Attenuation of spurious responses in electromechanical filters
View Patent ↗A spur cancelling, electromechanical filter includes a first resonator having a first resonant frequency and one or more first spurious responses, and it also includes, electrically connected to the first resonator, a second resonator having a second resonant frequency and one or more second spurious responses. The first and second resonant frequencies are approximately identical, but the first resonator is physically non-identical to the second resonator. The difference between the resonators makes the respective spurious responses different. This allows for filters constructed from a cascade of these resonators to exhibit reduced spurious responses.
1. A multiple-pole electromechanical filter having a center frequency F, comprising:
a first electroacoustic resonator configured to have a first resonant frequency substantially equal to F and configured to vibrate in a first mode shape in response to excitation at the first resonant frequency; and
electrically connected to the first electroacoustic resonator, a second electroacoustic resonator configured to have a second resonant frequency substantially equal to F and configured to vibrate in a second mode shape in response to excitation at the second resonant frequency;
wherein:
the first electroacoustic resonator at least partially constitutes a first pole of the multiple-pole electromechanical filter;
the second electroacoustic resonator at least partially constitutes a second pole of the multiple-pole electromechanical filter;
the first and second electroacoustic resonators have different geometrical configurations;
the first mode shape is different from the second mode shape;
each said electroacoustic resonator comprises one sub-resonator or two or more electrically connected sub-resonators;
the sub-resonators that constitute each electroacoustic resonator are connected to each other in parallel; and
the first and second electroacoustic resonators are connected to each other in series.
2. A multiple-pole electromechanical filter having a center frequency F, comprising:
a first electroacoustic resonator configured to have a first resonant frequency substantially equal to F and configured to vibrate in a first mode shape in response to excitation at the first resonant frequency; and
electrically connected to the first electroacoustic resonator, a second electroacoustic resonator configured to have a second resonant frequency substantially equal to F and configured to vibrate in a second mode shape in response to excitation at the second resonant frequency;
wherein:
the first electroacoustic resonator at least partially constitutes a first pole of the multiple-pole electromechanical filter;
the second electroacoustic resonator at least partially constitutes a second pole of the multiple-pole electromechanical filter;
the first and second electroacoustic resonators have different geometrical configurations;
the first mode shape is different from the second mode shape;
each said electroacoustic resonator comprises one sub-resonator or two or more electrically connected sub-resonators;
the sub-resonators that constitute each electroacoustic resonator are connected to each other in series; and
the first and second electroacoustic resonators are connected to each other in parallel.
3. A multiple-pole electromechanical filter having a center frequency F, comprising:
a first electroacoustic resonator configured to have a first resonant frequency substantially equal to F and configured to vibrate in a first mode shape in response to excitation at the first resonant frequency; and
electrically connected to the first electroacoustic resonator, a second electroacoustic resonator configured to have a second resonant frequency substantially equal to F and configured to vibrate in a second mode shape in response to excitation at the second resonant frequency;
wherein:
the first electroacoustic resonator at least partially constitutes a first pole of the multiple-pole electromechanical filter;
the second electroacoustic resonator at least partially constitutes a second pole of the multiple-pole electromechanical filter;
the first and second electroacoustic resonators have different geometrical configurations;
the first mode shape is different from the second mode shape;
each said electroacoustic resonator comprises one sub-resonator or two or more electrically connected sub-resonators;
each sub-resonator comprised by the first electroacoustic resonator includes an integer number n 1 of one or more electrode fingers; and
each sub-resonator comprised by the second electroacoustic resonator includes an integer number n 2 of one or more electrode fingers; wherein n 1 is not equal to n 2 .
4. The multiple-pole electromechanical filter of claim 3 , wherein:
the first electroacoustic resonator comprises an integer number m 1 of sub-resonators, each of which has n 1 electrode fingers;
the second electroacoustic resonator comprises an integer number m 2 of microresonators, each of which has n 2 electrode fingers; and
the product m 1 ×n 1 is about the same as the product m 2 ×n 2 .
5. The multiple-pole electromechanical filter of claim 4 , wherein the first electroacoustic resonator comprises sub-resonators each having three electrode fingers, and the second electroacoustic resonator comprises sub-resonators each having four electrode fingers.
6. The multiple-pole electromechanical filter of claim 4 , comprising at least one further electroacoustic resonator that comprises a plurality of sub-resonators having a total number of electrode fingers that is about the same as m 1 ×n 1 and about the same as m 2 ×n 2 .
7. The multiple-pole electromechanical filter of claim 4 , comprising at least two electroacoustic resonators that comprise pluralities of three-electrode-finger sub-resonators and at least two electroacoustic resonators that comprise pluralities of four-electrode-finger sub-resonators.
8. The multiple-pole electromechanical filter of claim 7 , wherein the sub-resonators that constitute each said plurality of sub-resonators are connected to each other in parallel, and wherein the electroacoustic resonators are connected to each other in series.
9. The multiple-pole electromechanical filter of claim 3 , wherein n 1 is at least two, n 2 is at least two, and the electrode fingers are interdigitated electrode fingers that alternate between input-signal-carrying fingers and output-signal-carrying fingers.
10. The multiple-pole electromechanical filter of claim 3 , configured as a parallel lattice filter.
11. The multiple-pole electromechanical filter of claim 3 , configured as a ladder filter.
12. The multiple-pole electromechanical filter of claim 3 , wherein the sub-resonators are conformed as contour-mode resonators.
13. The multiple-pole electromechanical filter of claim 3 , wherein the center frequency is in the range 400 MHz to 600 MHz.
14. The multiple-pole electromechanical filter of claim 3 , wherein the sub-resonators are all 5-100 μm wide.
15. The multiple-pole electromechanical filter of claim 3 , wherein a spurious response rejection greater than about 50 dBc is achieved over a bandwidth of 50 MHz to 2 GHz.
16. The multiple-pole electromechanical filter of claim 3 , wherein the sub-resonators each comprise an aluminum nitride piezoelectric layer.
17. The multiple-pole electromechanical filter of claim 16 , wherein the sub-resonators each comprise a temperature compensation layer that underlies the aluminum nitride layer.