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

Sat Apr 9 15:00:29 EDT 2016

Hi

Averaged over a long enough time (and without any hanging bridges) the frequency accuracy 
will be fine. The frequency accuracy of a 1 pps output on a GPS is “fine” on the same basis. Since
200 KHz is a “round division” off of any of the likely TCXO’s you will not have any jitter or spurs in the “static” 
case.

Bob

> On Apr 9, 2016, at 10:07 AM, Bert Kehren via time-nuts <time-nuts at febo.com> wrote:
> 
> I do not know what U blox does but I know when we use 200 KHz out of the 1  
> pps output on a $ 10 ublox 6 we consistently get better than 1 E-10 closer 
> to 1  E-11 out of the Morion have the data
> Bert Kehren
> 
> 
> In a message dated 4/9/2016 10:01:05 A.M. Eastern Daylight Time,  
> kb8tq at n1k.org writes:
> 
> Hi
> 
>> On Apr 8, 2016, at 9:39 PM, timenut at metachaos.net  wrote:
> 
>> 
>> 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.
> 
> The bigger problem is that the 24 MHz is *not* exact. It is  simply a free 
> running TCXO
> that happens to be in a GPS module. It has a  basic accuracy of +/- 1 ppm 
> or something 
> similar. It is no better or worse  than any other TCXO you could buy. 
> 
> To make it accurate they have two  choices:
> 
> 1) Put a voltage control input on the TCXO and turn it into a  TCVCXO, then 
> lock it up 
> with a loop.
> 
> 2) Let the oscillator free run  and “fix up” the output.
> 
> For a variety of reasons, none of the small  GPS modules go with option 
> number 1. They 
> all go with option number 2. The  24 Hz error on the (maybe)  24 MHz gets 
> taken out by dropping
> 24 edges  every second. That’s not a lot of edges, it’s not going to turn 
> the output  into absolute 
> garbage you can see on a scope. It is plenty of nonsense to  mess up a 
> radio or a piece of test gear. 
> 
> One easy way to look at it:  You have ~1 ppm jitter on the output (in the 
> example of 1 ppm of error). A  
> phase locked GPSDO with only simple filtering of a 1 pps would get you  
> down to 0.01 ppm of jitter. 
> A sawtooth corrected 1 pps would get you to  0.01 ppm. A good filter would 
> get you to <0.00001 ppm.
> Yes, I’m using a  very hand waving definition of jitter here, but it does 
> illustrate the point.  You could 
> look at the jitter on the pulse drop as 0.04 ppm.  
> 
> Bob
> 
>> 
>> 
>> 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
>> 
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