[time-nuts] GPSDO project
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,
> 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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