[ntp:questions] 500ppm - is it too small?

[David Woolley]

David J Taylor wrote:
> David Woolley wrote:
> []
>> A reasonable expectation of a cheap crystal is 50ppm static plus
>> <10ppm temperature dependent.  When one gets >500ppm it suggests the
>> problem is rather worse than poor crystal tolerancing.
> 
> I am inclined to agree.  One reference I found was this:
> 
>  http://www.maxim-ic.com/appnotes.cfm/an_pk/3566

This seems to suggest crystals have quadratic temperature dependency. 
My understanding is that normal cuts have cubic dependency.  Of course, 
32kHz crystals are often cut for wrist watch use, and using a cut with 
quadratic characteristics might give a flatter response close to body 
temperature, at the expense of poorer behaviour for signficant deviation.