Years ago I needed to lock a 16.777216 MHz oscillator to a 10 MHz
reference for a "Williams" DDS synthesizer.
Because 32768 is a subharmonic of 2^24 Hz, this same sort of scheme
should be adaptable.
I will quote my own posting to this group from Feb 2, 2012:
Clint wrote:
> Years ago (in the 80's) I needed to lock a homebrew DDS to an accurate,
> stable 10 MHz reference (a good TCXO in this case) that was set to WWV/H.
> Considering that the DDS was clocked at 2^24 Hz (16.777216 MHz) this was
> slightly awkward, but I did it using standard HC and 4000 logic.
>
> The convoluted path was:
>
> 10 MHz / 625 = 16 kHz (HC40103 as a div-by-125 and an HC4017 as a
div-by-5
> would work...)
>
> 16 kHz * 32 = 512 kHz (using a 4046 and 4040)
>
> 512 kHz /125 = 4096 Hz (using 40103 or similar)
>
> From there, it was a no-brainer to compare this with the 16.777216 MHz /
> 4096 with another 4046/integrator - but the same 'HC4040 that did
this also
> had a tap with 32768 kHz on it.
>
> With a fairly slow loop and a low-noise 2^24 Hz VCXO, the DDS's clock was
> both clean and stable - and tuned in 1 Hz steps! A cheap and more-common
> 4.194304 MHz crystal would work and I suppose that a similar scheme could
> be used to lock a 32768 Hz VCXO but I've never tried to 'VCXO a
tuning-fork
> crystal before:-)
73,
Clint
KA7OEI
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