[time-nuts] Need schoolin PPM vs E to the umth?

Chuck Harris cfharris at erols.com
Wed Dec 31 12:30:39 EST 2008


n3izn at aol.com wrote:
> So can any one bring it down a few clicks and explain the accuracy thing to this balding hippie?
> 
> I'm leaning towards ham radio applications. GPSDO are rated in one way like 1 X 10 to something. Ham radios are
> coming with "High Stability" oscillators that are rated in ? PPM.
> 
> In laymen's terms what do those numbers mean and how do they relate to each other?

They convey the same information as an accuracy percentage, only they
allow for an easier to understand number for higher stability sources.

For example, an oscillator that is accurate to 1%, is accurate to
one part in 100, or expressed in scientific notation, 1 part per 10E2.

Most people on this group consider that pretty (well awfully, actually)
crude.

As you get to more and more stable frequency sources, you might hear
of 1 part per million, or 1 PPM, or 1 part in 10E6.  In percentages,
that would be 0.0001%.

And as you become a time-nut, you start to talk of parts in 10E12, which
in percentages would be 0.0000000001%... Which is just too cumbersome
to use.  You have to count the zeros every time you look at the number,
and you still aren't sure what it means.

Any specification that tells of the accuracy of a frequency standard
needs some additional information... notably, a time frame over which
the accuracy is valid.  So you will hear things like 1PPM/year, or
1 part in 10E12 per second.

Some information about environmental conditions would also be useful.

Also, implicit in an accuracy specification is +/- which creates an
error band for the error to be contained within.

When one becomes a true time-nut, one starts to worry about trends in
stability issues, and at that point, statistical analysis becomes
important, and you start to talk of things like Allen-Variances.

-Chuck Harris
> 
> Hope this doesn't start to much controversy on the board.
> 
> 73 Chris
> 
> 
> 
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