Abstract:
In one embodiment, a cascaded monolithic crystal filter is provided. A first filter includes two resonators having a pair of electrodes with the monolithic crystal between. At least one electrode has a periphery which includes a feature capable of shifting a frequency associated with an anharmonic mode in the filter. The filter has a second resonator acoustically coupled to the first resonator. A second filter is cascaded with the first filter. The second filter includes a pair of acoustically coupled resonators.

Description:
BACKGROUND 
   MCF (monolithic crystal filter) based on quartz technology operating in the fundamental shear mode have been used over the past half century for radio communication applications. The center frequency of these filter are limited to about 10-250 MHz, due to the fabrication difficulty. In the last decade, the rapid progress in the wireless communication has created a strong demand for high performance Gigahertz filters with small dimension and low power consumption. 
   Currently, the commercially available monolithic crystal filters are usually limited to center frequency of about 250 MHz due to the fabrication difficulties. These relatively low frequency filters showed a rather high insertion loss of 6-7 dB when good out-of-band rejection of about 60 dB is required. At high frequency, the difference in the strength of the first two undesirable anharmonic modes observed in a resonator are usually less than 10 dB from the desirable fundamental shear mode. The suppression of these anharmonic modes has become a critical issue for a high performance Giga Hertz MCF. 
   To extend the existing MCF technology into the GHz range is not trivial. For a low frequency MCF, a good out-of-band rejection may be achieved by designing a quartz filter supporting only fundamental shear mode. However, at high frequency, greater than 1 GHz, a single mode quartz filter will have the dimension of smaller than a few microns by a few microns, and a gap of 1 micron or less. The tolerance for the fabrication error for such a filter may be costly and difficult to attain with conventional processing. Therefore, a highly effective technique for suppression of anharmonic modes is necessary. 
   Thus, what is needed is a high performance Giga hertz MCF capable of effectively suppressing anharmonic modes. 
   SUMMARY 
   In one embodiment, a cascaded monolithic crystal filter is provided. A first filter includes two resonators having a pair of electrodes with the monolithic crystal between. At least one electrode has a periphery which includes a feature capable of shifting a frequency associated with an anharmonic mode in the filter. The filter has a second resonator acoustically coupled to the first resonator. A second filter is cascaded with the first filter. The second filter includes a pair of acoustically coupled resonators. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  is a side view of a simplified illustration of one example of a typical 2-pole monolithic crystal filter. 
       FIG. 2  shows a plot illustrating the spectral response of a typical 2-GHz 2-pole MCF with dimensions of about 15 microns by 19 microns. 
       FIG. 3A  is a top view illustrating the acoustic energy of the fundamental mode ( 1 , 1 , 1 ) for a resonator. 
       FIG. 3B  is a top view illustrating the acoustic energy of the anharmonic mode ( 1 , 1 , 3 ) for a resonator. 
       FIG. 3C  is a top view illustrating the acoustic energy of the anharmonic mode ( 1 , 3 , 1 ) for a resonator. 
       FIG. 4  is a top view showing a simplified illustration of monolithic crystal filters with and without electrode tabs. 
       FIG. 5  is a plot of the spectral responses of monolithic crystal filter of  FIG. 4  with tabs and without tabs. 
       FIG. 6  is a top view showing a simplified illustration of monolithic crystal filters with and without electrode tabs. 
       FIG. 7  is a plot of the spectral responses monolithic crystal filter of  FIG. 6  with tabs and without tabs. 
       FIG. 8  is a top view of showing a simplified illustration of two cascaded monolithic crystal filters modified with the electrode tabs shown in  FIGS. 4 and 6 . 
       FIG. 9  is a plot of the spectral responses of the cascaded monolithic crystal filter of  FIG. 8 . 
       FIG. 10  is a top view of showing a simplified illustration of two cascaded monolithic crystal filters. 
       FIG. 11  is a plot of the spectral responses of the cascaded monolithic crystal filter of  FIG. 10 . 
   

   DESCRIPTION 
     FIG. 1  is a side view of a simplified illustration of one example of a typical 2-pole MCF or monolithic crystal filter  100 . The electrodes  112  and  122  are separated from electrodes  113  and  123 , respectively, by a piezoelectric material  130 , typically quartz. An input voltage signal at  110  is coupled to the output  120  by acoustical coupling through the piezoelectric material  130 . 
     FIG. 2  shows a plot illustrating the spectral response  200  of a typical 2-GHz 2-pole MCF with electrodes separated by 1.5 microns and having dimensions of approximately 15 microns by 19 microns. The out-of-band rejection for this filter is very poor due to the existence of several strong undesirable anharmonic modes, especially ( 1 ,  1 ,  3 ) and ( 1 ,  3 ,  1 ), in addition to a desirable fundamental shear mode ( 1 ,  1 ,  1 ). A simple cascade of MCF of the same kind will not significantly improve the out-of-band rejection. An effective way of suppressing anharmonic modes, while keeping the insertion loss of the filter low, is to cascade two or more filters with the same fundamental frequency but significantly different in the anharmonic modes. To achieve this, the differences in the characteristics of acoustic energy trapping of the fundamental and anharmonic modes is exploited as discussed below. 
     FIGS. 3A-C  are top views illustrating the acoustic energy trappings of the fundamental mode ( 1 , 1 , 1 ), shown in  FIG. 3A , and anharmonic modes ( 1 , 1 , 3 ) and ( 1 , 3 , 1 ), shown in  FIGS. 3B and 3C  respectively, for a resonator  300 .  FIGS. 3A-C  illustrate how features, such as tabs  360   at   1  and/or  360   at   2  ( FIG. 3A ), and/or cut-out portions  360   ar   1  and/or  360   ar   2  ( FIG. 3A ), formed along the periphery  360  of the electrode  312  affect the fundamental mode ( 1 , 1 , 1 ) shown in  FIG. 3A , differently from the anharmonic modes ( 1 , 1 , 3 ) and ( 1 , 3 , 1 ) shown in  FIGS. 3B and 3C . 
   The ( 1 ,  1 ,  1 ) mode, shown in  FIG. 3A , has acoustic energy confined in the center of the electrode  312   a . As such, the fundamental mode ( 1 , 1 , 1 ) is insensitive to the perturbation of the periphery  360   a  of the electrode  312   a . As a result, adding a small electrode tab  360   at   1  and/or  360   at   2 , or removing a small portions  360   ar   1  and/or  360   ar   2  of electrode  312   a , by laser trimming or FIB for example, will have essentially no effect on the fundamental mode ( 1 ,  1 ,  1 ) shown in  FIG. 3A . 
   The anharmonic modes ( 1 , 1 , 3 ) and ( 1 , 3 , 1 ), however, have acoustic energy spreading toward the peripheries  360   b  and  360   c  of the electrodes  312   b  and  312   c , respectively. As such, the anharmonic modes ( 1 , 1 , 3 ) and ( 1 , 3 , 1 ) are very sensitive to perturbation of the electrode periphery  360   b  and  360   c , respectively. Adding tabs  360   bt   2  and  360   ct   1 , or removing a small portions  360   br   2  and  360   cr   1  of the electrode  360   b  and  360   c  has a significant effect on the resonance frequency of respective anharmonic modes. For modes ( 1 , 1 , 3 ), ( 1 , 1 , 5 ), etc., adding the tab  360   bt   2  will shift the ( 1 , 1 , 3 ) ( 1 , 1 , 5 ) mode toward a lower frequency than an unperturbed electrode. For modes ( 1 , 1 , 3 ), ( 1 , 1 , 5 ), etc., removing a portion  360   br   2  will shift the ( 1 , 1 , 3 ) ( 1 , 1 , 5 ) toward a higher frequency than unperturbed electrode. Such a modification to electrode, however, will have negligible effect on the ( 1 ,  1 ,  1 ) and ( 1 ,  3 ,  1 ) modes. Similarly, adding tab  360   ct   1 , or removing portion  360   cr   1  has the significant effect on modes ( 1 , 3 , 1 ), ( 1 , 5 , 1 ), etc., but not on modes ( 1 ,  1 ,  1 ) and ( 1 ,  1 ,  3 ). 
     FIG. 4  is a top view showing a simplified illustration of electrodes  411  and  412  of monolithic crystal filters  405  and  415 . Shown in  FIG. 5  is a plot of the spectral response of filter  405  (pair of acoustically coupled resonators) with electrode tabs  460   t  along with a plot of filter  415  (pair of acoustically coupled resonators) without tabs, shown in  FIG. 4 . In the specific example embodiment of  FIG. 4  the electrodes  412  are separated by 2 microns and are each 16 microns by 16 microns with tabs  460   t  that extend 1.6 microns from the periphery of the electrode  412  and are each 4 microns wide. The electrodes  411  are similarly sized and spaced, but without tabs. 
     FIG. 6  is a top view showing a simplified illustration of electrodes  611  and  612  of monolithic crystal filters  605  and  615 .  FIG. 7  is a plot  700  of the spectral response of filter  605  (pair of acoustically coupled resonators) with electrode tabs  660   t  along with a plot of filter  615  (pair of acoustically coupled resonators) without tabs, shown in  FIG. 6 . In the embodiment of  FIG. 6  the electrodes  612  are separated by 2 microns and are each 16 microns by 16 microns with tabs  660   t  that extend 2 microns from the periphery of the electrode  612  and are each 3.2 microns wide. The electrodes  611  are similarly sized and spaced, but without tabs 
   Referring to  FIGS. 5 and 7 , in both plots  500  and  700 , the desirable ( 1 ,  1 ,  1 ) mode remains essentially unperturbed. However, the anharmonic resonance frequencies of ( 1 ,  1 ,  3 ) and ( 1 ,  3 ,  1 ) modes, respectively, have been shifted downward significantly by the acoustically coupled resonators  405  and  605  with the tabs  460   t  and  660   t , respectively. Thus, the shifted anharmonic frequencies can be filtered by cascading. 
     FIG. 8  is a top view of a simplified illustration of a 2 pole cascaded monolithic crystal filters embodiment  800 . This example embodiment improved out-of-band rejection to greater than 45 db as shown in the spectral response plot  900  of  FIG. 9 . In the embodiment of  FIG. 8 , the electrodes  811  are separated by 1.5 microns and are each 15 microns by 19.2 microns, with tabs  860   t  that extend 1.5 microns from opposite 19.2 micron sides of the electrodes  811  and are each 4.8 microns wide. Similarly, the electrodes  812  are separated by 1.5 microns and are each 15 microns by 19.2 microns, but with tabs  861   t  that extend 1.2 microns from adjacent 15 micron sides of the electrodes  812  and are each 6 microns wide. In this embodiment, the tabs  860   t  are shown on opposite sides of the electrodes  811  and the tabs  861   t  are shown on adjacent sides of the electrodes  812 . 
     FIG. 10  shows another example embodiment of a cascaded monolithic crystal filter  1000 . In this example, the out-of-band rejection was further improved to greater than 55 dB with less than 4 db insertion loss. In the embodiment of  FIG. 10 , the electrodes  1011  are separated by 1.5 microns and are each 15 microns by 15 microns, with tabs  1060   t  that extend 1.5 microns from opposite sides of the pair of electrodes  1011 , and are each 9.6 microns wide. Similarly, the electrodes  1012  are separated by 1.5 microns and are each 15 microns by 19.2 microns, but with tabs  1061   t  that extend 1.2 microns from adjacent 15 micron sides of the electrodes  1012  and are each 6 microns wide.  FIG. 11  shows the spectral plot  1100  for the example embodiment of  FIG. 10 . 
   In the cascaded monolithic crystal filter embodiments  800  ( FIG. 8 ), for example, the cross sectional area of the tabs  860   t  and  861   t  have the same cross sectional area to ensure that the fundamental mode remains unchanged, while all of the anharmonic modes are shifted. 
   In various embodiments discussed above, monolithic crystal filters may be easily fabricated to provide a significant difference in anharmonic modes, while the center frequency remains essentially unchanged. Cascading two or more of these filters can provide a band-pass MCF with low insertion loss (a few dB or less) and extreme high out-of-band rejection (70-80 db or more). 
   Further, various embodiments may provide extremely low insertion loss for filters up to a few GHz regardless of extremely narrow (much less than 1% of the center frequency) or very wide (greater than 10%) bandwidth due to an extremely high Q. Moreover, a steep and high out-of-band attenuation is possible. In addition, some embodiments can provide minimum ripple in transmission band (much less than 1 dB). 
   In certain applications, embodiments can be used to provide a passive filter, with no other power consumption, other than insertion loss. Further, embodiments may be easily miniaturized if desired. Thus, embodiments may also have a great potential for wireless communication application into a small, low cost component. 
   Although above embodiments are shown with tabs, other embodiments may have cut-outs of the peripheral edge of the electrodes instead of, or in addition to tabs. Furthermore, although generally rectangular tabs and cut-outs are shown for illustration purposes, other shapes, configurations, or features are possible, such as for example, arcuate, circular, tapered, triangular, trapezoidal, etc., or other features at or near the periphery of the electrode. A “cut-out” is as used herein may be formed during deposition without having to remove material by cutting, etching, or other removal technique. Also, a tab may be formed from an electrode by cutting, trimming, etching, or other removal technique, or be added to an electrode after electrode formation. 
   In addition, although one tab or cut-out is shown on each electrode in  FIGS. 4 ,  6 ,  8 , and  10 , it is possible that each electrode have more than one. For example, multiple tabs or cut-outs may be located on a same edge, or on opposing edges, of an electrode. 
   In alternate embodiments, a conventional monolithic crystal filter  100 , shown in  FIG. 1 , may include tabs and/or cut-outs. This can reduce the anharmonic signal by 5 to 10 dB in some such embodiments. Although not required in all embodiments, typically, resonator electrode pairs, such as  112  and  113 , will have the same feature at their peripheries. In one possible alternate embodiment, electrodes  112  and  113  may be configured as represented in  FIG. 3B , and the electrodes  122  and  123  may be configured as represented in  FIG. 3C . Or, in another alternate embodiment, electrodes  112  and  113  may be configured as shown in  FIG. 3A , and electrodes  122  and  123  without any feature at the periphery. 
   The example embodiments herein are not intended to be limiting, various configurations and combinations of features are possible. 
   Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not limited to the disclosed embodiments, except as required by the appended claims.