[SI-LIST] Re: PPM (2nd repost)
- From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 10 Mar 2004 12:48:04 -0500
Steve Rogers wrote:
> 2. You may find that the 50ppm includes everthing!
My opinion: the 50 ppm requirement in your design includes everything, but
the 50 ppm spec on the oscillator you want to buy, may not.
In my experience, an oscillator's spec usually does not include everything.
If you wait long enough, the aging part is technically not bounded and
eventually would exceed a single spec limit.
Maybe for really loose tolerance devices (100 ppm or more) it doesn't matter
because the aging part is insignificant by comparison for reasonable
lifetimes, so a manufacturer could claim that the spec is all-inclusive over
some lifetime. But for devices with somewhat tighter specs, I think they
are usually listed separately.
By the way, aging generally causes the frequency to decrease over time, as
if a small amount of material (a few molecules per year) is being added to
the crystal surfaces over time.
The important thing is to understand that all three of the processes Steve
listed, affect your oscillator's frequency. If you look at just the initial
tolerance spec and ignore the rest, you aren't going to get what you think
you're getting. When in doubt, ask the oscillator manufacturer.
A fourth variation, not often listed, is mechanical shock. Dropping or
otherwise shocking a crystal knocks molecules loose and thereby changes the
frequency, so don't handle them unusually roughly.
I concur with the other replies that a tolerance spec means "+/-" because
what you want to know is how far your frequency is from what it's supposed
to be, the value stamped on the case.
Regards,
Andy
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