[time-nuts] DMTD for different input frequencies

jimlux jimlux at earthlink.net
Sun Nov 14 19:28:31 UTC 2010


>> Cheers,
>> Magnus
>>
> One still has to ensure that close in spurs generated by the DDS sources 
> aren't an issue.
> If the zero crossings of the beat frequency signals at the outputs of 
> the 2 final mixers arent closely aligned then such spurs can be 
> problematic.
> 

How close in for the spurs?

For most DDSes the spurs come from phase truncation errors, and to a 
lesser extent from harmonics aliased into the passband.  There's a 
couple ways to reduce this for basically fixed frequencies..  You can 
drive the DDS clock from a variable frequency source (e.g. another PLL 
or DDS) so that the frequency you're generating doesn't have spurs in 
"bad" places.  Another approach is to do your DDS with a FPGA/DAC, and 
make the phase accumulator and sin/cos LUT not a power of 2 long.

And finally, there's all kinds of clever ways to suppress close in spurs 
with things like error feedback.



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