[time-nuts] GPSDO project

[Hal Murray]

You want 66.6666 MHz from 10 MHz.

I can think of several approaches.

1) Patch the radio stuff to work with 10 MHz.  Since 10 MHz is common from 
things like GPSDOs or Telco surplus rubidium clocks, somebody may have done 
that already.

2) Build a PLL.  The first step is probably to find a 66.666 MHz oscillator 
that has an external fine tuning pin.  Then it's divide by 20 and 3, compare, 
filter...

3) Get to 66.666 MHz by dividing by 3 then multiplying by 2 and 5.  I don't 
know much about this area, but there was a lot of discussion here a few 
months ago.  Check the archives.

4) Use a DDS chip to synthesize 66.666 MHz.  Analog Devices makes lots of 
nice ones.  One problem with DDSes is that they normally make spurs.  But 
they aren't a problem if the target frequency is a clean multiple of the 
source frequency.  20/3 doesn't sound clean, but I'd have to do a lot of work 
to check the details.  There may be a clean frequency that is close enough to 
66.666 MHz and/or one that has spurs that are far enough out so you can 
filter them.

5) Use a low cost 66.666 MHz oscillator and live with the error.  You may be 
able to correct any errors.  The key step would be to feed the 66.666 MHz to 
a counter running off the T-Bolt clock so you know the real frequency of your 
66.666 MHz osc.  Suppose your 66.666 MHz is 73 ppm fast and you want to 
listen to 12.123 MHz.  You would set the radio to listen to 73 PPM below 
12.123 MHz.



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