Abstract:
Filter devices and duplexer devices are disclosed. A filter device includes two or more surface acoustic wave resonators, including at least a first shunt resonator, formed on a surface of a substrate. A ground conductor formed on the surface of the substrate connects the first shunt resonator to a ground pad. At least a portion of an edge of the ground conductor is shaped as a plurality of serrations.

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation in part of application Ser. No. 14/495,494, filed Sep. 24, 2014, entitled HIGH REJECTION SURFACE ACOUSTIC WAVE FILTER, now U.S. Pat. No. 9,077,312, which claims priority from Provisional Patent Application No. 62/029,279, filed Jul. 25, 2014, titled HIGH ISOLATION DUPLEXER, both of which are included by reference. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     1. Field 
     This disclosure relates to radio frequency filters using surface acoustic wave (SAW) resonators, and specifically to filters and duplexers incorporating SAW resonators to provide very high rejection or isolation in a predetermined frequency band. 
     2. Description of the Related Art 
     As shown in  FIG. 1 , a SAW resonator  100  may be formed by thin film conductor patterns formed on a surface of a substrate  105  made of a piezoelectric material such as quartz, lithium niobate, lithium tantalate, or lanthanum gallium silicate. A first inter-digital transducer (IDT)  110  may include a plurality of parallel conductors. A radio frequency or microwave signal applied to the first IDT  110  via an input terminal IN may generate an acoustic wave on the surface of the substrate  105 . As shown in  FIG. 1 , the surface acoustic wave will propagate in the left-right direction. A second IDT  120  may convert the acoustic wave back into a radio frequency or microwave signal at an output terminal OUT. The conductors of the second IDT  120  may be interleaved with the conductors of the first IDT  110  as shown. In other SAW resonator configurations (not shown), the conductors forming the second IDT may be disposed on the surface of the substrate  105  adjacent to, or separated from, the conductors forming the first IDT. 
     The electrical coupling between the first IDT  110  and the second IDT  120  may be frequency-dependent. The electrical coupling between the first IDT  110  and the second IDT  120  typically exhibits both a resonance (where the impedance between the first and second IDTs is very high) and an anti-resonance (where the impedance between the first and second IDTs approaches zero). The frequencies of the resonance and the anti-resonance are determined primarily by the pitch and orientation of the interdigitated conductors, the choice of substrate material, and the crystallographic orientation of the substrate material. 
     Grating reflectors  130 ,  132  may be disposed on the substrate to confine most of the energy of the acoustic waves to the area of the substrate occupied by the first and second IDTs  110 ,  120 . However a portion of the energy of the acoustic wave, represented by the dashed arrows  140 , may leak or escape and propagate across the surface of the substrate. An acoustic wave propagating across the surface of the substrate may reflect at the edges of the substrate. Additionally, since the velocity of an acoustic wave is different between regions of the substrate that are and are not covered by conductors, a portion of the energy of an acoustic wave will reflect each time the acoustic wave encounters the edge of a conductor. 
     SAW resonators are used in a variety of radio frequency filters including band reject filters, band pass filters, and duplexers. A duplexer is a radio frequency filter device that allows simultaneous transmission in a first frequency band and reception in a second frequency band (different from the first frequency band) using a common antenna. Duplexers are commonly found in radio communications equipment including cellular telephones. 
     Filter circuits commonly incorporate more than one SAW resonator. For example,  FIG. 2  shows a schematic diagram of a radio frequency filter circuit  200  incorporating nine SAW resonators, labeled Xa through Xi. The use of nine SAW resonators is exemplary and a filter circuit may include more or fewer than nine SAW resonators. The filter circuit  200  may be, for example, a band pass filter, a band reject filter, or a combination band pass/band reject filter depending on the characteristics of the SAW resonators. 
     In the filter circuit  200 , SAW resonators Xa, Xc, Xe, Xg, and Xi may be referred to as “series resonators” since these five SAW resonators are connected in series between the two ports of the filter circuit. SAW resonators Xb, Xd, Xf, Xh may be referred to as “shunt resonators” since these four SAW resonators are each connected from a node between two series resonators and ground (and thus “shunt” some radio frequency energy to ground). While not used in the exemplary circuit of  FIG. 2 , a SAW resonator connected from one of the ports of a filter to ground would also be considered a “shunt resonator”. 
     The nine SAW resonators Xa through Xi are typically fabricated in close proximity on a common substrate. Since the SAW resonators are in close proximity, acoustic energy that leaks from a first resonator may impinge upon one or more other resonators, either directly or after reflection from an edge of the substrate or an edge of a conductor pattern. The one or more other resonators that receive the leaked acoustic energy may convert some or all of the leaked acoustic energy into electrical signals. For example, acoustic energy leaking from SAW resonator Xa may impinge on SAW resonator Xg, as indicated by the dashed arrow  210 , and acoustic energy leaking from SAW resonator Xb may impinge on SAW resonator Xf, as indicated by the dashed arrow  220 . Leaked acoustic energy may effectively provide sneak paths by which RF signals can bypass portions of the filter circuit. 
       FIG. 3  shows a graph  300  comparing the simulated and measured performance of a combination band pass/band reject filter circuit similar to the filter circuit  200  shown in  FIG. 2 . The graph  300  plots |S(1,2)| (the magnitude in dB of the transfer function between port  1  and port  2  of the filter) as a function of frequency. The solid line  310  is the expected filter performance based on electromagnetic modeling of the filter circuit. The dashed line  320  is the measured performance of a prototype filter. The measured transfer function (dashed line) closely approximates the modeled performance (solid line) over the band pass region centered at 1.785 GHz. The measured transfer function (dashed line) deviates substantially (i.e. as much as 18 dB) from the modeled performance (solid line) over the frequency band from 1.805 GHz to 1.85 GHz. The unexpectedly low insertion loss of the actual filter in this frequency band may result, at least in part, from acoustic leakage paths that are not included in the electromagnetic modeling. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a SAW resonator. 
         FIG. 2  is a schematic circuit diagram of a filter incorporating multiple SAW resonators. 
         FIG. 3  is a graph comparing modeled and measured performance of a SAW band pass/band reject filter. 
         FIG. 4  is a schematic plan view of a SAW duplexer filter device. 
         FIG. 5  is a schematic circuit diagram of a duplexer filter device. 
         FIG. 6  is a schematic plan view of a SAW duplexer filter device incorporating serrated ground electrodes. 
         FIG. 7  is a detail view comparing portions of the SAW duplexer filter devices of  FIG. 4  and  FIG. 6 . 
         FIG. 8  is a graph comparing measured performance of the SAW duplexer filter devices of  FIG. 4  and  FIG. 6 . 
         FIG. 9  is a schematic plan view of another SAW duplexer filter device incorporating serrated ground electrodes. 
     
    
    
     Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator. 
     DETAILED DESCRIPTION 
       FIG. 4  is greatly magnified schematic plan view of a duplexer  400  fabricated by depositing thin film conductors on a piezoelectric substrate  410 . Each of the blocks labeled X 1  through X 19  represents a SAW resonator formed by interdigitated conductors too fine to show in the figure. As is known in the art, each SAW resonator X 1 -X 19  may have a resonance at a first frequency and an anti-resonance at a second frequency. The frequencies of the resonance and anti-resonance of each SAW resonator X 1 -X 19  are determined primarily by the pitch and orientation of the interdigitated conductors and the choice of substrate material and its orientation. 
     The nineteen SAW resonators X 1 -X 19  are interconnected by thin film conductors to form the duplexer circuit. The duplexer  400  includes an input pad, an antenna pad, and an output pad (labeled “in”, “ant”, and “out”, respectively) to couple RF signals to/from the duplexer. When the duplexer is incorporated into a device such as a cellular telephone, a transmitter may be connected to the input pad, an antenna may be connected to the antenna pad, and a receiver may be connected to the output pad. The duplexer  400  also includes five ground pads (labeled “G 1 ” through “G 5 ”) to connect the duplexer to an external ground plane. The five ground pads G 1  to G 5  are not formed separately, but are predetermined regions of ground conductors  420 ,  422 ,  424 ,  426 , and  428 . 
       FIG. 5  is a schematic circuit diagram of a duplexer  500  which may be the duplexer  400  shown in  FIG. 4 . The duplexer  500  includes a transmit filter  510  connected between an input pad (“In”) and an antenna pad (“Antenna”) and a receive filter  520  connected between the antenna pad and an output pad (“Out”). The transmit filter  510  includes SAW resonators X 1 -X 9 . SAW resonators X 1 , X 3 , X 5 , X 7 , and X 9  are series resonators and SAW resonators X 2 , X 4 , X 6 , and X 8  are shunt resonators. The receive filter  520  includes SAW resonators X 10 -X 19 . SAW resonators X 10 , X 12 , X 14 , X 16 , and X 18  are series resonators and SAW resonators X 11 , X 13 , X 15 , X 17 , and X 19  are shunt resonators. The input pad, the antenna pad, the output pad, the series resonators, and a first end of each of the shunt resonators are interconnected by signal conductors (such as signal conductors  442 ,  444 , and  446  in  FIG. 4 ). A second end of each of the shunt resonators is connected to one of the ground pads G 1  to G 5  by respective ground conductors ( 420 ,  422 ,  424 ,  426 ,  428  in  FIG. 4 ). Note that ground pads G 1 , G 2 , and G 3  are associated with the transmit filter and ground pads G 4  and G 5  are associated with the receive filter. On the substrate  410  of the duplexer  400 , the ground pads and ground conductors associated with the transmit filter may be electrically isolated from the ground pads and ground conductors associated with the receive filter. 
     The transmit filter  510  may be designed to convey RF signals within a transmit frequency band from the transmitter connected to the input pad of the duplexer to the antenna connected to antenna pad, while blocking RF signals in other frequency bands. The receive filter  520  may be designed to convey RF signals within a receive frequency band from the antenna connected to the antenna pad to a receiver connected to the output pad, while blocking RF signals in other frequency bands. 
     The RF signal from the transmitter introduced at the input pad may be substantially higher power than the receive signals from the antenna introduced at antenna pad. To prevent leakage of the transmit signal into the input of the receiver connected to output pad, the duplexer  400 / 500  may be designed to provide very high isolation between the input pad and the output pad, particular for the receive frequency band, but also for the transmit frequency band. 
       FIG. 6  is greatly magnified schematic plan view of an improved duplexer  600 . The improved duplexer has 19 SAW resonators arranged similarly to those of the duplexer  400  of  FIG. 4 . The electrical schematic diagram of the duplexer  600  is essentially the same as shown in  FIG. 5 . Resonators X 1 -X 9  form a transmit filter and resonators X 10 -X 19  form a receive filter. Resonators X 1 , X 3 , X 5 , X 7 , X 9 , X 10 , X 12 , X 13 , X 4 , X 16 , and X 18  are series resonators. Resonators X 2 , X 4 , X 6 , X 8 , X 11 , X 13 , X 15 , X 17 , and X 19  are shunt resonators. One end of each shunt resonators is connected to one of five grounds pads G 1 -G 5  by respective ground conductors  620 ,  622 ,  624 ,  626 ,  628 . 
     At least some edges of the ground conductors  620 ,  622 ,  624 ,  626 ,  628  are serrated, which is to say at least some edges of the ground conductors are formed into plural teeth or serrations, such as serrations  630 ,  632 ,  634 ,  636 ,  638 . The serrated edges of the ground conductors may face one or more of the SAW resonators. All or portions of the serrated edges of the ground conductors may face signal conductors interconnecting the SAW resonators. For example, some of the serrations on ground conductor  620  face signal conductor  640 , and some of the serrations on ground conductor  622  face signal conductor  642 . Although not present in this example, serrated edges of the ground conductors may face other ground conductors. 
     The serrations along the edges of the ground conductors may be triangular, as shown in  FIG. 6 . Some or all of the serrations may be or include convex curved portions, concave curved portions, or combinations of straight, convex, and/or concave portions. Curved portions may be circular, parabolic, sinusoidal, or some other shape. 
     The multiple serrations along at least some edges of the ground conductors are not necessarily uniform in size or shape. In the case of triangular serrations, the width w, depth d, and internal angle Θ of the serrations may vary. The width w and depth d of the serrations may be large compared to the wavelength of the acoustic waves propagating on the surface of the substrate. For example, the width w and depth d of each serration may be between 10 microns and 100 microns. The internal angle Θ of each serration may be from 45 degrees to 135 degrees. Serrations such as serrations  630 ,  632 ,  634 ,  636 ,  638  may scatter acoustic waves to reduce undesired acoustic coupling between SAW resonators. Triangular serrations having an internal angle Θ near 90 degrees may function to retro-reflect at least a portion of incident acoustic waves. 
     Ground conductors  620 ,  622 ,  624 ,  626 ,  628  extend over much of the usable surface area of the piezoelectric substrate not occupied by SAW resonators and signal conductors. A gutter  615  around the perimeter of the piezoelectric substrate  610  may be required to facilitate excising the duplexer  600  from a larger wafer. The gutter  615  is not considered usable surface area. The ground conductors  620 ,  622 ,  624 ,  626 ,  628 , in aggregate, may cover at least 50% of the usable surface area of the piezoelectric substrate  610  not occupied by SAW resonators, and signal conductors. 
       FIG. 8  shows a graph  800  comparing the |S(1,3)| parameter for the conventional duplexer of  FIG. 4  (dashed curve  810 ) and the improved duplexer of  FIG. 6  (solid curve  820 ). The |S(1,3)| parameter is the magnitude of the coupling between the input pad and the output pad of the duplexer. The improved duplexer of  FIG. 6  provides about 10 dB to 15 dB lower coupling (i.e. 10 dB to 15 dB greater isolation) between port  1  and port  3  over the frequency range from 1.805 to 1.88 GHz. 
       FIG. 9  is greatly magnified schematic plan view of another duplexer  900 . The electrical schematic diagram of the duplexer  900  is similar to that shown in  FIG. 5  except for the arrangement of ground pads and ground conductors. Resonators X 1 -X 9  form a transmit filter connected between an input pad (“In”) and an antenna pad (“Antenna”). Resonators X 10 -X 19  form a receive filter connected from the antenna pad to an output pad (“Out”). Resonators X 1 , X 3 , X 5 , X 7 , X 9 , X 10 , X 12 , X 13 , X 4 , X 16 , and X 18  are series resonators. Resonators X 2 , X 4 , X 6 , X 8 , X 11 , X 13 , X 15 , X 17 , and X 19  are shunt resonators. 
     Ground conductor  920  connects ground pad G 1  to shunt resonators X 2  and X 6 . Ground conductor  920  includes extended portions  920   a ,  920   b , and  920   c  that fill areas of the piezoelectric substrate  910  not occupied by resonators or signal conductors. Extended portions  920   a ,  920   b , and  920   c  are not required to connect ground pad G 1  to resonators X 2  and X 6 , but are specifically intended to reduce undesired coupling between resonators in the duplexer  900 . Ground conductor  922  connects ground pad G 2  to shunt resonators X 4  and X 8 . Ground conductor  922  includes extended portions  922   a  and  922   b  that fill areas of the piezoelectric substrate  910  not occupied by resonators or signal conductors. Extended portions  922   a  and  922   b  are not required to connect ground pad G 2  to resonators X 4  and X 8 . 
     Ground conductor  924  connects ground pad G 3  to shunt resonators X 11  and X 13 . Ground conductor  926  connects ground pad G 4  to shunt resonators X 15  and X 19 . Ground conductor  928  connects ground pad G 5  to shunt resonator X 17 . Each of ground conductors  924 ,  926 , and  928  includes one or more extended portions (shown but not identified) that fill areas of the piezoelectric substrate  910  not occupied by resonators or signal conductors. Note that an edge  932  of ground conductor  922  faces an edge  938  of ground conductor  928 . Both edges  932  and  938  may be serrated as shown. Each of the ground pads G 1 -G 5  is electrically isolated from the other ground pads on the piezoelectric substrate  610 . The ground conductors  920 ,  922 ,  924 ,  926 ,  928 , in aggregate, may cover at least 70% of the usable surface area (i.e. the surface area exclusive of the gutter  915 ) of the piezoelectric substrate  910  not occupied by SAW resonators, and signal conductors. 
     While the previously discussed examples demonstrate the effectiveness of serrated ground conductors to improve isolation in a duplexer, serrated ground conductors may also be employed to reduce undesired acoustic coupling between SAW resonators to increase rejection in band reject filters.