PPS over USB

[Hal Murray]

James said:
> I haven't tested it yet, as I'm attempting to 'improve' my PPS over USB
> setup.

Do you know what you are getting into? 1/2 ;)
It's either fun if you are in the right mood, or a pain in the butt if you 
are unlucky.

Do you know about hanging bridges?
  http://www.leapsecond.com/pages/m12/sawtooth.htm

You can get the same thing with the USB clock beating against the PPS 
clock.  The USB clock is derived from the CPU clock.  That varies with 
temperature.  I have one setup that gets a good hang on a cold winter 
night.

If your clocks aren't close to in-sync, then ntpd will average over 
several/many samples with a random offseet.  That's the common case.  Your 
clock will be 1/2 ms off if you are using real slow USB and 1/2 of 125 
microseconds if it's using the faster USB.  I forget what USB calls them.  
Most GPS USB gizmos are slow and don't have PPS.

If the clocks get very very close to in-sync, then the offset isn't 
random.  If the clocks are close enough to in-sync, then all the samples 
will be close to the same offset.

Putting ntpd on top of that adds another layer of complexity.


-- 
These are my opinions.  I hate spam.