[time-nuts] How to measure Allan Deviation?

Didier Juges didier at cox.net
Sun Oct 22 21:17:43 EDT 2006


Dr Bruce Griffiths wrote:
>
> Reading between the lines on the Jupiter GPS receiver datasheet it would 
> appear that the 10KHz output is phase modulated at 1Hz to realign it to 
> successive PPS output pulses. As the PPS jitters about so does the 10KHz 
> signal.
> Most GPS receivers with higher frequency outputs than 1Hz, phase 
> modulate the high frequency output in this way and the datasheets 
> explicitly indicate this.
>
> Thus there would appear to be little advantage in phase locking to the 
> 10KHz signal with a short loop time constant.
>
> To be absolutely sure you will need to use an oscilloscope to observe 
> the synchronous jitter in the 10KHz waveform.
>
> Bruce
>   
I believe the main benefit of using the 10 kHz output is the easier 
filtering, and the optional capability of speeding up the loop for the 
initial lock, which may only be useful for experimentation (to verify 
that the loop does lock), as any loop speed up will degrade the short 
term stability.

However, if the 10 kHz is the open loop crystal for .9999 second, then 
resynced at 1 Hz, there is no advantage whatsoever, since the main 
error/correction component will still be at 1 Hz. That should be easy to 
check.

I am not sure what the advantage of the 10 kHz output is, then?


Didier


More information about the time-nuts mailing list