[SI-LIST] Re: Parallel Resonance of crystall oscillator

  • From: Ed Miguel <emiguel@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 14 Jan 2003 13:44:44 -0600


Hi all,
Below is a graphical view of a xtal's // resonance.   What challenge=20
plotting like this!
In a nutshell, parallel resonance, which I believe is a coined term, occurs=
=20
between the series and anti-resonant frequencies.  Within this area, the=20
series C will set the frequency.  Low C values will result in frequencies=20
near anti-resonance, Higher C values will put the frequency closer to=20
series resonance.  The phase between series and antiresonance will swing=20
-180 degrees.  The inverter amplifier will make up the remainder of the 180=
=20
degrees to meet Barkhausen's criterion for oscillation, 0 or n360 degrees=20
with gain greater than 1.   Parallel resonant crystals are calibrated with=
=20
a known load capacitance like 12 pf for example.  The user ckt needs to=20
duplicate this as with 2 -24 pf PI caps which are really in series with the=
=20
crystal. Once oscillation is achieved and easy way to set frequency is to=20
tweek the cap values or use a trimmer.  Usually you get many ppm per pf of=
=20
capacitance change.

                                     fa
Xtal reactance curve                -
                                    --
                                  -  -
Inductive                         -     -
                               -         -
                        fs         -             -
                 - - - - -                   -                 - - - - - -=
=20
- - - -
         -                           -      -            freq >>>
      -                                      - -
   -     capacitive                  --
-                                   -
                                 ^
                             // res area
0 ----------------------------          \/     =
 -------------------------------
                         -                   -
Phase plot              -                   -
                         -                   -
-180                    ----------------


I hope this helps,

Ed Miguel
Connor Winfield


At 12:50 PM 1/14/03 +0800, =3D?gb2312?q?Bi=3D20Han?=3D wrote:

>The phase shift is most mentioned in ring oscillators.
>
>  Parthasarathy Sampath <parthsv@xxxxxxxxx> wrote:
>Hi all,
>I have a question regarding crystall oscillator
>operating at parallel resonance.
>At parallel resonance, the impedance offered by
>series resonant ckt within crystal oscillator is
>inductive. This inductive reactance is cancelled by
>the capacitive reactance offered by parallel
>capacitance in the crystall oscillator and the load
>capacitance. So there should not be any phase in the
>circuit since the reactances get cancelled. However a
>lot of articles say load capacitance provide a phase
>shift of 180 in order to oscillate in closed loop.
>Since the load capacitance is included in the parallel
>resonance to cancel inductive reactance, how can it
>contribute 180 degress phase shift.
>
>Thanks in advance,
>Partha
>
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