[time-nuts] nubie querie

[Bob Camp]

Hi

A clock and a hazmat site all rolled into one.....

Bob

On Mar 5, 2010, at 2:41 PM, paul swed wrote:

> OK then if you ground up cesium azide and put it in the hour glass wouldn't
> you have a cesium clock at much lower cost (And accuracy) then an HP??
> Might last quite a while also.
> 
> On Fri, Mar 5, 2010 at 1:53 PM, Tom Van Baak <tvb at leapsecond.com> wrote:
> 
>> Even defining when the sand timer is "done" is not a real
>>> simple thing. Waiting for that very last particle to drop may
>>> not be the best approach.
>>> Bob
>>> 
>> 
>> Correct. Marking time with an hour glass is not that different
>> from marking time with a 1PPS. Each signal has a rise time;
>> one picks the appropriate live trigger level or sampled slope
>> waveform model to minimize jitter.
>> 
>> Waiting for the last grain is like waiting for the last millivolt of
>> a TTL 1PPS pulse; no one does that.
>> 
>> Note that [this] hourglass interval has a standard deviation on
>> the order of 10 seconds. So my initial goal is 1 second timing
>> resolution, which turns out to be pretty easy to do optically.
>> 
>> Whether sand or cesium, phase comparators have a minimum
>> resolution. While lower resolution is better, if the ADEV of the
>> DUT is too far above the ADEV of the comparator then that
>> resolution is wasted. So detection to a granularity of 1 second
>> is sufficient for this application.
>> 
>> 
>> /tvb
>> 
>> 
>> 
>> 
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