[SI-LIST] Re: Crystal Oscillator

In my recent posting on crystal oscillators it appears that I may have
put out some info that isn't strictly correct. Thanks to Merrick
Moeller for pointing out my error.

I stated that in the parallel resonant mode a crystal resonator provided
180 degrees phase shift. Yes and No. 180 degrees phase shift is
provided to sustain oscillation, but the phase shift is provided by
the shunt C elements that are on each side of crystal (90 degrees for
each) while the steep phase slope of the quartz resonator provides the
stability. 


See this representative app-note for more details:

http://foxonline.com/pdfs/osctheoryoper.pdf

also, here are Merrick's comments:

---start of included text----------

        The other 180 deg of phase shift comes from
Cload. The amount of shift depents on the configuration.
The typical Cap --- XTAL/Inverter --- Cap config. would 
have 90 degrees of phase shift from each cap, ideally.
The load of these caps forces the crystal to run at some
point above or below the first resonant frequency, at
a point where Xcload = Xlcrystal. If you can look at the
crystal respone (ZREF IMAG) on a VNA you should see the resonant points
and be able to find the operating point on the crystal 
responce by calculating the impedance of the load at frequency
and finding the offseting inductive load on the responce.
If the crystal is unloaded it will run on the series resonant
frequency. You should be able to fabricate your own VNA socket
to accomidate the crystal package and standards with relativly
similar lead length to the crystal package. This is a very 
rough calibration, but it will give you the proper shape to
the curve if not the exact values. 

---End of included text---------

Sorry for any confusion. (one shouldn't post to a technical list
on a Friday afternoon after 4 straight hours of meetings :)

-Ray

>
>Hi Raymond,
>    Thanks for the detailed explanation. As you quoted
>">When operated at parallel resonance the quartz resoantor 
>contributes
>>180 degrees phase shift."
>    Could you Pl. explain how the quartz resoantor  contributes 
>180 degree phase shift?
>
>Thanks/-
>Pankaj
>
>

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