The oscilation frequency of crystal oscilators is effected by the load
capacitance of the crystal.  The crystal manufacturer specifies the required
load capacitance to get the stated frequency.  Changing from the specified
load capacitance will simply change the frequency of your oscilator a small 
amount.  Probably much less than 0.1%, probably less than 0.01%

Use the values that the ic maker recomends to get it running, then if you want
tweak the freq, you can vary the caps.  You will undoubtably have stray
capacitance on the order of 10-20 pf, so its hard to design for a specific

Hint: the circuit will look something like this:

                    |                      || C1
              output| -----------+---------||-----Vss
                    |            |         ||
                    |            |
                    |         +--+--+
                    |         |     |
                    |         |     |crystal
                    |         |     |            
                    |         +--+--+        C2
                    |            |         ||
              input | -----------+---------||-----Vss
                    |                      ||

if you make C1 = twice C2, then you will get some voltage gain from
input to output.  This can help.

Opinions expressed herein are my own and may not represent those of my employer.

Date: 17 May 1996 20:32:44 GMT

Original Subject: Re: Q: Crystal circuit capacitors ?

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