[time-nuts] DMTD for different input frequencies

[Magnus Danielson]

On 11/14/2010 08:11 PM, Bruce Griffiths wrote:
> 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.

Playing some tricks with choice of frequency could avoid the issues. 
Since the beat frequencies does not have to be spot on a particular 
frequency, some of the available frequency range could be allocated to 
adjustment of DDS frequencies to avoid problematic spurs.

This can also to some degree be used as a medicin to handle 
phase-truncation issues, but the approach I described is really 
orthogonal to these issues. However, the approach would allow the LO1 
and LO2 to be mis-tuned to some degree to get a better performance. LO1 
should be tuned near the nominal average of the two input frequencies. 
LO2 could be tuned to be nominally the mid of the average IF1 
frequencies plus or minus the target beat frequency. Another approach is 
to offset LO1 suitably such that LO2 is nominally midways the IF1 
frequencies which is spread about 2xnominal beat frequency. There are 
thus several strategies to avoid problematic spurs to attempt, all 
within the possibilities of the DDSes chosen.

Cheers,
Magnus