Crystal oscillator circuit and method of operation

1. A crystal oscillator circuit comprising:

a crystal resonator;

a voltage source configured to apply a voltage step across the crystal resonator where a polarity of the voltage source applied to the crystal resonator is switched in response to a sign of a current passing through the crystal resonator and in response thereto a self-timed energy injection waveform is applied to the crystal resonator;

wherein the injection waveform switches polarity at each zero-crossing of a motional branch current of the crystal resonator.

2. The crystal oscillator circuit of claim 1 , wherein the crystal resonator is initially coupled to a start-up oscillator circuit comprising the voltage source configured to start oscillations by the crystal resonator and thereafter switchably coupled to a steady state oscillator circuit.

3. The crystal oscillator circuit of claim 1 , wherein the voltage source generating the self-timed energy injection waveform is coupled to a switch control circuit and a plurality of switches controlled by the switch control circuit to switch the polarity of the voltage source is formed in one of: a H bridge circuit; a single ended.

4. A crystal oscillator circuit comprising:

a crystal resonator;

a voltage source configured to apply a voltage step across the crystal resonator where a polarity of the voltage source applied to the crystal resonator is switched in response to a sign of a current passing through the crystal resonator and in response thereto a self-timed energy injection waveform is applied to the crystal resonator;

wherein the crystal resonator is initially coupled to a start-up oscillator circuit comprising the voltage source configured to start oscillations by the crystal resonator and thereafter switchably coupled to a steady state oscillator circuit.

5. The crystal oscillator circuit of claim 4 , wherein the injection waveform switches polarity at each zero-crossing of a motional branch current of the crystal resonator.

6. The crystal oscillator circuit of claim 4 wherein the crystal oscillator is coupled to at least one load capacitor and the steady state oscillator circuit 1300 comprises at least one self-quenched NMOS transistor coupled to a delay circuit configured to pull down a voltage across the at least one load capacitor.

7. The crystal oscillator circuit of claim 6 , wherein the voltage source generating the self-timed energy injection waveform is coupled to a switch control circuit and a plurality of switches controlled by the switch control circuit to switch the polarity of the voltage source is formed in one of: a H bridge circuit; a single ended.

8. The crystal oscillator circuit of claim 4 , wherein the voltage source generating the self-timed energy injection waveform is coupled to a switch control circuit and a plurality of switches controlled by the switch control circuit to switch the polarity of the voltage source is formed in one of: a H bridge circuit; a single ended.

9. A crystal oscillator circuit comprising:

a crystal resonator;

a voltage source configured to apply a voltage step across the crystal resonator where a polarity of the voltage source applied to the crystal resonator is switched in response to a sign of a current passing through the crystal resonator and in response thereto a self-timed energy injection waveform is applied to the crystal resonator;

wherein the voltage source generating the self-timed energy injection waveform is coupled to a switch control circuit and a plurality of switches controlled by the switch control circuit to switch the polarity of the voltage source is formed in one of: a H bridge circuit; a single ended.

10. The crystal oscillator circuit of claim 9 wherein each respective switch of the plurality of switches of the H bridge circuit that is coupled to a power source is connected across a comparator and configured to be activated in an alternating manner in response to a control signal from the switch control circuit.

11. The crystal oscillator circuit of claim 10 , wherein a sign of the comparator(s) is detected at a beginning of a start-up phase, and is used to swap the comparator inputs and invert the output, such that a compactor voltage offset is always negative.

12. The crystal oscillator circuit of claim 10 , wherein the comparator is a single, shared comparator with a switch matrix of inputs that is controlled by the switch control circuit.

13. The crystal oscillator circuit of claim 10 wherein each respective switch of the plurality of switches of the H bridge circuit that is coupled to a power source and is connected across a comparator is a high ohmic switch.

14. The crystal oscillator circuit of claim 10 , wherein the plurality of switches is a plurality of non-zero on-resistance switches, such that a measured voltage over a switch in an ‘on’ state indicates a measure of a current flowing through it.

15. The crystal oscillator circuit of claim 9 wherein each respective switch of the plurality of switches of the H bridge circuit that is coupled to a power source and is connected across a comparator is a high ohmic switch.

16. The crystal oscillator circuit of claim 15 wherein comparator inputs are alternately connected to a supply in order to detect a sign of an offset current monitored by the respective comparator.

17. The crystal oscillator circuit of claim 16 wherein the sign of the offset current is detected at a beginning of a start-up phase, and the sign is used to swap comparator inputs such that a comparator offset is always negative.

18. The crystal oscillator circuit of preceding claim 9 , wherein the plurality of switches is a plurality of non-zero on-resistance switches, such that a measured voltage over a switch in an ‘on’ state indicates a measure of a current flowing through it.

19. The crystal oscillator circuit of claim 18 , wherein, a measure of a current flowing through the switch indicates a zero-crossing point of the motional current I m .

20. A method for operating a crystal oscillator circuit comprising:

applying a voltage step across a crystal resonator by a voltage source;

determining a polarity of the voltage source in response to the determined sign of the current passing through the crystal resonator;

applying a self-timed energy injection waveform to the crystal resonator; and

switching the self-timed energy injection waveform in response to the determined sign of the current, wherein the injection waveform switches polarity at each zero-crossing of a motional branch current of the crystal resonator.