Crystal oscillator design

1. Pierce oscillator is the most popular one; pros: easy startup; cons: needs to pins for external resonance connection;
1.1. Isolated Pierce oscillator is even better than simple Pierce oscillator;

2. Negative resistance: gme*Xce1*Xce2; here e means equivalent, indicating contribution not only from a single device, needing some (simple) calculation to obtain; can change gm or cap values to change Rneg;

3. Gain stage can be either inverter based or CS/CE mosfet/BJT with load (load can be current source, resistor or inductor);

4. Inverter based one is easy to design, however, it can perturb the power supply too much; its current consumption also varies over PVT;

5. Current-controlled mosfet can have precise control on current (and thus can be very low current), and does not perturb the power supply; however, it needs some calculation to check how much current it requires.

6. Designers shall check following:
1) Rneg and total loss, and make sure safety factor is moderate (not too large or too small);
2) Power burned by external resonator is within a limit;
3) Harmonics are properly filtered out before entering external resonator  (this post only discusses the case of crystal oscillator utilizing fundamental freq); otherwise, external resonator may have been excited at other freqs;
4) Device reliability: during transient, e.g., voltage cross mosfet oxide does not exceed certain value;
5) PSRR;
6) Phase noise;
7) Current consumption;
8) etc;

Ref:
Benjamin Parzen, Design of Crystal and Other Harmonic Oscillators

Crystal oscillator design