[SI-LIST] Re: Crystal Oscillator
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 25 Jul 2003 13:33:27 -0700 (PDT)
Pankaj-
A quartz crystal resonator has two resonant modes, series and parallel.
(The series resonant frequency is (to a first order approximation)
determined by the series combination of the motional inductance and
motional capacitance of the quartz crystal.
The parallel resonant frequency is determined by the series connection
of the motional inductance and motional capacitance in parallel with
the shunt holder capacitance. The series and parallel frequencies are
very close to each other, typically separated by only a few kilohertz.
Also note that when operated in the series resonant mode, the quartz
resonator contributes ZERO degrees phase shift at resonance. When
operated at parallel resonance the quartz resoantor contributes
180 degrees phase shift.
So an oscillator whose active element contributes 180 degrees phase
shift (perhaps a single inverter) may be used at the parallel resonant
frequency of the quartz resonator to sustain oscillation. (180 + 180 =
0 degrees or positive feedback).
If your active elements contribute 0 degrees phase shift (say, two
invertors), then the quartz resonator will operate in the series resonant
mode (with zero degrees phase shift contributed by the crystal) to
sustain oscillation at the series resonant frequency.
Note that capacitive loading by the oscillator circuit can 'pull'
the actual frequency of oscillation slightly, and may contribute
a small amount of phase shift that may cause the oscillator to actually
operate slightly off the series or parallel resonant frequencies of
the unloaded quartz resonator.
I hope this explanation helps out.
Ray Anderson
Staff SI Engineer
Sun Microsystems Inc.
>
>Hi Raymond,
> I have some querry about crystal oscillator at parallel resonance. The
>reason, I am asking you personally is because I posted the same question
>in SI list, but unfortunately didn't get any response. Your help will be
>really appreciated.
> I understand in parallel resonance mode 180 degree(infact >180)
>phase shift comes from inverter. Also, other 180 degree phase shift will
>come from crystal and load. Now, crystal is modelled as inductor above
>series resonance. That means, we are having inductor and capacitor, which
>acts like tank circuit. But, tank circuit will give 0 degree phase shift.
>So, I am wondering how we are getting the remaining 180 degree phase shift?
> Any insight will be great help to me. Also, please forward me some
>good links.
>
> Thanks in Advance.
>
>Pankaj Kumar
>IO Design Engineer
>ARTISAN COMPONENTS, INC.
>
>www.artisan.com
>
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