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I want to design a crystal oscillator (pierce) but I'm having trouble. Proposed oscillation frequency is around 39 MHz and I used crystal L~4mH C~4.1fF R~10ohm; Co~2pF

C1,C2=20pF (shunt cap for drain to GND & gate to GND)

Feedback resistor ~ 1 Mohm; Vdd~1.5V Transistor gm~80u

However, my oscillator input/output settles to same voltage, and does not oscillate.

When I change crystal cap to 200 fF then it starts oscillation.

  • Can crystal motional cap affect to oscillation?
  • What is the cause for non oscillating?

pierce oscillator This is the schematic of my design.

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    \$\begingroup\$ Shall we provide solutions to school assignments? \$\endgroup\$ Commented Oct 12, 2016 at 16:20
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    \$\begingroup\$ @AliChen Shall we provide solutions to job assignments? \$\endgroup\$
    – pipe
    Commented Oct 12, 2016 at 16:23
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    \$\begingroup\$ @pipe, that's one good question. If a person starts with "I want to design" and does not know about negative impedance concept in Pierce Oscillator, I think my answer is NO. \$\endgroup\$ Commented Oct 12, 2016 at 16:31
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    \$\begingroup\$ Show your circuit diagram. \$\endgroup\$
    – Andy aka
    Commented Oct 12, 2016 at 21:58
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    \$\begingroup\$ Sure, I do understand your curioucity. That's why you are using a professional-grade transistor-level ASIC design tool Virtuoso from Cadence, with SPICE license? Then you have selected probably the worst design topic for your self-learning. \$\endgroup\$ Commented Oct 13, 2016 at 4:41

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The only reason for lack of oscillation is insufficient gain to cover attenuation loss in parallel resonant circuit with 180 phase inversion in CLC resonator and 180 deg inverting amplifier.

All circuit elements shape the Q of the resonator ( Typ ~10k) especially the motional capacitance which is very small with a very large inductance.

I could analyze your Pierce design but I would bet money that Xtal impedance in resonant mode is lower than driver impedance, thus high insertion loss and insufficient gain in transistor.

Pierce Osc.

enter image description here enter image description here

Hartley Osc. enter image description here

For more details ref> http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snaa065&fileType=pdf

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  • \$\begingroup\$ A pierce oscillator uses series resonance not parallel as far as I know. \$\endgroup\$
    – Andy aka
    Commented Oct 12, 2016 at 22:00

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