On 11/14/2010 08:28 PM, jimlux wrote:
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.
The simplicity of power of 2 DDSes and it's effects on spurs seems to be
not fully understood. I have just not taken then effort to write up a
DDS approach with variable length, but it is definitively on my list of
things to do.
And finally, there's all kinds of clever ways to suppress close in spurs
with things like error feedback.
Feed forward corrections have also been seen.
But all this is really a complete side-track to what I was trying to
discuss, a method for dual frequency DMTD.
Cheers,
Magnus
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