[time-nuts] Advise on building a DIY GPSDO?
Bob Camp
kb8tq at n1k.org
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
>>>>
>>>>
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com
>>>>> To unsubscribe, go to
>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>> and follow the instructions there.
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>
>>
>>
>> --
>> Best regards,
>> Timenut mailto:timenut at metachaos.net
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts
mailing list