At 10:08 PM 6/11/01 +0200, Niklas Wennerstrand wrote:
>Dale wrote: I'm just curious.  Am I the only one who has never used
>capacitors with
>the crystal, and never had one fail to start?  Or is this one of those "it
>will work for more repetitions than you can test for one-offs and
>prototypes but don't trust it for production" things?
>
>Answer: If you use a crystal it is just plain luck that makes it to work or
>maybe the capacitance in the wire that makes things rock.

No, it's gain and phase shift.
In a closed loop system, with gain >1, you will oscillate at the frequency
where the phase shift = 360 degrees, including whatever shift is in the
amplifier.

The crystal and the caps provide the phase shift.
The crystal provides 180 degrees of phase shift at one particular
frequency, and the amplifier in the chip provides the gain, and the other
180 degrees of shift.

The caps trim it out to the proper frequency, because the circuit interacts
with the crystal. Rather than leave this to chance, we apply a larger value
of C to swamp out the parasitics. The crystal is designed to work with this
larger C value.

--
Dave's Engineering Page: http://www.dvanhorn.org

I would have a link to http://www.findu.com/cgi-bin/find.cgi?KC6ETE-9 here
in my signature line, but due to the inability of sysadmins at TELOCITY to
differentiate a signature line from the text of an email, I am forbidden to
have it.

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads