Bandpass filters using transversely-excited film bulk acoustic resonators
Bandpass filters for a target communications band extending between a lower band edge and an upper band edge are disclosed. A bandpass filter includes one or more shunt resonators and one or more series resonators connected in a ladder filter circuit, wherein a relative difference between the anti-resonance and resonance frequencies of each acoustic resonator is greater than a fractional bandwidth of the target communications band. A first capacitor is connected in parallel with a first shunt resonator from the one or more shunt resonators, and a second capacitor connected in parallel with a first series resonator from the one or more series resonators.
1. A bandpass filter for a communication band extending between a lower band edge and an upper band edge, the bandpass filter comprising:
a plurality of transversely excited bulk acoustic resonators (XBARs) including a plurality of shunt resonators and a plurality of series resonators connected in a ladder filter circuit, wherein a relative frequency difference between anti-resonance and resonance frequencies of each transversely excited bulk acoustic resonator is greater than a fractional bandwidth of the communications band;
a first capacitor connected in parallel with a first shunt resonator of the plurality of shunt resonators to form a first shunt subcircuit; and
a second capacitor connected in parallel with a first series resonator of the plurality of series resonators to form a first series subcircuit,
wherein the relative frequency difference of each of the XBARs is greater than or equal to 15%, the relative frequency difference being a difference between the resonance and the anti-resonance frequencies divided by an average of the resonance and the anti-resonance frequencies,
wherein at least one of the first capacitor and the second capacitor is a metal-insulator-metal (MIM) capacitor,
wherein a first conductor layer includes a portion forming interleaved fingers of an interdigital capacitor of at least one of the first and second capacitors,
wherein a pitch between two adjacent fingers of the interleaved fingers is greater than 0.05 times a width of at least one finger of the interleaved fingers and less than 0.5 times the width of the at least one finger, and
wherein the MIM capacitor is formed by a portion of a second conductor layer that overlaps a portion of the first conductor layer with an intervening dielectric layer therebetween.
2. The bandpass filter of claim 1 ,
wherein the first capacitor has a capacitance value that is between 1% and 50% of a static capacitance of the first shunt resonator, and the second capacitor has a capacitance value that is between 1% and 50% of a static capacitance of the first series resonator.
3. The bandpass filter of claim 1 , wherein:
a resonance frequency of the first shunt resonator is less than and proximate the lower band edge, and
a capacitance of the first capacitor is set such that an anti-resonance frequency of the parallel combination of the first capacitor and the first shunt resonator is within the target communications band.
4. The bandpass filter of claim 1 , further comprising:
one or more additional capacitors connected in parallel with respective resonators from the plurality of acoustic resonators.
5. The bandpass filter of claim 1 , wherein all of the plurality of XBARs include lithium niobate piezoelectric plates with Euler angles 0°, β, 0° where 28°≤β≤38°.
6. The bandpass filter of claim 5 , wherein
a fractional bandwidth of the communications band is less than or equal to 13%, the fractional bandwidth being a difference between a frequency of the upper band edge of the bandpass filter and a frequency of the lower band edge of the bandpass filter divided by a center frequency of the bandpass filter.
7. The bandpass filter of claim 1 , wherein the second shunt subcircuit has a thickness that is greater than a thickness of the first shunt subcircuit.
8. A bandpass filter for a communication band extending between a lower band edge and an upper band edge, the bandpass filter comprising:
a plurality of transversely excited bulk acoustic resonators (XBARs) including a plurality of shunt resonators and a plurality of series resonators connected in a ladder filter circuit;
a first capacitor connected in parallel with a first shunt resonator of the plurality of shunt resonators to form a first shunt subcircuit; and
a second capacitor connected in parallel with a first series resonator of the plurality of series resonators to form a first series subcircuit,
wherein a relative frequency difference of each of the XBARs is greater than or equal to 15%, the relative frequency difference being a difference between a resonance frequency and an anti-resonance frequency of the respective XBAR divided by an average of the resonance and the anti-resonance frequencies,
wherein a first conductor layer includes a portion of at least one of the first and second capacitors,
wherein at least one of the first capacitor and the second capacitor is a metal-insulator-metal (MIM) capacitor that is formed by a portion of a second conductor layer that overlaps a portion of the first conductor layer with an intervening dielectric layer therebetween, and
wherein each of the XBARs includes a plurality of interleaved fingers and a pitch between two adjacent fingers of the plurality of interleaved fingers is greater than 0.05 times a width of at least one finger of the plurality of interleaved fingers and less than 0.5 times the width of the at least one finger.
9. The bandpass filter of claim 8 , wherein the relative frequency difference between the anti-resonance and resonance frequencies of each transversely excited bulk acoustic resonator is greater than a fractional bandwidth of the communications band.
10. The bandpass filter of claim 8 , wherein the portion of the first conductor forms interleaved fingers of an interdigital capacitor of the at least one of the first and second capacitors.
11. The bandpass filter of claim 8 , wherein the first capacitor has a capacitance value that is between 1% and 50% of a static capacitance of the first shunt resonator, and the second capacitor has a capacitance value that is between 1% and 50% of a static capacitance of the first series resonator.
12. The bandpass filter of claim 8 , wherein:
a resonance frequency of the first shunt resonator is less than and proximate the lower band edge, and
a capacitance of the first capacitor is set such that an anti-resonance frequency of the parallel combination of the first capacitor and the first shunt resonator is within the target communications band.
13. The bandpass filter of claim 8 , further comprising one or more additional capacitors connected in parallel with respective resonators from the plurality of acoustic resonators.
14. The bandpass filter of claim 8 , wherein all of the plurality of XBARs include lithium niobate piezoelectric plates with Euler angles 0°, β, 0° where 28°≤β≤38°.
15. The bandpass filter of claim 8 , wherein:
a fractional bandwidth of the communications band is less than or equal to 13%, the fractional bandwidth being a difference between a frequency of the upper band edge of the bandpass filter and a frequency of the lower band edge of the bandpass filter divided by a center frequency of the bandpass filter.