Patent Publication Number: US-2009231060-A1

Title: Ladder resonator filter and related system

Description:
TECHNICAL FIELD 
     This disclosure is generally directed to filters and more specifically to a ladder resonator filter and related system. 
     BACKGROUND 
     Surface acoustic wave (SAW) ladder resonator filters are routinely used to filter signals. The SAW ladder resonator filters often include multiple SAW resonators that are arranged to provide desired filtering functionality. A typical SAW ladder resonator filter includes five to ten resonators that are coupled together. One problem with conventional SAW ladder resonator filters is that each of the resonators is placed in its own acoustic track on a substrate. This is done to help reduce or prevent interactions between the resonators. However, this increases the size of the SAW ladder resonator filters and the associated costs of the SAW ladder resonator filters. Also, this type of layout increases the length and complexity of interconnections between the resonators in the SAW ladder resonator filters. 
     SUMMARY 
     This disclosure provides a ladder resonator filter and related system. 
     In a first embodiment, an apparatus includes a plurality of resonators forming a filter. At least one of the resonators includes (i) a plurality of interdigital transducers positioned in a common acoustic track and (ii) a plurality of reflectors configured to reflect acoustic waves from the interdigital transducers back to the interdigital transducers. At least one reflector is positioned between adjacent interdigital transducers. 
     In particular embodiments, the interdigital transducers in one of the resonators are coupled in parallel or in series between an input and an output of that resonator. 
     In other particular embodiments, the interdigital transducers in a first of the resonators are coupled in parallel between an input of the first resonator and ground. The interdigital transducers in a second of the resonators are coupled in series between the input of the first resonator and an output of the second resonator. The interdigital transducers in a third of the resonators are coupled in parallel between the output of the second resonator and ground. Each of the first, second, and third resonators could include two interdigital transducers and three reflectors. 
     In yet other particular embodiments, each of the interdigital transducers includes multiple sets of electrodes. At least some of the reflectors are electrically coupled to at least some of the sets of electrodes in the interdigital transducers. At least some of the reflectors are electrically coupled together. 
     In still other particular embodiments, the interdigital transducers in a first of the resonators are coupled in series between an input of the first resonator and an output of the first resonator. The interdigital transducers in a second of the resonators are coupled in parallel between the output of the first resonator and ground. The interdigital transducers in a third of the resonators are coupled in series between the output of the first resonator and an output of the third resonator. Each of the first, second, and third resonators could include two interdigital transducers and three reflectors. 
     In additional particular embodiments, the plurality of resonators forming the filter include resonators forming at least two Π sections or at least two T sections in the filter. 
     In a second embodiment, a system includes a signal source configured to provide a signal and a filter configured to filter the signal. The filter includes a plurality of resonators. At least one of the resonators includes (i) a plurality of interdigital transducers positioned in a common acoustic track and (ii) a plurality of reflectors configured to reflect acoustic waves from the interdigital transducers back to the interdigital transducers. At least one reflector is positioned between adjacent interdigital transducers. 
     In particular embodiments, the system further includes a signal processor configured to process a filtered signal provided by the filter. 
     In other particular embodiments, the signal source includes an antenna. 
     In a third embodiment, a resonator includes a plurality of interdigital transducers positioned in a common acoustic track. The resonator also includes a plurality of reflectors configured to reflect acoustic waves from the interdigital transducers back to the interdigital transducers. At least one reflector is positioned between adjacent interdigital transducers. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  illustrate example ladder resonator filters according to this disclosure; 
         FIGS. 2 and 3  illustrate example resonators within the ladder resonator filters of  FIGS. 1A and 1B  according to this disclosure; 
         FIGS. 4 and 5  illustrate example operation of an interdigital transducer within the ladder resonator filters of  FIGS. 1A and 1B  according to this disclosure; 
         FIGS. 6 and 7  illustrate additional details of the ladder resonator filter of  FIG. 1A  according to this disclosure; 
         FIG. 8  illustrates additional details of the ladder resonator filter of  FIG. 1B  according to this disclosure; and 
         FIG. 9  illustrates an example system using a ladder resonator filter according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 9 , discussed below, and the various embodiments used to scribe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system. 
       FIGS. 1A and 1B  illustrate example ladder resonator filters according to this disclosure. The embodiments of the ladder resonator filters shown in  FIGS. 1A and 1B  are for illustration only. Other embodiments of the ladder resonator filters could be used without departing from the scope of this disclosure. 
     As shown in  FIG. 1A , a ladder resonator filter  100  receives an input signal IN and produces a filtered output signal OUT. The ladder resonator filter  100  in this example includes five resonators  102 - 110 . The resonators  102 - 104  are coupled in series with each other, and the resonators  106 - 110  are coupled in parallel with each other. In this example, the ladder resonator filter  100  is said to include two “Π” sections, where “Π” denotes the shape of the two sections. One “Π” section includes the resonators  106 ,  102 , and  108 . The other “Π” section includes the resonators  108 ,  104 , and  110 . 
     As shown in  FIG. 1B , a ladder resonator filter  150  is similar in structure to the ladder resonator filter  100 . The ladder resonator filter  150  in this example includes five resonators  152 - 160 . The resonators  152 - 156  are coupled in series with each other, and the resonators  158 - 160  are coupled in parallel with each other. In this example, the ladder resonator filter  150  is said to include two “T” sections. One “T” section includes the resonators  152 ,  154 , and  158 . The other “T” section includes the resonators  154 ,  156 , and  160 . 
     While each of the resonators  102 - 110  appears as a single unit in  FIG. 1A , each of these resonators  102 - 110  could represent a collection of resonators. Also, as described in more detail below, the resonators  102 - 110  in the ladder resonator filter  100  could be formed in the same acoustic track of a substrate. Similarly, while each of the resonators  152 - 160  appears as a single unit in  FIG. 1B , each of these resonators  152 - 160  could represent a collection of resonators. Moreover, as described in more detail below, the resonators  152 - 160  in the ladder resonator filter  150  could be formed in the same acoustic track of a substrate. Among other things, this may allow the ladder resonator filters  100  and  150  to occupy less physical space, resulting in reduced area and lower associated manufacturing costs. This may also help to simplify the connections between resonators in the ladder resonator filters  100  and  150 . 
     Although  FIGS. 1A and 1B  illustrate examples of ladder resonator filters  100  and  150 , various changes may be made to  FIGS. 1A and 1B . For example, each of the ladder resonator filters  100  and  150  could include any suitable number of sections, and filters having non-uniform or no sections could be used. Also, each of the sections in the ladder resonator filters  100  and  150  could include any suitable number and arrangement of resonators. 
       FIGS. 2 and 3  illustrate example resonators within the ladder resonator filters of  FIGS. 1A and 1B  according to this disclosure. The embodiments of the resonators shown in  FIGS. 2 and 3  are for illustration only. Other embodiments of the resonators could be used without departing from the scope of this disclosure. 
     As shown in  FIG. 2 , a resonator  200  includes multiple interdigital transducers (IDTs)  202   a - 202   m  and multiple reflectors  204   a - 204   n.  Each of the interdigital transducers  202   a - 202   m  typically includes multiple sets of conductive electrodes that are interleaved. Each of the interdigital transducers  202   a - 202   m  includes any suitable structure having multiple sets of interleaved conductive electrodes. 
     The interdigital transducers  202   a - 202   m  are located between pairs of the reflectors  204   a - 204   n.  For example, the interdigital transducer  202   a  is located between the reflectors  204   a - 204   b.  In other words, adjacent interdigital transducers are separated by at least one of the reflectors. Each of the reflectors  204   a - 204   n  includes any suitable structure for reflecting acoustic waves, such as a non-interleaved set of electrodes. 
     In this example, the interdigital transducers  202   a - 202   m  are generally coupled in parallel with one another between a resonator input IN R  and a resonator output OUT R . In other words, the interdigital transducers  202   a - 202   m  all have “inputs” that are coupled to the resonator input IN R  and “outputs” that are coupled to the resonator output OUT R . During operation of the resonator  200 , each of the interdigital transducers  202   a - 202   m  generally produces acoustic waves that propagate in the resonator  200 . The reflectors  204   a - 204   n  generally operate to reflect waves from an interdigital transducer back to that interdigital transducer. In this way, multiple interdigital transducers  202   a - 202   m  can be located in the same acoustic track without unduly interfering with each other. 
     As shown in  FIG. 3 , a resonator  300  includes multiple interdigital transducers  302   a - 302   m  and multiple reflectors  304   a - 304   n.  Each of the interdigital transducers  302   a - 302   m  is located between a pair of the reflectors  304   a - 304   n.  Each of the interdigital transducers  302   a - 302   m  includes any suitable structure having multiple sets of interleaved conductive electrodes. Each of the reflectors  304   a - 304   n  includes any suitable structure for reflecting acoustic waves, such as a non-interleaved set of electrodes. 
     In this example, the interdigital transducers  302   a - 302   m  are generally coupled in series with one another between a resonator input IN R  and a resonator output OUT R . In other words, the interdigital transducer  302   a  has an “input” that is coupled to the resonator input IN R , and each remaining interdigital transducer  302   b - 302   m  has an “input” that is coupled to an “output” of the prior interdigital transducer. During operation of the resonator  300 , each of the interdigital transducers  302   a - 302   m  generally produces acoustic waves that propagate in the resonator  300 , and the reflectors  304   a - 304   n  generally operate to reflect waves from an interdigital transducer back to that interdigital transducer. In this way, multiple interdigital transducers  302   a - 302   m  can be located in the same acoustic track without unduly interfering with each other. 
     In some embodiments, the resonator  200  of  FIG. 2  could be used as the resonators  106 - 110  in the ladder resonator filter  100  of  FIG. 1A  and as the resonators  158 - 160  in the ladder resonator filter  150  of  FIG. 1B . Also, the resonator  300  of  FIG. 3  could be used as the resonators  102 - 104  in the ladder resonator filter  100  of  FIG. 1A  and as the resonators  152 - 156  in the ladder resonator filter  150  of  FIG. 1B . 
     An example operation of the interdigital transducers and reflectors in the ladder resonator filters  100  and  150  is illustrated in  FIGS. 4 and 5 . In  FIG. 4 , an interdigital transducer  402  is formed from two sets  404 - 406  of interleaved conductive electrodes. Two reflectors  408 - 410  are positioned on opposite sides of the resonator  402 . The reflectors  408 - 410  include conductive electrodes that are generally parallel to the sets  404 - 406  of interleaved conductive electrodes in the interdigital transducer  402 . By applying a voltage across the interdigital transducer  402 , the interdigital transducer  402  generates acoustic waves that propagate substantially normal to the sets  404 - 406  of electrodes. The reflectors  408 - 410  reflect those waves back to the interdigital transducer  402 . As shown in  FIG. 5 , when the waves being reflected and the waves being generated by the interdigital transducer  402  are in phase, a standing wave is created at a resonance frequency. Away from its resonance frequency, the interdigital transducer  402  acts like a static capacitor, while a large current is provided at the resonance frequency. 
     In general, the reflectors  204   a - 204   n  and  304   a - 304   n  have a reflection band where the reflection coefficient is almost one. A stop band of the interdigital transducers  202   a - 202   m  and  302   a - 302   m  is the frequency range where the transmission coefficient is almost zero and waves can propagate. In the resonators  200  and  300 , the reflection coefficient of the reflectors is small outside of the reflection band, while the reflection coefficient of the reflectors is strong within the stop band of the interdigital transducers. Outside of the reflection band, this allows the resonator  200  or  300  to work as a resonator with long ID T . Inside of the reflection band, this allows the resonator  200  or  300  to work as several separated short resonators. This could represent optimal operations both inside and outside of the reflection band. 
     In particular embodiments, the ladder resonator filters  100  and  150  are formed on a lithium tantalite (LiTaO 3 ) substrate. Each interdigital transducer in the resonators  102 - 110  and  152 - 160  (implemented as shown in  FIGS. 2 and 3 ) may include between  150 - 250  electrodes, and each reflector in the resonators  102 - 110  and  152 - 160  may include between 30-70 electrodes. Further, the interdigital transducers and the reflectors may have identical periods for the electrodes, and there may be no shift between the electrodes in the interdigital transducers and the reflectors. In addition, the aperture (the space between electrodes in the interdigital transducers and the reflectors) could have a size of between 10λ-15λ, where λ represents the center wavelength of the signal being filtered. 
     Although  FIGS. 2 and 3  illustrate examples of resonators within the ladder resonator filters of  FIGS. 1A and 1B , various changes may be made to  FIGS. 2 and 3 . For example, the resonator  200  could include any suitable number of interdigital transducers coupled in parallel and separated by reflectors, including two or more interdigital transducers. Similarly, the resonator  300  could include any suitable number of interdigital transducers coupled in series and separated by reflectors, including two or more interdigital transducers. 
       FIGS. 6 and 7  illustrate additional details of the ladder resonator filter  100  of  FIG. 1A  according to this disclosure. In particular,  FIGS. 6 and 7  illustrate one example implementation of a single “Π” section of the ladder resonator filter  100  of  FIG. 1A . The additional details shown in  FIGS. 6 and 7  are for illustration only. The ladder resonator filter  100  of  FIG. 1A  could be implemented in any other suitable manner without departing from the scope of this disclosure. 
     As shown in  FIG. 6 , the resonator  106  in  FIG. 1A  is implemented using the same technique shown in  FIG. 2 . Here, two interdigital transducers  602   a - 602   b  are isolated by three reflectors  604   a - 604   c.  The interdigital transducers  602   a - 602   b  are coupled in parallel between an input IN Π  of the section and ground. The reflectors  604   a - 604   c  operate to generally reflect waves from each interdigital transducer  602   a - 602   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  602   a - 602   b.    
     The resonator  102  in  FIG. 1A  is implemented using the same technique shown in  FIG. 3 . Here, two interdigital transducers  606   a - 606   b  are isolated by three reflectors  608   a - 608   c.  The interdigital transducers  606   a - 606   b  are coupled in series between an output of the resonator  106  and an input of the resonator  108 . The reflectors  608   a - 608   c  operate to generally reflect waves from each interdigital transducer  606   a - 606   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  606   a - 606   b.    
     The resonator  108  in  FIG. 1A  is implemented using the same technique shown in  FIG. 2 . Here, two interdigital transducers  610   a - 610   b  are isolated by three reflectors  612   a - 612   c.  The interdigital transducers  610   a - 610   b  are coupled in parallel between an output of the resonator  102  and an output OUT N  of the section. The reflectors  612   a - 612   c  operate to generally reflect waves from each interdigital transducer  610   a - 610   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  610   a - 610   b.    
     One example layout of the “Π” section of the ladder resonator filter  100  from  FIG. 6  is shown in  FIG. 7 . It may be noted that various ones of the reflectors  604   a - 604   c,    608   a - 608   c,    612   a - 612   c  are electrically connected to portions of the interdigital transducers  602   a - 602   b,    606   a - 606   b,    610   a - 610   b.  For example, the reflector  604   a  is electrically connected to one of the sets of electrodes in the interdigital transducer  602   a,  and the reflector  604   c  is electrically connected to one of the sets of electrodes in the interdigital transducer  602   b.  The reflector  604   b  is electrically connected to both (i) another of the sets of electrodes in the interdigital transducer  602   a  and (ii) another of the sets of electrodes in the interdigital transducer  602   b.  Moreover, various ones of the reflectors  604   a - 604   c,    608   a - 608   c,    612   a - 612   c  are electrically connected to each other. For example, the reflector  604   b  is electrically connected to the reflector  608   c,  and the reflector  608   a  is electrically connected to the reflector  612   b.    
     As noted above,  FIGS. 6 and 7  illustrate one example implementation of a single “Π” section of the ladder resonator filter  100  of  FIG. 1A . The same or similar implementation could be used to form one or more additional “Π” sections of the ladder resonator filter  100  of  FIG. 1A  (to thereby form a complete filter). 
     Implementing one, some, or all of the “Π” sections of the ladder resonator filter  100  in this way allows the resonators within the ladder resonator filter  100  to be located in a single acoustic track. This may help the ladder resonator filter  100  to have a smaller size. This may also help to provide simpler and shorter interconnections between resonators. 
     Although  FIGS. 6 and 7  illustrate additional details of one example implementation of the ladder resonator filter  100  of  FIG. 1A , various changes may be made to  FIGS. 6 and 7 . For example, each of the resonators  102 ,  106 , and  108  in  FIG. 6  could include any suitable number of interdigital transducers (such as more than two). 
       FIG. 8  illustrates additional details of the ladder resonator filter  150  of  FIG. 1B  according to this disclosure. In particular,  FIG. 8  illustrates one example implementation of a single “T” section of the ladder resonator filter  150  of  FIG. 1B . The additional details shown in  FIG. 8  are for illustration only. The ladder resonator filter  150  of  FIG. 1B  could be implemented in any other suitable manner without departing from the scope of this disclosure. 
     As shown in  FIG. 8 , the resonator  152  in  FIG. 1B  is implemented using the same technique shown in  FIG. 3 . Here, two interdigital transducers  802   a - 802   b  are isolated by three reflectors  804   a - 804   c.  The interdigital transducers  802   a - 802   b  are coupled in series between an input IN T  of the section and an input of the resonator  158 . The reflectors  804   a - 804   c  operate to generally reflect waves from each interdigital transducer  802   a - 802   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  802   a - 802   b.    
     The resonator  158  in  FIG. 1B  is implemented using the same technique shown in  FIG. 2 . Here, two interdigital transducers  806   a - 806   b  are isolated by three reflectors  808   a - 808   c.  The interdigital transducers  806   a - 806   b  are coupled in parallel between an output of the resonator  152  and ground. The reflectors  808   a - 808   c  operate to generally reflect waves from each interdigital transducer  806   a - 806   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  806   a - 806   b.    
     The resonator  154  in  FIG. 1B  is implemented using the same technique shown in  FIG. 3 . Here, two interdigital transducers  810   a - 810   b  are isolated by three reflectors  812   a - 812   c.  The interdigital transducers  810   a - 810   b  are coupled in series between an output of the resonator  158  and an output OUT T  of the section. The reflectors  812   a - 812   c  operate to generally reflect waves from each interdigital transducer  810   a - 810   b  back to that interdigital transducer, which helps to isolate the interdigital transducers  810   a - 810   b.    
     An example layout of the “T” section could be similar to that shown in  FIG. 7 , with modifications to support the different structure of the resonators  152 ,  154 ,  158  and to support the appropriate connections between the resonators  152 ,  154 ,  158 . Also, as noted above,  FIG. 8  illustrate one example implementation of a single “T” section of the ladder resonator filter  150  of  FIG. 1B . The same or similar implementation could be used to form one or more additional “T” sections of the ladder resonator filter  150  of  FIG. 1B  (to thereby form a complete filter  150 ). 
     Implementing one, some, or all of the “T” sections of the ladder resonator filter  150  in this way allows the resonators within the ladder resonator filter  150  to be located in a single acoustic track. This may help the ladder resonator filter  150  to have a smaller size and to provide simpler and shorter interconnections between resonators. 
     Although  FIG. 8  illustrates additional details of one example implementation of the ladder resonator filter  150  of  FIG. 1B , various changes may be made to  FIG. 8 . For example, each of the resonators  152 ,  154 , and  158  in  FIG. 8  could include any suitable number of interdigital transducers (such as more than two). 
       FIG. 9  illustrates an example system  900  using a ladder resonator filter according to this disclosure. The embodiment of the system  900  shown in  FIG. 9  is for illustration only. Other embodiments of the system  900  could be used without departing from the scope of this disclosure. 
     In  FIG. 9 , a signal source  902  provides a signal to a ladder resonator filter  904 , which filters the signal for processing by a signal processor  906 . The signal source  902  represents any suitable source of a signal that is filtered by the ladder resonator filter  904 . The signal source  902  could, for example, represent a component that generates the signal being filtered by the ladder resonator filter  904 . In this case, the signal source  902  could represent any suitable structure that generates a signal. The signal source  902  could also represent a component that receives from an external source the signal being filtered by the ladder resonator filter  904 . In this case, the signal source  902  could represent any other suitable structure that receives a signal, such as an antenna. 
     The ladder resonator filter  904  represents a filter for filtering the signal from the signal source  902 . The ladder resonator filter  904  could represent any suitable filter that includes multiple resonators coupled in series and/or in parallel, where the resonators are separated by reflectors and can be placed in a single acoustic track of a substrate. This includes the ladder resonator filters  100  and  150  described above. 
     The signal processor  906  processes the filtered signal from the ladder resonator filter  904 . The signal processor  906  represents any suitable component that can further process the signal filtered by the ladder resonator filter  904 . The signal processor  906  could, for example, represent a digital processing component, such as a microprocessor, microcontroller, or digital signal processor (DSP). In this case, an analog-to-digital converter could be used to digitize the filtered signal from the filter  904 . The signal processor  906  could also represent analog processing components, such as mixers or amplifiers. The signal processor  906  may generally include any component(s) for processing the signal from the ladder resonator filter  904 . The actual makeup and arrangement of the signal processor  906  depends, among other things, on the specific application for the system  900 . 
     Although  FIG. 9  illustrates one example of a system  900  using a ladder resonator filter, various changes may be made to  FIG. 9 . For example, the ladder resonator filters  100  and  150  described above could be used in any other suitable system. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “transmit,” “receive,” and “communicate,” as well as derivatives thereof, encompass both direct and indirect communication. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.