Bandpass filter with improved upper band edge sharpness
Bandpass filters are disclosed. A bandpass filter includes a plurality of series resonators and a plurality of shunt resonators. The series resonators are connected in series between a first port and a second port. The plurality of series resonators includes a first series resonator and a second series resonator connected at a node. The plurality of shunt resonators includes a first shunt resonator coupled between the node and a ground. A capacitor bridges the first and second series resonators.
1. A bandpass filter comprising:
a plurality of series resonators connected in series between a first port and a second port, the plurality of series resonators including a first series resonator and a second series resonator connected at a node;
a plurality of shunt resonators including a first shunt resonator coupled between the node and a ground;
a first capacitor bridging the first and second series resonators; and
a second capacitor connected in series with the first shunt resonator between the node and the ground, wherein a series combination of the first shunt resonator and the second capacitor has a resonance frequency greater than an upper limit of a passband of the passband filter.
2. The bandpass filter of claim 1 , wherein the first and second series resonators are electrically identical.
3. The bandpass filter of claim 1 , wherein a resonance frequency of the first shunt resonator is greater than a resonance frequency of the first and second series resonators.
4. The bandpass filter of claim 3 , wherein the resonance frequency of the first shunt resonator is proximate to the upper limit of the passband.
5. The bandpass filter of claim 1 , wherein
the plurality of series resonators includes at least a third series resonator, and
the plurality of shunt resonators includes at least three shunt resonators.
6. A bandpass filter comprising:
a plurality of series resonators connected in series between a first port and a second port, the plurality of series resonators including a first series resonator and a second series resonator connected at a node;
a first capacitor bridging the first and second series resonators; and
a series combination of a first shunt resonator and second capacitor coupled between the node and a ground, wherein
the series combination of the first shunt resonator and the second capacitor has a resonance frequency greater than an upper limit of a passband of the bandpass filter.
7. The bandpass filter of claim 6 , wherein the plurality of series resonators includes at least a third series resonator.
8. The bandpass filter of claim 6 , further comprising:
at least a second shunt resonator.
9. The bandpass filter of claim 6 , wherein the first and second series resonators are electrically identical.
10. The bandpass filter of claim 6 , wherein the resonance frequency of the first shunt resonator is proximate to the upper limit of the passband.
11. A bandpass filter comprising:
a plurality of series resonators connected in series between a first port and a second port, the plurality of series resonators including a first series resonator connected between a first node and a second node and a second series resonator connected between the second node and a third node;
a first shunt resonator coupled between the second node and a ground; and
a capacitor connected between the first node and the third node,
wherein a resonance frequency of the first shunt resonator is greater than a resonance frequency of the first and second series resonators.
12. The bandpass filter of claim 11 , wherein the first and second series resonators are electrically identical.
13. The bandpass filter of claim 12 , wherein a capacitance of the capacitor is configured such that a parallel combination of a series combination of the first series resonator and the second series resonator in parallel with the capacitor has an antiresonance frequency lower than an upper limit of a passband of the bandpass filter.
14. The bandpass filter of claim 13 , wherein the capacitance of the capacitor is configured such that the parallel combination significantly sharpens a transition from high transmission to low transmission for frequencies at the upper limit of the passband.
15. The bandpass filter of claim 11 , wherein a resonance frequency of the first shunt resonator is greater than an upper limit of a passband of the bandpass filter.
16. The bandpass filter of claim 15 , wherein the resonance frequency of the first shunt resonator is proximate to the upper limit of the passband.
17. The bandpass filter of claim 11 , wherein a resonance frequency of the first shunt resonator, in isolation, is lower than an upper limit of a passband of the bandpass filter.
18. The bandpass filter of claim 11 , wherein the plurality of series resonators includes at least a third series resonator.
19. The bandpass filter of claim 18 , further comprising:
at least a second shunt resonator.