[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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