[SI-LIST] Re: PPM
- From: "Steve Rogers" <SRogers@xxxxxxxxxxxxx>
- To: <ingo.kupper@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Mar 2004 09:33:42 -0000
Ingo,
1. I think you will find that 50ppm means +/- 50ppm.
2. You may find that the 50ppm includes everthing! So you are clear on =
this I will explain. There are basically three reasons why your xtal =
frequency is not exactly where it should be:
(a)Tolerance [manufacturing differences if you like]
(b)Tempco [how the frequency varies over temperature]
(c)Ageing [longe term changes in frequency]
You may find that the 50ppm you need includes all of the above (or maybe =
not)? If it does then you need to know all of the above. These are =
always given on a basic xtal specification. Bog standard HC49 xtals will =
do tolerance of typically +/- 30ppm with temp stability of +/-50 ppm, =
ageing will be +/- 3ppm in year one and dropping off approx 50% of this =
each year after. These basic xtals are probably not good enough for your =
job. However, you can get tighter tolerance and tempco down to +/- 5ppm =
which is much more than you need! Finally, rememgber that you need to =
know what your operating temperature range is in order to fully spec =
your xtal.
Have a look at CMAC, GOLLEDGE, EUROQUARTZ sites to find out more
STEVE =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: ingo.kupper@xxxxxxxxxxxxxx [mailto:ingo.kupper@xxxxxxxxxxxxxx]
Sent: Monday, March 08, 2004 11:22 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] PPM
Content-Type: text/plain;
charset=3D"iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Hi There,
=3D20
I am designing an LOM (LAN on motherboard) and wonder about the =3D
oscillator
requirement in the data sheet.
It says: "... driven by an external 25MHz crystal of 50PPM or better..."
Now, I have seen data sheets saying =3DB150ppm or 50ppm - is that the =
=3D
same? I
think, that 50ppm could actually mean =3DB125PPM - which is getting more
precision for a higher purchase price.
=3D20
Any opinion or real definition appreciated!
=3D20
Thanks,
=3D20
Ingo
=3D20
=3D20
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