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

Bob Camp kb8tq at n1k.org
Sat Apr 9 08:20:29 EDT 2016


Hi

> On Apr 8, 2016, at 10:35 PM, Herbert Poetzl <herbert at 13thfloor.at> wrote:
> 
> 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.

This is exactly what the GPS module is doing when it creates the output from the 24 MHz clock. 
Unless there is another clock in the system, all the output transitions take place on a 24 MHz edge.
The net result is a drop / add of pulses.The alternatives are:

1) Use a higher clock rate (24 MHz was given earlier)

2) Phase lock an oscillator (the modules do not do this)

3) Do the phasing / DSB / SSB stuff that NIST likes (again, not done in these modules). 

4) Do a full DDS and put out an analog waveform.

5) Run a DDS through a very narrow filter

That’s pretty much a complete list. Number 2, 3 and 5 are narrowband, so that makes them unattractive 
for a general purpose output. The pulse drop and add stuff is cheap and easy.  Number 4 does not
produce a logic output and at a ratio like 24 to 10 has many of the same spur issues. 

Bob


> 
> 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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