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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
