[time-nuts] Advise on building a DIY GPSDO?
Herbert Poetzl
herbert at 13thfloor.at
Fri Apr 8 22:35:45 EDT 2016
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.
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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