[time-nuts] 10 MHz to 32.768 kHz converter

Clint Turner turner at ussc.com
Mon Mar 21 12:46:39 EDT 2016


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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