[time-nuts] How close can you trim a Cs?

John Ackermann N8UR jra at febo.com
Wed Mar 9 14:09:24 EST 2005

Chuck Harris wrote:
> John Ackermann N8UR wrote:

> The problem with this method is the counter cannot be relied apon not to
> add(or subtract) a certain amount of time (less than its resolution) to
> each measurement.  If this was simply jitter, it should average out to 
> zero,
> but if it is a delay time, it will remain as a bias on your result.  This
> bias will survive any amount of averaging.
> The only way I know of reducing the affect of this sort of error is to 
> start
> your timer at the beginning of the C-beam's first 1 second pulse, and 
> stop the
> timer at the beginning of the GPS's pulse that comes nearest to the 
> beginning
> of the 100th 1 second pulse that comes from the C-beam.
> This will give you a true average of the differences between the two 
> 1pps rates,
> +/- the 2ns resolution of your counter.  The best you can hope for with 
> this
> counter based technique is 2 parts in 10E11.  If you want 10E13, you 
> will have
> to up your time window to 10,000 seconds, or find a counter with finer 
> resolution.

Hi Chuck --

But if the bias is constant, it won't affect the results because I'm 
looking at the change in time interval over time, not the absolute value 
of the reading.  So, if the initial time difference reading is 1.100us 
and 24 hours later it's 1.200us, the offset is 100 nanoseconds in 86,400 
seconds, or 1.16x10e-12.

The fact that there's a bias in both the initial and the final values 
doesn't matter because it's "common mode" to both the starting and 
ending readings and thus cancels out (assuming, of course, that the bias 
doesn't change during the course of the measurement).


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