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

[timenut at metachaos.net]

Hello Bob,

Friday, April 8, 2016, 6:13:07 PM, you 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.
If you know you are doing a 24Mhz and a 10Mhz, why not divide the first by 12
and the second by 5 and then phase lock the resulting 2Mhz? Or divide by 24
and 10, respectively and lock the 1Mhz? That way, everything is exact.


Mike

> 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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-- 
Best regards,
 Timenut                            mailto:timenut at metachaos.net