Patent Publication Number: US-2007120627-A1

Title: Bandpass filter with multiple attenuation poles

Description:
DESCRIPTION OF THE INVENTION  
     Field of the Invention  
      The present invention relates to a bandpass filter, and more specifically to a bandpass filter having multiple attenuation poles.  
     BACKGROUND OF THE INVENTION  
      In recent years, marked advances in the miniaturization of mobile communication terminals, such as mobile phones and Wireless LAN (Local Area Network) routers, has been achieved due to the miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is the filter.  
      One type of bandpass filter used in such communication applications is disclosed in U.S. Pat. No. 6,424,236 (Kato) and is shown in  FIG. 1A .  FIG. 1A  depicts a bandpass filter utilizing three inductor-capacitor (LC) resonators. The filter further includes three inductors, three capacitors, two input/output (I/O) capacitors, two coupling capacitors and a pole adjustment pattern  47  facing the coupling capacitor pattern.  
      As seen in  FIG. 1B , by changing the size of pole adjustment pattern  47 , the position of poles at the lower attenuations band is adjusted. For example, when the area of the overlapping portion between the coupling capacitors patterns and pole adjustment pattern is increased, an electrostatic capacitor generated between them is increased, which increases the spacing between poles. By changing the size of pole adjustment pattern  47 , the distance between the two poles are controlled. However, as a result, when the attenuation closer to the lower-passband side of the frequency band is improved, the very low frequency band attenuation is deteriorated. In addition, while altering pole adjustment pattern  47  controls the distance between the lower-passband side poles, it does not allow for individual control of the poles.  
      While the Kato bandpass filter is generally acceptable for the creation of an additional attenuation pole at the lower-passband side of the filter, the requirement for I/O capacitors increases the size of the filter and makes it less suitable for application in smaller communication devices. For wide band filters, the size of these capacitors should be big enough to provide required external circuit coupling. Such capacitors can increase the size and cost of the filter. In addition, the Kato filter lacks the ability to individually control the lower-passband side attenuation poles and completely lacks an upper-passband side attenuation pole.  
     SUMMARY OF THE INVENTION  
      The invention provides a bandpass filter having multiple attenuation poles.  
      According to one embodiment of the invention, the bandpass filter includes a combline bandpass filter including tapped-line input and output terminals, at least three resonators, and a loading capacitor for each resonator. The bandpass filter further includes a plurality of loading inductors, each loading inductor being connected between one of the resonators and its respective loading capacitor; and a direct coupling capacitor connected between any two of the at least three resonators that are separated by at least one other resonator.  
      Reduced size of the filter is achieved by using tapped-line input and output terminals rather than I/O capacitors typically found on conventional combline filters. By adding a direct coupling capacitor to a combline bandpass filter, an additional lower-passband side attenuation pole is created. The attenuation and rolloff characteristics of the lower-passband side can be controlled by altering the value of the direct coupling capacitance. By adding loading inductors to a combline bandpass filter, an upper-passband side attenuation pole is created. The attenuation and rolloff characteristics of the upper-passband side can be controlled by altering the value of the loading inductors.  
      According to another embodiment of the invention, the bandpass filter includes a combline bandpass filter including tapped-line input and output terminals, at least three resonators, and a loading capacitor for each resonator. The bandpass filter further includes a direct coupling capacitor connected between any two of the at least three resonators that are separated by at least one other resonator.  
      According to another embodiment of the invention, the bandpass filter includes a combline bandpass filter including tapped-line input and output terminals, at least three resonators, and a loading capacitor for each resonator. The bandpass filter further includes a direct coupling capacitor connected between any two of the at least three resonators that are separated by at least one other resonator. This bandpass filter adds an attenuation pole at the lower-passband side to the frequency response of the combline filter.  
      The frequency response of each of the embodiments described above can further be altered by adjusting the location of the tapped-line input and output terminals. Typically, the tapped-line input and output terminals are connected to the open end of the resonators. Out-of-band attenuation at both the lower- and upper-passband sides of the frequency response can be further improved by moving the location of the I/O terminals to some point below the open end of the resonators.  
      It is to be understood that the descriptions of this invention herein are exemplary and explanatory only and are not restrictive of the invention as claimed.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  depicts the physical layout of a prior art bandpass filter.  
       FIG. 1B  depicts the frequency response of the prior art bandpass filter shown in  FIG. 1 .  
       FIG. 2A  depicts the schematic of a conventional combline bandpass filter.  
       FIG. 2B  depicts the frequency response of a conventional combline bandpass filter.  
       FIG. 3  depicts the schematic of a bandpass filter having loading inductors and a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 4  depicts the physical layout of a bandpass filter having loading inductors and a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 5  depicts the frequency response of a bandpass filter having loading inductors and a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 6  depicts the frequency response, in relation to direct coupling capacitance, of a bandpass filter having loading inductors and a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 7  depicts the frequency response, in relation to loading inductance, of a bandpass filter having loading inductors and a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 8  depicts a schematic of a bandpass filter having lowered I/O terminals according to one embodiment of the invention.  
       FIG. 9  depicts the physical layout of a bandpass filter having lowered I/O terminals according to one embodiment of the invention.  
       FIG. 10  depicts the frequency response, in relation to I/O terminal location, of a bandpass filter according to one embodiment of the invention.  
       FIG. 11  depicts a schematic of a bandpass filter having four resonators according to one embodiment of the invention.  
       FIG. 12  depicts a schematic of a bandpass filter having a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 13  depicts the physical layout of a bandpass filter having a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 14  depicts the frequency response of a bandpass filter having a direct coupling capacitor according to one embodiment of the invention.  
       FIG. 15  depicts a schematic of a bandpass filter having loading inductors according to one embodiment of the invention.  
       FIG. 16  depicts a physical layout of a bandpass filter having loading inductors according to one embodiment of the invention.  
       FIG. 17  depicts a frequency response of a bandpass filter having loading inductors according to one embodiment of the invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
      The present invention utilizes and modifies a conventional combline bandpass filter to create a new bandpass filter that exhibits multiple attenuation poles.  FIG. 2A  depicts the schematic of a conventional combline bandpass filter. Combline bandpass filter  100  includes three resonators  110 ,  111 , and  112 . Typically, the resonators are transverse electromagnetic (TEM) quarter-wave resonators. The short end of each of the resonators is connected to ground, while the open end of each of the resonators is connected to loading capacitors  121 ,  131  and  141 , respectively. Internal coupling capacitor  117  connects the open end of first resonator  110  to the open end of second resonator  111 . Internal coupling capacitor  118  connects the open end of second resonator  111  to the open end of third resonator  112 . Input terminal  114  and input capacitor  113  are connected to the open end of first resonator  110 , while output terminal  115  and output capacitor  116  are connected to the open end of third resonator  112 .  
       FIG. 2B  depicts the frequency response of the combline bandpass filter shown in  FIG. 2A . As shown in  FIG. 2B , a conventional combline bandpass filter has only one attenuation pole at the lower-passband side. There is no pole at the upper-passband side and the rolloff is relatively shallow.  
       FIG. 3  depicts the schematic of bandpass filter  105  according to one embodiment of the invention. As can be seen in the schematic, bandpass filter  105  resembles combline bandpass filter  100  of  FIG. 2A . However, in addition to the components of a conventional combline bandpass filter, bandpass filter  105  includes a direct coupling capacitor  150  and loading inductors  121 ,  131 , and  141 . In addition, bandpass filter  105  may operate without input and output capacitors. As shown in  FIG. 3 , input terminal  114  and output terminal  115  are tapped-line I/O terminals. That is, the input and output terminals connect directly to the resonators. In this way, space in the filter package may be saved.  
      The short end of first resonator  110 , second resonator  111 , and third resonator  112  are each connected to ground. The open end of the first, second, and third resonators is connected in series with a first LC pair  120 , a second LC pair  130 , and a third LC pair  140 , respectively. The open end of first resonator  110  is connected to the open end of second resonator  111  by internal coupling capacitor  117 , and likewise, the open end of second resonator  111  is connected to the open end of third resonator  113  by internal coupling capacitor  118 . Direct coupling capacitor  150  connects the open end of first resonator  110  to the open end of third resonator  112 . In addition, input terminal  114  is connected to the open end of first resonator  110  and output terminal  115  is connected to the open end of third resonator  113 . Each of resonators  110 ,  111 , and  112  are preferably transverse electromagnetic quarter-wave resonators.  
      First LC pair  120  consists of a first loading capacitor  121  and a first loading inductor  122 . Likewise, second LC pair  130  consists of a second loading capacitor  131  and a second loading inductor  132 , and third LC pair  140  consists of a third loading capacitor  141  and a third loading inductor  142 . The LC pairs are connected between the open end of their respective resonators and ground. As shown in  FIG. 3 , the loading capacitors are directly connected to ground while the loading inductors are directly connected to a resonator, however this orientation may be reversed.  
       FIG. 4  depicts one example of a physical layout for the circuit shown in  FIG. 3 . Typically, for application in communication system and wireless LAN&#39;s, a multilayer structure would be employed. Preferably, the filter is created utilizing a low temperature co-fired ceramic (LTCC process), however any process for creating the multilayer structure may be employed, including thin-film processes, liquid crystal polymer processes, and other cell material technologies. In the description of  FIG. 4  below, each of the metal regions may be formed from any suitable conductive material and is preferably, silver, copper, or gold. Likewise, all of the vias described below may be formed from any suitable conductive material, and are preferably formed from a conductive paste containing silver, copper, or gold.  
      As shown in  FIG. 4 , bandpass filter layout  200  includes metal regions  201 ,  202 , and  203  form the system ground, first floating ground and second floating ground, respectively. The ground metal regions are connected to each other by vias  204 ,  205 ,  206 ,  207 ,  208 , and  209 . Metal regions  224 ,  225 , and  226  form the first, second, and third resonators, respectively. This configuration of metal regions  224 ,  225 , and  226  is sometimes referred to as a strip-line structure. The short ends of the resonators connect to ground through vias  204 ,  205 , and  206 .  
      Metal regions  210 ,  211 , and  212  form the first, second, and third inductors, respectively. These are typically referred to as shunt inductors. As shown, metal regions  210 ,  211 , and  212  are generally line-shaped metal regions, with metal regions  210  and  212  exhibiting one 90 degree turn. However, the shape depicted for the loading inductors is only exemplary and any shape of metal region that produced the desired level of inductance may be used. Metal regions  210 ,  211 , and  212  (loading inductors) connect to the open end of metal regions  224 ,  225 , and  226  (resonators) through vias  221 ,  222 , and  223 .  
      Metal regions  210 ,  211 , and  212  (loading inductors) also connect to metal regions  213 ,  214 , and  215 . Metal regions  213 ,  214 , and  215  in conjunction with metal region  203  (second floating ground) and metal region  201  (system ground) form the first, second, and third loading capacitors, respectively. These are typically referred to as shunt capacitors. As can be seen from the configuration, by utilizing both the second floating ground and the system ground, the loading capacitors are sandwiched capacitors. By utilizing this configuration, the size of the capacitors, and hence the size of the filter, can be reduced.  
      Metal regions  217  and  218  in conjunction with metal region  216  form the first and second internal coupling capacitors, respectively. These are parallel plate capacitors. Metal region  217  (first internal coupling capacitor) is connected to the open end of metal region  224  (first resonator) through via  221 , while metal region  218  (second internal coupling capacitor) is connected to the open end of metal region  226  (third resonator) through via  223 . Metal region  216  (forming part of both the first and second internal coupling capacitor) is directly connected to the open end of metal region  225  (second resonator).  
      Metal regions  219  and  220  form the direct coupling capacitor. These are also parallel plate capacitors. Metal region  219  is connected to the open end of metal region  226  (third resonator) through via  223 , and metal region  220  is connected to the open end of metal region  224  (first resonator) through via  221 .  
      Metal region  227  forms the input terminal is connected directly to the open end of metal region  224  (first resonator). Likewise, metal region  228  forms the output terminal and is connected directly to the open end of metal region  226  (third resonator). In this form, both the input and output terminals are tapped-line I/O terminals.  
       FIG. 5  depicts the frequency response of the circuit depicted in  FIG. 3 . Through the inclusion of the loading inductors and the direct coupling capacitors, three attenuation poles are achieved in the frequency response. Poles P 1  and P 2  on the lower-passband side, and pole P 3  on the upper-passband side. As can be seen in  FIG. 5 , the passband (roughly 4.50 to 6.50 GHz) is substantially flat. The frequency performance depicted in  FIG. 5  would be generated from a circuit that utilized loading capacitor values of 0.6 pF, loading inductor values of 0.4 nH, internal coupling capacitor values of 0.5 pF, direct coupling capacitor values of 0.15 pF, and resonator lengths and widths of 1100 μm and 100 μm, respectively. Preferably, the height of the LTCC substrate is 500 μm and the dielectric constant (ε r ) of the ceramic material is 7.5. However, this schematic is applicable for use in bandpass filters with any desired range of frequency response, and as such, the capacitance values, inductance values, resonator lengths, substrate height, and dielectric constant may be adjusted to suit a particular application.  
       FIG. 6  depicts the frequency response of the circuit depicted in  FIG. 3  for varying values of the direct coupling capacitor. The values of the other components remain the same as described above with reference to  FIG. 5 . As shown in  FIG. 6 , when the value of the direct coupling capacitor is dropped to zero (effectively no capacitor), frequency response  600  only includes one pole P 1  at the lower-passband side. When the direct coupling capacitor has a capacitance of 0.1 pF, frequency response  601  shows two poles (P 1  and P 2 ) on the lower-passband side. If the capacitance of the direct coupling capacitor is increased to 0.15 pF, frequency  602  also contains two poles at the lower-passband side. In addition, frequency response  602  exhibits a sharper rolloff for pole P 2  than is exhibited by frequency response  601 . As such, the frequency responses in  FIG. 6  show that a second attenuation pole can be achieved at the lower-passband side by adding a direct coupling capacitor to a combline bandpass filter as shown in  FIG. 3 . In addition, by changing the capacitance value of the direct coupling capacitor, the frequency response of the bandpass filter can be adjusted to produce a steeper (higher capacitance) or less steep (lower capacitance) rolloff response on the lower-passband side.  
       FIG. 7  depicts the frequency response of the circuit depicted in  FIG. 3  for varying values of the loading inductors. The values of the other components remain the same as described above with reference to  FIG. 5 . As shown in  FIG. 7 , when the value of the loading inductors is dropped to zero (effectively no loading inductors), frequency response  603  includes two poles on the lower-passband side, but no pole on the upper-passband side. In fact, the frequency response on the upper-passband side when there are no loading inductors exhibits a fairly shallow rolloff. When the loading inductors have an inductance of 0.2 nH, frequency response  604  shows an additional pole P 3  on the upper-passband side. If the inductance of the loading inductors is increased to 0.3 nH, frequency  605  also contains the additional pole P 3  on the upper-passband side. In addition, frequency response  605  exhibits a sharper rolloff for pole P 3  than is exhibited by frequency response  604 . As such, the frequency responses in  FIG. 7  show that a third attenuation pole can be achieved at the upper-passband side by adding loading inductors to a combline bandpass filter as shown in  FIG. 3 . In addition, by changing the inductance value of the loading inductors, the frequency response of the bandpass filter can be adjusted to produce a steeper (higher inductance) or less steep (lower inductance) rolloff response on the upper-passband side.  
       FIG. 8  depicts another embodiment of bandpass filter according to the invention. This embodiment is virtually identical to the filter depicted in  FIG. 3  except for the placement of the input and output terminals. As can be seen in  FIG. 8 , bandpass filter  106  includes input terminal  125  and output terminal  126  that are positioned below the open end of first resonator  110  and third resonator  112  respectively.  
       FIG. 9  depicts the physical layout of the bandpass filter of  FIG. 9 . Bandpass filter layout  250  is identical to bandpass filter layout  200  of  FIG. 4  in every respect except for the metal regions forming the input and output terminals. Metal region  229  forms the input terminal and is connected to metal region  224  (first resonator) at a point approximately 200 μm below the open end of metal region  224 . Likewise, metal region  230  forms the output terminal and is connected to metal region  226  (third resonator) at a point approximately 200 μm below the open end of metal region  226 . The input and output terminals may be positioned at any distance below the open end of the resonators, up to half the length of the resonator.  
       FIG. 10  depicts the frequency response of the circuit depicted in  FIGS. 3 and 8  for varying positions of the input and output terminals. The values of the inductive and capacitive components, length of the resonators, height of the substrate, and dielectric constant of the ceramic materials remain the same as described above with reference to  FIG. 5 . As shown in  FIG. 10 , the steepness of the rolloff and the attenuation on the upper- and lower-passband sides is increased as the input and output terminals are moved back from the open end of the resonators. Frequency response  606  shows the response when the input and output terminals are at the open end of the resonators, frequency response  607  shows the response when the input and output terminals are set back 200 μm from the open end of the resonators, and frequency response  608  shows the response when the input and output terminals are set back 400 μm from the open end of the resonators.  
      The bandpass filters described with reference to FIGS.  3  to  10  need not be limited to circuits with only three resonators. Circuits with four or more resonators are also acceptable. All that is required is that there is one LC pair connected in series with each resonator and one internal coupling capacitor between the open ends of each successive resonator. In addition, at least one direct coupling capacitor may be connected between any two resonators that are separated by at least one other resonator. For example, in a circuit that utilizes four resonators, the direct coupling capacitor may be connected between the first and third resonators or between the second and fourth resonators.  
       FIG. 11  depicts a schematic of bandpass filter  107  that includes four resonators. This circuit is similar to the circuit depicted in  FIG. 3 . The four resonator circuit adds a fourth resonator  123 , a fourth LC pair  160  connected to the open end of fourth resonator  123 , and a third internal coupling capacitor  119  connected between the open end of fourth resonator  123  and first resonator  110 . Fourth LC pair  160  includes fourth loading capacitor  161  and fourth loading inductor  162 . As shown in  FIG. 11 , direct coupling capacitor  150  is connected between resonator  110  and resonator  112  (which are separated by resonator  111 ). As discussed above, it would also be acceptable to connect direct coupling capacitor  150  between resonator  123  and resonator  111  (which are separated by resonator  110 ).  
      As discussed above, the addition of a direct coupling capacitor to a combline bandpass filter creates an additional attenuation pole at the lower-passband side in the frequency response of the filter. The addition of loading inductors to a combline bandpass filter creates an attenuation pole at the upper-passband side in the frequency response of the filter. The circuits described thus far have included both the direct coupling capacitor and loading inductors. However, inclusion of both types of these components is not necessary for applications in which improved out-of-band attenuation is only desired for one side of the passband.  
       FIG. 12  depicts the schematic for a bandpass filter  108  that includes the direct coupling capacitor  150 , but no loading inductors. In this case, first loading capacitor  121 , second loading capacitor  131 , and third loading capacitor  141  are connected to the open ends of first resonator  110 , second resonator  111 , and third resonator  112 , respectively. First internal coupling capacitor  117  is connected between the open ends of first resonator  110  and second resonator  111  and second internal coupling capacitor  118  is connected between the open ends of second resonator  111  and third resonator  112 . Direct coupling capacitor  150  is connected between the open ends of first resonator  110  and third resonator  112 . Input terminal  114  is connected to the open end of first resonator  110  and output terminal  115  is connected to the open end of third resonator  112 .  
      As before, more than three resonators may be used so long as there is one loading capacitor connected in series with each resonator and one internal coupling capacitor between the open ends of each successive resonator. In addition, the direct coupling capacitor may be connected between any two resonators that are separated by at least one other resonator.  
       FIG. 13  shows the physical layout of the circuit shown in  FIG. 12 . Bandpass filter layout  300  includes metal regions  301 ,  302 , and  303  that form the system ground, first floating ground, and second floating ground, respectively. The ground metal regions are connected to each other by vias  304 ,  305 ,  306 ,  307 ,  308 , and  309 . Metal regions  324 ,  325 , and  326  form the first, second, and third resonators, respectively. This configuration is often referred to as a strip-line structure. The short end of the resonators is connected to ground through vias  304 ,  305 , and  306 , respectively.  
      Metal regions  313 ,  314 , and  315 , in conjunction with metal region  303  (second floating ground) and metal region  301  (system ground), form the first, second, and third loading capacitors, respectively. This configuration is referred to as a sandwiched capacitor. Metal regions  313 ,  314 , and  315  (loading capacitors) connect to metal regions  324 ,  325 , and  326  (resonators) through vias  321 ,  322 , and  323 .  
      Metal regions  317  and  318 , in conjunction with metal region  316  form the first and second internal coupling capacitors, respectively. This configuration is referred to as a parallel plate capacitor. Metal region  317  (first internal coupling capacitor) is connected to the open end of metal region  324  (first resonator) through via  321 , while metal region  318  (second internal coupling capacitor) is connected to the open end of metal region  326  (third resonator) through via  323 . Metal region  316  (forming part of both the first and second internal coupling capacitor) is directly connected to the open end of metal region  325  (second resonator).  
      Metal regions  319  and  320  form the direct coupling capacitor. This configuration is referred to as a parallel plate capacitor. Metal region  319  is connected to the open end of metal region  326  (third resonator) through via  323 , and metal region  320  is connected to the open end of metal region  324  (first resonator) through via  321 .  
      Metal region  327  forms the input terminal is connected directly to the open end of metal region  324  (first resonator). Likewise, metal region  328  forms the output terminal and is connected directly to the open end of metal region  326  (third resonator). In this form, both the input and output terminals are tapped-line I/O terminals.  
       FIG. 14  shows the frequency response of the circuit depicted in  FIG. 12 . As can be seen, the addition of a direct coupling capacitor to a combline bandpass filter produces two attenuation poles at the lower-passband side of the frequency response.  
       FIG. 15  depicts the schematic for a bandpass filter  109  that includes loading inductors  122 ,  132 , and  142 , but no direct coupling capacitor. In this case, first LC pair  120  (including first loading capacitor  121  and first loading inductor  122 ), second LC pair  130  (including second loading capacitor  131  and second loading inductor  132 ), and third LC pair  140  (including third loading capacitor  141  and third loading inductor  142 ) are connected to the open ends of first resonator  110 , second resonator  111 , and third resonator  112 , respectively.  
      First internal coupling capacitor  117  is connected between the open ends of first resonator  110  and second resonator  111  and second internal coupling capacitor  118  is connected between the open ends of second resonator  111  and third resonator  112 . Input terminal  114  is connected to the open end of first resonator  110  and output terminal  115  is connected to the open end of third resonator  112 .  
      As before, more than three resonators may be used so long as there one LC pair connected in series with each resonator and one internal coupling capacitor between the open end of each successive resonator.  
       FIG. 16  shows the physical layout of the circuit shown in  FIG. 15 . Bandpass filter layout  400  includes metal regions  401 ,  402 , and  403  that form the system ground, first floating ground, and second floating ground, respectively. The ground metal regions are connected to each other by vias  404 ,  405 ,  406 ,  407 ,  408 , and  409 . Metal regions  424 ,  425 , and  426  form the first, second, and third resonators, respectively. This configuration is referred to as a strip-line structure. The short ends of the resonators connect to ground through vias  404 ,  405 , and  406 .  
      Metal regions  410 ,  411 , and  412  form the first, second, and third inductors, respectively. These are referred to as shunt inductors. As shown, metal regions  410 ,  411 , and  412  are generally line-shaped metal regions, with metal regions  410  and  412  exhibiting one 90 degree turn. However, the shape depicted for the loading inductors is only exemplary and any shape of metal region that produced the desired level of inductance may be used. Metal regions  410 ,  411 , and  412  (loading inductors) connect to the open end of metal regions  424 ,  425 , and  426  (resonators) through vias  421 ,  422 , and  423 .  
      Metal regions  410 ,  411 , and  412  (loading inductors) also connect to metal regions  413 ,  414 , and  415 . Metal regions  413 ,  414 , and  415  in conjunction with metal region  403  (second floating ground) and metal region  401  (system ground) form the first, second, and third loading capacitors, respectively. These configurations are referred to as sandwiched capacitors.  
      Metal regions  417  and  418  in conjunction with metal region  416  form the first and second internal coupling capacitors, respectively. These configurations are referred to as parallel plate capacitors. Metal region  417  (first internal coupling capacitor) is connected to the open end of metal region  424  (first resonator) through via  421 , while metal region  418  (second internal coupling capacitor) is connected to the open end of metal region  426  (third resonator) through via  423 . Metal region  416  (forming part of both the first and second internal coupling capacitor) is directly connected to the open end of metal region  425  (second resonator).  
      Metal region  427  forms the input terminal is connected directly to the open end of metal region  424  (first resonator). Likewise, metal region  428  forms the output terminal and is connected directly to the open end of metal region  426  (third resonator). In this form, both the input and output terminals are tapped-line I/O terminals.  
       FIG. 17  shows the frequency response of the circuit depicted in  FIG. 15 . As can be seen, the addition of loading inductors to a combline bandpass filter produces an attenuation pole at the upper-passband side of the frequency response.  
      Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and embodiments disclosed herein. Thus, the specification and examples are exemplary only, with the true scope and spirit of the invention set forth in the following claims and legal equivalents thereof.