[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Dr Bruce Griffiths]

Tom Van Baak wrote:
>> On the subject of Brooks Shera's design, the one thing that troubles me is
>>     
> the
>   
>> use of a 24 MHz oscillator to count the width of the 1PPS signal.
>> This yields a precision of 4.16e-8, but does it really?
>>     
>
> No, with averaging it's much better than that.
>
>   
>> This oscillator is uncontrolled and any drift would exist as noise that
>>     
> would
>   
>> have to be filtered (He uses a software low pass filter).
>>     
>
> No, when an oscillator is used as a timebase for what
> is essentially a short period time interval counter the
> XO drift rate does not affect the result like you think.
>
> Suppose you use a cheap XO with a huge drift rate of
> 100 ppm per year or even 1 ppm per day to make TI
> measurements between the OCXO and GPS. So an
> average measurement that is, say 12.34 ns today,
> will be off by 1 ppm tomorrow: it will be 12.34001 ns
> instead. Do you see now why it doesn't matter how
> bad the XO is?
>
> Secondly, someone can double check me here -- but
> it seems to me that any GPSDO that uses a built-in TIC
> to monitor the deviation between the GPS 1PPS and
> the OCXO 1PPS is a closed loop system and so the
> actual accuracy of the TIC timebase has no effect on
> the function of the GPSDO. I mean, the 24 MHz clock
> could drift down to 20 MHz or up to 30 MHz and the
> GPSDO would still work fine (hey, maybe even better).
>
> /tvb
>
>
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>   
Tom

Yes, frequency changes in the TIC oscillator only change the phase 
detector gain, if the loop attempts to lock somewhere near zero phase 
shift then the phase error at lock will be relatively unaffected by 
phase detector gain changes of a few percent. However if the phase 
detector gain changes by too much the loop dynamics will be compromised. 
If the loop locks at say 90 degrees phase shift then changes in the 
phase detector gain will affect the static phase error when the loop is 
locked. Even then the frequency offset will be zero for a second order loop.

Bruce