[time-nuts] GPSDO project - 66.667MHz from 10MHz

Luis Cupido cupido at mail.ua.pt
Mon Jun 29 05:23:56 EDT 2009

```Both for locking with a 10MHz or 1pps you have a
single chip solution using one of my reflock designs.

http://w3ref.cfn.ist.utl.pt/cupido/reflock.html

For a 1pps lock of fractional 1/3 frequencies you have
already a code. (check the list of files for reflock 1)

For locking a 66.666(6) VCXO to 10MHz you can use the same
reflock I design and the configuration is kind of trivial
(but no one asked for it before) and I can make a file
for you.

Luis Cupido
ct1dmk.

> ________________________________
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Sunday, 28 June, 2009 11:04:49 PM
> Subject: Re: [time-nuts] GPSDO project
>
> 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
>>
>> 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
>> 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.
>>
>>
>>
>>
>
> Bruce
>
>
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