[time-nuts] Long period variation of GPS PPS timing?

Joseph M Gwinn gwinn at raytheon.com
Thu Jul 1 15:42:51 EDT 2010


time-nuts-bounces at febo.com wrote on 07/01/2010 02:01:25 PM:

> From:
> 
> Paul Nicholson <vlf0308 at abelian.org>
> 
> To:
> 
> Discussion of precise time and frequency measurement 
<time-nuts at febo.com>
> 
> Date:
> 
> 07/01/2010 02:02 PM
> 
> Subject:
> 
> Re: [time-nuts] Long period variation of GPS PPS timing?
> 
> Sent by:
> 
> time-nuts-bounces at febo.com
> 
[snip]
> 
> Joe Gwinn wrote:
>  > The centroid of pulses triggered by the leading edge of the
>  > 1PPS pulse will vary with the width of the pulses.
> 
> I see.  It is a passive network but unprotected from temperature
> changes.  It is shut in a box with 2 PCs and a bunch of other
> stuff.  Various fans on and off. Temperature varies 26-30C in
> the box.   I need to look at temperature, but gut feeling tells
> me the cycle is a little too regular and is some subtle software
> defect, aliasing or something.   I must plot soundcard drift
> rate against the slow cycle, etc.   I can tackle this with
> confidence now that I know it is not a GPS limitation.  Thanks
> for that.

If the room temperature varies (due to the cycling of the heating system), 
the temperatures within the box will vary more or less in parallel. 

PC power dissipation varies with computational load, so periodic 
processing can cause periodic temperature variation.

The quickest way to tell is to log temperature while logging PPS timing, 
and see if they correlate.  The temperature measurement need not be all 
that accurate, so long as the sensor is stable and has good resolution.

Joe Gwinn




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