[time-nuts] GPSDO project

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 28 18:04:49 EDT 2009

Hal Murray wrote:
> 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.

Actually need to multiply 3.333.. MHz by 20 (5 x 2 x 2)
No need to multiply by 2 or 4, if the output of the divide by 3 is a 1/3
duty cycle square wave, one can extract the 2nd (or 4th) harmonic of the
square wave repetition rate with a filter.
Amplify and multiply by 5 (can use the same approach as used in the
5370A/B frequency multiplier chain (1 transistor per multiplier) and filter.

A high level injection locked divider can have lower close in phase
noise than a digital one.

> 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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