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.griffi...@xtra.co.nz>
To: Discussion of precise time and frequency measurement <time-nuts@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
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.
Bruce
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