[time-nuts] Advise on building a DIY GPSDO?

Graham / KE9H ke9h.graham at gmail.com
Sat Apr 9 02:31:31 UTC 2016


The lowest jitter way to do this kind of conversion is to multiply the
signal up to some common multiple frequency, then divide it back down to
where you want to be.  For instance, with 8 or 24 MHz, multiply up to 240
MHz, then divide by 24 to get 10 MHz.

Modern clock generator chips have this capability built in.  As an example,
the TI LMK04100 series clock chip.  It actually has an on-board 1200 MHz
VCO, and all the phase-lock loop hardware to multiply up, and, and five
different divider chains, so you can get up to five different output
frequencies as long as the math works out.  Everything is constrained to
integer multiples and integer division, so there is none of the dithering
discussed above.  But you have a lot of integer options when the common
multiple is up at 1200 MHz. Much lower sidebands and phase noise.  Also the
ability to add a crystal VCO as a clean-up filter loop if your input
reference is dirty to begin with.

In my application, I am looking at taking a 10 MHz Oven-VCXO input and
putting out both a 24 MHz clock for the master clock of a BeagleBone Black,
and 480 kHz for a Shera style control loop for GPS disciplining.

--- Graham

==

On Fri, Apr 8, 2016 at 5:13 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> If you start from a 24 MHz TCXO (different modules use different TCXO’s):
>
> On an 8 MHz output, most of the time you divide by three.
>
> On a 10 MHz output, you need to divide by 2.4. The net result is that you
> divide by 2 sometimes and 3 other times.
>
> In the 10 MHz case, there is a *lot* of energy at 12 MHz and 8 MHz, along
> with
> the 10 MHz output.
>
> In the 8 MHz case, most of the RF energy is at 8 MHz.
>
> ====
>
> To correct the output by 1 ppm on the 8 MHz output, you need to either
> drop or
> add one pulse out of every million pulses. Effectively you divide the 24
> MHz by
> 2 or by 4 when you do that. You get a bit of 12 MHz or a bit of 6 MHz as a
> result.
> That can be filtered out with a RF filter. The same is true with a
> (somewhat more
> complex) filter on the 10 MHz output.
>
> In addition to the “big” RF spurs, you get a low frequency component to
> the output
> modulation. You are “phase hitting” the output eight times a second. That
> gives you
> an 8 Hz sideband along with the further removed stuff. Since it’s not
> simple / clean
> phase modulation, there are more sidebands than just the few mentioned
> above.
>
> What messes things up even more is that you never are quite doing one ppm.
> You are doing
> corrections like 0.12356 ppm this second and 0.120201 ppm the next second.
> The pattern of pulse drop and add is not as simple as you might hope. The
> low
> frequency part of the jitter (and it will be there) is no different than
> the noise on
> a 1 pps output. You still need to do very long time constant (or very
> narrow band)
> filtering to take it out.
>
> Bob
>
> > On Apr 8, 2016, at 7:06 AM, Herbert Poetzl <herbert at 13thfloor.at> wrote:
> >
> > On Mon, Apr 04, 2016 at 06:07:54PM -0700, Alexander Pummer wrote:
> >> and it is relative easy to make 10MHz from 8MHz with analog
> >> frequency manipulation, which generates less jitter
> >
> > Could you elaborate on this a little if time permits?
> > I'm more a 'digital person' but it sounds interesting.
> >
> > Thanks in advance,
> > Herbert
> >
> >> 73
> >
> >> On 4/4/2016 4:27 PM, Attila Kinali wrote:
> >>> On Mon, 4 Apr 2016 17:56:29 -0400
> >>> Bob Camp <kb8tq at n1k.org> wrote:
> >
> >>>> The variable frequency output on the uBlox (and other) GPS
> >>>> receivers has come up many times in the past.
> >
> >>>> If you dig into the archives you can find quite a bit of
> >>>> data on the (lack of) performance of the high(er) frequency
> >>>> outputs from the various GPS modules. They all depend on
> >>>> cycle add / drop at the frequency of their free running TCXO.
> >>>> Regardless of the output frequency, that will put a *lot* of
> >>>> jitter into the output.
> >>> That's why you should put the output frequency of the ublox modules
> >>> to an integer divisor of 24MHz. Ie 8MHz works but not 10MHz.
> >
> >>>                     Attila Kinali
> >
> >
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