[time-nuts] DDS in GPSDO design?

[Michael Tharp]

On 05/27/2012 06:23 PM, Bruce Griffiths wrote:
> The principal problem with conventional DDS implementations is phase
> truncation spurs which can occur close to the desired carrier.
> Virtually all commercial DDS chips produce such phase truncation spurs.
>
> It is possible to eliminate such spurs if one implements a custom DDS
> using an FPGA and an external DAC.
> In this case the performance is limited by the DAC.

How specifically does the FPGA resolve the problem? I have a FPGA 
already for the phase comparator, it's just a "simple" matter of 
figuring out how big of a DAC to get and what data to feed it...

> Another approach is to use a cascaded mix and divide technique
> <http://www.karlquist.com/FCS95.pdf> to restrict the effective tuning
> range of the DDS.
> The amplitude of DDS generated spurs is thereby significantly reduced.

Great paper, this looks like it could be interesting as a standalone 
filter. It's just a little over my head (self-taught digital guy) but 
since I don't need to hit a home run on the first try, I'll keep it in mind.