Although DDS sources seem to more-or-less dominate
where high resolution / accuracy is needed, there is another route that
could be worth investigating: Fractional-N synthesis with addtional
For example, the LMX2541 with a 10MHz reference phase
comparator input will generate steps as low as 2.38Hz at its minimum
internal VCO frequency of 2.2GHz. A programmable output divider goes
to divide-by 63, allowing steps of 0.038Hz at 35MHz . Add a single
chip binary divider of 256 (for the 137kHz band) and you're down to
150uHz steps - which are completely deterministic.
Since phase noise and Fract-N sidebands are hardly an issue with
this degree of output division, the phase comparator could be operated
at a much lower frequency, say 500kHz, allowing steps of lower than
10uHz. Which is a significantly higher resolution than the 2.3mHz
steps the traditional AD9850 32 bit DDS offers.