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

Herbert Poetzl herbert at 13thfloor.at
Sat Apr 9 02:35:45 UTC 2016


On Fri, Apr 08, 2016 at 06:13:07PM -0400, Bob Camp 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.

Thanks for the input, but alternating between dividing
by two and three doesn't really sound like "analog 
frequency manipulation" to me.

Maybe I'm completely wrong here, maybe I just need to
see an analog circuit which does this.

Best,
Herbert

> ====

> 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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