[time-nuts] DMTD for different input frequencies
magnus at rubidium.dyndns.org
Sun Nov 14 19:55:34 UTC 2010
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
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.
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