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:-)
* * *
I, too, have an older (Philips) DVR that has lost its time sync since
the analogs went dark. For a while, I used the XDS time code that
happened to be in the vertical interval of one of its standard
definition DTV PBS station's sub-channels (received on a set-top box and
modulated onto a TV channel to which the DVR would "look" for its time
code) but this has code since been dropped.
Before I discovered this, I dug up the line 21 (IIRC) code
specifications and noted that even a PIC could probably generate the
proper code, synchronized either from a GPS or a WWVB receiver. I'd
thought about putting it on multiple lines and then RF modulating it for
the DVR to see, but lost enthusiasm after I discovered the time code on
the sub-channel. Since that went away (about a year ago) I've just
remembered to set the clock once a month, not being able to quickly find
the specs for the time code again online...
73,
Clint
KA7OEI
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