[SI-LIST] Re: PPM (2nd repost)
- From: "Steve Rogers" <SRogers@xxxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Mar 2004 08:42:52 -0000
Just to add a bit more to the mix
1. Ageing can be an issue if as Andrew has said you are using a tight =
tol xtal. However, most xtal manufacturers will tell you that the ageing =
change is largest over the first six months to one year. After that =
things settle down. Typically you will find the figures on this to be =
about 50% less each year after year one. One point that has not been =
made regarding ageing is that you can get the xtals pre-aged. They =
simply cook the thing for a while!=20
2. One other thing to watch out for that is probably a lot more =
significant than all the other issues we have discussed thus far is load =
capacitance! You will find that the xtal manufacturer gives a =
recommended load capacitance for the xtal. All the tolerances etc that =
are given for the particular xtal are only valid for this load =
capacitance. Often you will see oscillators with the wrong load =
capacitance. They may start up and run ok but by ignoring the load cap =
value you will introduce another error. I recently encountered a poor =
oscillator design that was off frequency by about 100ppm due to the =
wrong load capacitance. This is not to say you should not use other =
capacitor values on oscillators as you can often improve things such as =
harmonic content of the oscillator etc.
Im sure we could go on..........!!=20
Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer
Micromill Electronics Limited
Leydene House
Waterberry Drive=20
Waterlooville Hampshire
PO7 7XX
Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673
Registered No. 1456922 (England). =20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ
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-----Original Message-----
From: Ingraham, Andrew [mailto:a.ingraham@xxxxxxxx]
Sent: Wednesday, March 10, 2004 5:48 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PPM (2nd repost)
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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