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

Sat Apr 9 08:01:59 EDT 2016

Am I missing something? KISS. If you start out with a 10 MHz OCXO use a tvb 
 PIC to divide down to a lower frequency a 74HC74 to get symmetrical output 
of a  $ 10 u-blox 6 and the PIC and 86 XOR. If no PIC capability ebay has 
LS90's for a  one off. 
Bert Kehren

In a message dated 4/9/2016 7:06:26 A.M. Eastern Daylight Time,  
ke9h.graham at gmail.com writes:

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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