Patent Publication Number: US-6657518-B1

Title: Notch filter circuit apparatus

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to filters and more particularly to a notch filter circuit apparatus. 
     BACKGROUND OF THE INVENTION 
     In many circuits it is desirable to operate the circuit so that one frequency signal is highly attenuated, while a desired frequency signal is left unattenuated. A circuit input, for example, may include not only a fundamental frequency signal, but may also include second, third, fourth, and higher harmonic frequency signals. In some circuit implementations it may be required to pass the fundamental frequency signal while blocking a specific harmonic signal. A notch, or bandstop, filter is the most appropriate filter to meet this requirement. A bandpass filter that discriminates against a wide range of frequency signals outside the passband may not provide the desired results. 
     Notch filters are often realized using distributed transmission line stubs, which can occupy significant substrate space. In conventional coplanar waveguide circuits, a notch filter may be created by symmetrically placing shunt stubs on opposite sides of the coplanar waveguide line. Conventional methods for reducing stub length, and therefore scarce substrate space, include using bent shunt stubs, meander structures, or capacitive loading. Notch filters employing these methods may be difficult to control over a broad frequency band or in more than one narrow frequency band of interest. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, a notch filter circuit includes a coplanar waveguide that is located on a silicon substrate and at least one shunt stub bent at an angle to the coplanar waveguide. The notch filter circuit further includes at least one capacitor bridging at least one discontinuity of the shunt stub. 
     Some embodiments of the invention provide numerous technical advantages. Other embodiments may realize some, none, or all of these advantages. For example, according to one embodiment, a notch filter circuit utilizes at least one metal-insulator-metal capacitor in place of an air bridge or wire-bond to reduce the physical size of the notch filter. In some embodiments, the metal-insulator-metal capacitor also provides coplanar waveguide ground equalization. In addition the notch filter circuit may be implemented on a high-resistivity silicon substrate. In some embodiments, multiple metal-insulator-metal capacitors are located at specific positions along the length of stub to allow the filter pass-band and stop-band to be properly selected. 
    
    
     Other advantages may be readily ascertainable by those skilled in the art from the following FIGURES, description, and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numbers represent like parts, and which: 
     FIG. 1 illustrates a notch filter circuit in one embodiment of the present invention; 
     FIG. 2 graphically illustrates a simulated signal transmission curve and a simulated signal reflection curve for a conventional notch filter circuit containing air bridges; 
     FIG. 3 illustrates a schematic diagram of a notch filter circuit in one embodiment of the present invention; 
     FIG. 4 graphically illustrates signal transmission curves and signal reflection curves for a notch filter circuit in one embodiment of the present invention; 
     FIG. 5 illustrates a notch filter circuit that contains one metal-insulator-metal capacitor located in a straight shunt stub; and 
     FIG. 6 graphically illustrates signal transmission curves and signal reflection curves for a notch filter containing one metal-insulator-metal capacitor located in a straight shunt stub. 
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
     Embodiments of the invention are best understood by referring to FIGS. 1 through 6 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 is a diagram illustrating a notch filter circuit  100  in one embodiment of the present invention. Notch filter circuit  100  includes an input port  110  and an output port  112 . Notch filter circuit  100  also includes a Coplanar Waveguide (CPW)  120  located on a substrate  122 . Notch filter circuit  100  further includes at least one shunt stub  130 . In one embodiment of the present invention, notch filter circuit  100  includes two symmetrical shunt stubs  130  located on opposite sides of CPW  120 . 
     CPW  120  may be formed by placing metal layers (the light regions of FIG. 1) on a substrate  122  (the dark regions of FIG.  1 ). In one embodiment of the present invention, CPW  120  is formed from chromium-silver-chromium-gold (Cr—Ag—Cr—Au) metal layers total thickness, approximately one micron (μm); however, a CPW  120  formed from any suitable material and dimension is within the scope of the present invention. CPW  120  is formed by placing the metal layers on a silicon substrate  122 , which in one embodiment is highly resistive. In one embodiment the silicon substrate is approximately 400 μm thick. Shunt stub  130  may also be formed by placing Cr—Ag—Cr—Au metal layers on silicon substrate  122 . A shunt stub  130  formed from any suitable material is within the scope of the present invention. In one embodiment of the present invention, shunt stub  130  will be patterned in the same plane as CPW  120  and bent at an angle of ninety degrees relative to the longitudinal axis of CPW  120 . Other configurations of shunt stub  130  may also be utilized. Shunt stub  130  includes at least one metal-insulator-metal (MIM) capacitor  132  located at a discontinuity of shunt stub  130 ; however, other types of capacitor  132  are within the scope of the present invention. 
     In one embodiment, symmetrical shunt stubs  130  are located on opposite sides of CPW  120 . Input port  110  of notch filter circuit  100  is operable to receive an incoming microwave or millimeter-wave electronic signal and direct the signal into CPW  120 . Shunt stubs  130  filter the signal, and the filtered signal will be output from CPW  120  at output port  112 . For purposes of illustration shunt stubs  130  and CPW  120  are discussed as forming a notch filter circuit  100  operable to pass signals of 21 GHz and stop, or notch, signals of 42 GHz. For this example 21 GHz is the fundamental frequency signal, and 42 GHz is the second harmonic frequency signal. Notch filter circuit  100  may be designed to pass frequencies and to stop other particular frequencies, and it is envisioned that other notch filter circuits  100  so designed are also within the scope of the present invention. 
     In conventional shunt stub designs air bridges are placed at discontinuities within shunt stub  130  to suppress the propagation of undesired modes. A conventional shunt stub design locates air bridges where MIM capacitors  132  are located in notch filter circuit  100  of FIG.  1 . When properly designed with an adequate bridge-height and minimum bridge-width, the air bridge introduces minimal parasitic effects to the conventional notch filter circuit. Conventional notch filter circuits implemented using air bridges occupy significant surface area in a circuit design as will be described below in greater detail. 
     FIG. 2 graphically illustrates the response of a conventional notch filter circuit wherein each shunt stub  130  includes a first air bridge located a distance  140  from CPW  120 , a second air bridge located a distance  142  from the first air bridge, and a third air bridge located a distance  144  from the second air bridge and a distance  146  from the end of the conventional shunt stub. The air bridges of conventional notch filter circuits are not illustrated in FIG. 1 for reasons of clarity. In order to obtain a pass-band at a fundamental frequency and a stop-band at a second harmonic frequency, the total physical length of the conventional shunt stub is determined by dividing the guided wavelength of the fundamental frequency by four. Accordingly, in order to obtain a pass-band at 21 GHz and a stop-band at 42 GHz, the total physical length of the conventional shunt stub is approximately 1490 μm. Thus, distances  140 ,  142 ,  144 , and  146  add to a total distance of 1490 μm. 
     Referring now to FIG. 2, there are graphically illustrated a simulated signal transmission curve  202  and simulated signal reflection curve  204  for a conventional notch filter circuit. An electromagnetically-simulated signal transmission curve  202  illustrates a high signal transmission at approximately 20 GHz and a very low signal transmission at approximately 40 GHz. An electromagnetically-simulated signal reflection curve  204  illustrates a high signal reflection at approximately 40 GHz and a very low signal reflection at approximately 20 GHz. Thus, a conventional notch filter circuit may be made to effectively pass a fundamental frequency signal while blocking a second harmonic frequency signal, although a conventional shunt stub length of 1490 μm is required. 
     According to the teachings of the invention, shunt stub  130  in one embodiment of the present invention is illustrated in FIG. 1 as including three MIM capacitors  132 . A first MIM capacitor  132  is located a distance  140  from CPW  120 , and a second MIM capacitor  132  is located a distance  142  from first MIM capacitor  132 . A third MIM capacitor  132  is located a distance  144  from second MIM capacitor  132  and a distance  146  from the end of shunt stub  130 . In one embodiment a silicon-oxide (SiO) layer 0.58 μm thick may be used as a dielectric  134  in MIM capacitors  132 . Any suitable material or thickness of dielectric is within the scope of the present invention. By using MIM capacitors, notch filter circuit  100  is operable to attenuate a selected frequency with little effect on other frequencies. In some embodiments of the present invention, multiple MIM capacitors  132  are located at specific positions along the length of shunt stub  130  to allow the pass-band and stop-band of notch filter circuit  100  to be properly selected. 
     FIG. 3 illustrates a circuit model equivalent of notch filter circuit  100  of FIG.  1 . In the illustrated embodiment MIM capacitors  132  are sized at 0.082 pF, and the locations of MIM capacitors  132  are indicated by distances  140 ,  142 ,  144 , and  146 . Through proper selection of shunt stub  130  parameters and MIM capacitor  132  values, it is possible to obtain an effective notch filter circuit  100  with a pass-band response at 21 GHz (Z in, stub= infinity Ω) and a stop-band response at 42 GHz (Z in, stub =0 Ω). Z in, stub  is the shunt stub impedance with respect to a particular frequency signal. The total physical length of each shunt stub  130  in this embodiment is 735 μm. By replacing the air bridges with three MIM capacitors  132 , therefore, shunt stub  130  may be reduced in size from 1490 μm to 735 μm. 
     The required surface area for notch filter circuit  100  may be significantly reduced by replacing the conventional air bridges with MIM capacitors  132  in shunt stubs  130 . In microwave and millimeter-wave integrated circuits, compact layout is an important issue that is limited by both circuit cross-talk and component size. Filter size is particularly important, because the filters are often realized using distributed transmission line stubs that can occupy significant substrate space. 
     MIM capacitors  132  serve an additional function within notch filter circuit  100 . MIM capacitors  132  are, in one embodiment, operable to provide CPW  120  ground equalization through the underlying metal by providing a direct current contact between the two ground paths of CPW  120 . Ground equalization in conventional notch filter circuits has been accomplished using air bridges. 
     Referring now to FIG. 4 there is graphically illustrated a comparison between electromagnetic simulation results and the measured response of notch filter circuit  100  employing MIM capacitor-loaded shunt stubs  130 . A measured signal transmission curve  408  substantially matches the simulated signal transmission curve  406 . Similarly, a measured signal reflection curve  404  substantially matches the simulated signal reflection curve  402 . Measured signal transmission curve  408  illustrates a high signal transmission level at approximately 20 GHz and a low signal transmission level at approximately 40 GHz. Measured signal reflection curve  404  illustrates a high signal reflection level at approximately 40 GHz and a low signal reflection level at approximately 20 GHz. In one embodiment, the 3-dB pass-band bandwidth of notch filter circuit  100  is approximately 55 percent. The insertion loss is approximately 1 dB at 21 GHz and the rejection at 42 GHz is 30 dB. FIG. 4 illustrates that one embodiment of notch filter circuit  100  is operable to transmit a fundamental signal frequency and block a second harmonic frequency signal. Notch filter circuit  100  is operable to do so with shunt stubs  130  approximately 50 percent smaller than the shunt stubs in a conventional notch filter circuit. 
     Referring now to FIG. 5 there is illustrated a notch filter circuit  510  embodying a MIM capacitor-loaded straight shunt stub topology. In this embodiment a single MIM capacitor  132  is located in each straight shunt stub  500 . Neglecting parasitic effects, the impedance seen looking into straight shunt stub  500  is given by          Z     in   ,   stub       =       j                   Z   0        tan                 θ       1   -     ω                   CZ   0        tan                 ϑ                         
     In the equation ω is 2nf, where f is the frequency, C is the capacitance of MIM capacitor  132 , Z 0  is the characteristic impedance, and θ is the electrical length of shunt stub  500 . The above equation assumes that MIM capacitor  132  is located at the exact junction between CPW  120  and straight shunt stub  500 . This means MIM capacitor  132  is located a zero distance  502  from CPW  120 . To obtain the pass-band filter response at 21 GHz Z in, stub =infinity Ω) a fixed C and Z 0  are used in the following equation:        θ   =       tan     -   1            (     1     ω                   CZ   0         )                       
     From this equation it is seen that θ decreases with increasing C, and θ will be less than 90° for any non-zero value of C. With C and Z 0  fixed however, it will not be possible to satisfy the filter stop-band response at the second harmonic frequency of 42 GHz (Z in, stub =0 Ω), which requires θ to 180° . 
     An analysis of the circuit illustrated in FIG. 5, in which distance  502  is allowed to be non-zero, reveals that a single MIM capacitor  132  in a straight shunt stub  500  is operable to provide the desired responses at the pass-band and stop-band frequencies. Since MIM capacitor  132  serves a dual purpose of capacitive-loading of CPW  120  and ground plane equalization, it is important that MIM capacitor  132  be placed near the junction between shunt stub  500  and CPW  120  in this embodiment. Therefore, distance  502  should be minimized to the extent possible. Decreasing distance  502  requires that the size of MIM capacitor  132  increase. In one embodiment of the present invention, the correct pass-band and stop-band responses were obtained in notch filter circuit  510  with distances  502  and  504  equaling 110 μm and 300 μm, respectively, and a MIM capacitor  132  value of 0.65 pF. By way of contrast, notch filter circuit  100  as illustrated in FIG. 1 required only MIM capacitors  132  sized at 0.082 pF. Parasitic effects in MIM capacitor  132  of size 0.65 pF become noticeable in the 40-60 GHz range, however, which complicates the process of establishing the null at the desired second harmonic frequency. 
     Referring now to FIG. 6, there is graphically illustrated a comparison between a measured response and electromagnetic simulation results for a notch filter circuit  510  embodying the straight shunt stub topology. In this design distances  502  and  504  were 110 μm and 470 μm, respectively, and MIM capacitor  132  was sized at 0.275 pF. In FIG. 6 measured signal transmission curve  608  is similar to simulated signal transmission curve  606 . Measured signal reflection curve  604  is similar to simulated signal reflection curve  602 . Although not an optimal design for this application, the results illustrated in FIG. 6 demonstrate the presence of a controllable stop-band response at approximately 58 GHz. 
     Although the present invention has been described with several example embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass those changes and modifications as they fall within the scope of the claims.