[time-nuts] Time Interval Algebra?

Brooke Clarke brooke at pacific.net
Mon Dec 20 01:35:41 EST 2004

Hi Bill:

Getting every reading is only important if you want to calculate the 
Allan variance.  For a simple drift measurement it's not required.
I've been watching a PIC generate a 1 PPS output from a DIP oscillator 
that's in a home made oven for over a year and I just manually enter the 
period measurement into an Excel spreadsheet when I get to my computer 
each morning along with the data and time.

My Gibbs 10 Mhz lab standard oscillator finally aged to the point where 
it could not be brought back on frequency.  But when dividing an 
oscillator down to 1 PPS there is no practical limit.  Also note that 
aging overwhelms all other sources of error in crystal oscillators.

Have Fun,

Brooke Clarke, N6GCE

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

Bill Hawkins wrote:

>Why is it important to get every measurement? If I turn the counter on
>Monday afternoon, get a reading of 0.500000 seconds, turn it off and 
>next Monday afternoon get a reading of 0.500027, then I have an offset
>or error of 27 +/- 1 microseconds in 7 days. Each reading has been accurate
>to 1 microsecond, which requires that the counter clock drift less than
>0.1 microsecond per second.
>I suppose that the problem is that this doesn't allow an Allan Variance
>calculation based on one calculation per second. Why must it be one per
>second, not one in 10 or 100 seconds? Or why not 100 per second?
>A larger problem may be the variance in the counter that is dividing the
>Rb output into 1 pulse per second. Are the counters all clocked by the
>Rb clock or is it a simple cascade of decade counters?
>Bill Hawkins
>time-nuts mailing list
>time-nuts at febo.com

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