[time-nuts] Interesting paper: Don't GPSD' your Rb...

Hal Murray hmurray at megapathdsl.net
Sun May 6 02:12:12 UTC 2012


phk at phk.freebsd.dk said:
> If you cannot apply the negative sawtooth, you will get better results by
> disciplining almost any random quartz xtal, ovenized or not to the GPS,
> divide it down to PPS and then discipline the PRS10 to that. 

I don't understand that.  What am I missing?

In the simple case, the main parameter for a PLL is the time constant or 
cutoff frequency on the low pass filter.  Normally, if the time constant is 
low enough the PLL will track the incoming wobbles.  If the time constant it 
long enough, the PLL will ignore the short term wobbles yet track the long 
term wanders.

In this context, the Rb is very stable so the time constant can be very long. 
 Unfortunately, the time constant of hanging bridges can also be very long, 
especially if your temperature regulation is good (which is probably a good 
first step for anything like this).

What does putting an extra crystal between GPS and Rb do?  What time 
constants do I use on the PLL from GPS to xtal and xtal to Rb?


-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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