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