[time-nuts] GPSDO project - 66.667MHz from 10MHz
g4hup at btinternet.com
Sun Jun 28 22:15:44 UTC 2009
You can also use a DFS (Direct Frequency Synthesiser) to produce 66.667MHz in two ways - either 50 + 16.667MHz, or 70 - 3.33MHz. Information on the 50+16.667 is on my web site (http://g4hup.com), and has been implemented as a reference source for an SDR-IQ receiver. Another customer of mine has also implemented the 70-3.33MHz version for the same application - I don't have the filter component values or detail design posted, but could provide them on request..
My personal preference is for the slightly more complex 50+16.667Mhz option, since the filtering is a little easier at VHF - but both solutions work, and give a good clean, stable low-noise output (depending on the quality of your 10MHz reference input, of course!)
Kits are available.
Hope this helps
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
> 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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