[time-nuts] Advise on building a DIY GPSDO?
EWKehren at aol.com
EWKehren at aol.com
Sat Apr 9 10:07:07 EDT 2016
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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