[time-nuts] 4046 experiment for gpsdo

Will ZL1TAO at gmx.com
Sun Sep 27 05:53:31 EDT 2015


Thanks,

Yes, apologies, I didn't look at the schematic, just the text  of
message.  Makes more sense.

I'm looking at using a Neo7 GPS with the output set to something. I
gather that an integer divider gives the least jitter so will probably
be set 8Mhz and divide that by 8 to give 1MHz at 50% duty cycle.  It
will be locking a Trinble 10MHz OCXO. Divided  by 5 and then 2 to also
give 1MHz with 50% duty cycle. Though as Magnus points out I might have
to use some hold off until the oven heats up and stabilises.

Would there be any advantage in setting the Neo7 to say 1MHz or even
lower and dividing both Neo and Trimble down to 1pps or some other low
frequency?

Cheers,
Will

On 27/09/15 00:36, Chuck Harris wrote:
> You missed the part about the 10MHz being divided by 10 million
> to produce a 1PPS signal that is compared to the second 1PPS signal...
>
> -Chuck Harris
>
> Will wrote:
>> Hi,
>>
>> I'm new and trying to get to grips with things.
>>
>> If I understand correctly, please forgive if I have it wrong,  This
>> locks a 10MHz signal  to a 1Hz (1pps) signal.  What makes it lock to 10
>> 000 000Hz instead of 999 999Hz or 10 000 001Hz?  Just the hope that the
>> 10MHz is exactly that?
>>
>> Cheers,
>> Will
>>
>> On 26/09/15 08:32, Jim Harman wrote:
>>> To further demonstrate the Diode - R- C- approach, here  (hopefully)
>>> is a
>>> screenshot of the raw DAC output vs time on my Arduino Micro (32u4)
>>> based
>>> system. For this test the oscillator is free running with an error
>>> of about
>>> 1 usec per 460 sec or 2.17x10^-9. The horizontal scale is 125
>>> sec/div (1000
>>> sec total) and the vertical is 1024  DAC counts (0-2.56 V) which
>>> corresponds to 1 usec of offset between the oscillator and the
>>> reference.
>>>
>>> You can see that there is some curvature because the capacitor is being
>>> charged through a resistor and not a true current source, but as I
>>> mentioned earlier this does not affect the system's ability to lock the
>>> oscillator to the pps reference. When locked with a time constant of
>>> 1000
>>> sec, the phase detector output is almost always less than +/- 100
>>> counts
>>> from the setpoint of 500.
>>>
>>> The noise is due mostly to jitter in my PPS reference, which is
>>> generated
>>> by an Adafruit GPS module. Presumably it would be less if I had a real
>>> timing receiver.
>>>
>>>
>>> ​.
>>> If the inserted image does not come through, I will re-send as an
>>> attachment.
>>>
>>>> -- 
>>> --Jim Harman
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