 bits for tuning on the fpga, but we truncate 
> it to 14 - unless there's a good reason for that, we could be much more 
> precise in tuning the NCO... again, unless Matt has a good reason for that.
> ciao
> Matteo

Good question.  I've wanted to revisit it myself too.
As I recall it was to reduce the spurs in the DDC output.
If you get a chance, please see if you can find relevant papers.
I suggest searching for "DDS spurs", or something like that.

In looking at the DDS's from Analog Devices, etc., they maintain a
full precision frequency tuning word and phase accumulator, and then
use only the high part to feed into the sin/cos generator.  We also
do that in the FPGA.  The question is whether we should be doing the
coarse quanitization of the frequency tuning word the way we are, or
some other way, or not at all.

Anybody else got something to contribute to this discussion?


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