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

[Bob Camp]

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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