Temperature controlled acoustic resonator comprising feedback circuit
An acoustic resonator device includes an annular acoustic resonator, a heater coil and a heat sensor. The annular acoustic resonator is positioned over a trench formed in a substrate of the acoustic resonator device. The heater coil is disposed around a perimeter of the annular acoustic resonator, the heater coil including a resistor configured to receive a heater current. The heat sensor is configured to adjust the heater current in response to a temperature of the heater coil. A feedback circuit is used to control a temperature of the acoustic resonator device.
1. An acoustic resonator device, comprising:
an annular acoustic resonator positioned over a trench formed in a substrate;
a heater disposed around at least a portion of a perimeter of the annular acoustic resonator, the heater configured to receive a heater current; and
a heat sensor resistor disposed adjacent to the heater;
a reference resistor;
an amplifier configured to receive a first input from the reference resistor and a second input from the heat sensor resistor, and to provide an output proportional to a difference between the first input and the second input, wherein the heater current is adjusted based on the difference;
a pillar disposed in the trench, the pillar supporting the annular acoustic resonator at a central location; and
a conductor wound around the pillar for providing the heater current to a heater coil, wherein the heater coil is disposed around an inner perimeter of the annular acoustic resonator.
2. The acoustic resonator device of claim 1 , further comprising:
a plurality of tethers connected to the annular acoustic resonator, the plurality of tethers being configured to limit transfer of externally applied forces to the annular acoustic resonator.
3. The acoustic resonator device of claim 2 , wherein a first tether of the plurality of tethers comprises a first conductive trace for providing the heater current to a heater coil, and the heater coil is disposed around an outer perimeter of the annular acoustic resonator.
4. The acoustic resonator device of claim 3 , wherein a second tether of the plurality of tethers comprises a second conductive trace for providing heat sensor current to a heat sensor, a change in a temperature of the acoustic resonator device causing a corresponding change in sensor resistance of the heat sensor resistor.
5. The acoustic resonator device of claim 4 , wherein the reference resistor has a predetermined reference resistance, and the amplifier is a transconductance amplifier configured to sense a voltage caused by a difference between a sensor impedance and the predetermined reference resistance; and the acoustic resonator device further comprises a transistor configured to receive the output from the transconductance amplifier and to provide the heater current.
6. The acoustic resonator device of claim 5 , wherein for a predetermined temperature of the annular acoustic resonator, a reference voltage (Vref) across the reference resistor is set to be equal to a sensor voltage (Vsense) across the heat sensor resistor at the predetermined temperature.
7. The acoustic resonator device of claim 6 , wherein the predetermined temperature is substantially equal to a turn-over-temperature (TOT), or a TOT range.
8. The acoustic resonator device of claim 6 , wherein the transistor is a pass field effect transistor (pass FET), and the output from a transconductance amplifier (TCA) is an error current proportional to a difference between the reference voltage (Vref) and the sensor voltage (Vsens), the pass FET configured to change a magnitude of a current (I heat ) to the heater.
9. The acoustic resonator device of claim 3 , wherein the first tether comprises:
a membrane, the first conductive trace being formed on a first surface of the membrane; and
a second conductive trace formed on a second surface of the membrane, opposite the first surface, the second conductive trace being configured to provide a return path for the heater current, wherein the membrane defines a via through which the first conductive trace being formed on the first surface of the membrane electrically connects with a heater coil.
10. The acoustic resonator device of claim 2 , wherein the plurality of tethers includes four tethers, each tether being approximately equal in length and having two lever arms, and each lever arm extending along approximately one quarter of a circumference of the annular acoustic resonator.
11. The acoustic resonator device of claim 2 , wherein the plurality of tethers includes two tethers, each tether being approximately equal in length and having two lever arms, and each lever arm extending along approximately one half of a circumference of the annular acoustic resonator.
12. The acoustic resonator device of claim 1 , wherein the heat sensor resistor has a temperature coefficient of resistance such that its sensor impedance (R sense ) varies in response to a temperature of the annular acoustic resonator.
13. A feedback circuit configured to control a temperature of an acoustic resonator, the feedback circuit comprising:
a heater disposed around at least a portion of a perimeter of the acoustic resonator, the heater configured to receive a heater current; and
a heat sensor resistor disposed adjacent to the heater;
a reference resistor having a predetermined reference resistance; and
a transconductance amplifier (TCA) configured to: receive a first input from the reference resistor and a second input from the heat sensor resistor; sense a voltage caused by a difference between a sensor impedance and the predetermined reference resistance; and provide an output proportional to a difference between the first input and the second input, wherein the heater current is adjusted based on the difference; and
a transistor configured to receive the output from the TCA and to provide the heater current.
14. The feedback circuit of claim 13 , wherein the heat sensor resistor has a temperature coefficient of resistance such that its sensor impedance (R sense ) varies in response to a temperature of the acoustic resonator.
15. The feedback circuit of claim 14 , wherein for a predetermined temperature of the acoustic resonator, a reference voltage (Vref) across the reference resistor is set to be equal to a sensor voltage (Vsense) across the heat sensor resistor at the predetermined temperature.
16. The feedback circuit of claim 15 , wherein the predetermined temperature is substantially equal to a turn-over-temperature (TOT), or a TOT range.
17. The feedback circuit of claim 16 , further comprising a steering digital to analog converter (DAC) configured to determine the reference voltage (Vref).
18. The feedback circuit of claim 13 , wherein the transistor is a pass field effect transistor (pass FET), and the output from the TCA is an error current proportional to a difference between a reference voltage (Vref) and a sensor voltage (Vsens), the pass FET configured to change a magnitude of a current (I heat ) to the heater.