Abstract:
The present invention relates to a low pass filter incorporating coupled inductors to enhance stop band attenuation. In one embodiment, the coupled inductors are provided along with various capacitors to provide for superior performance within a smaller surface area of a semiconductor or ceramic integrated device. In a further specific embodiment, the capacitors are formed on an integrated device within an area on which entirely intertwined inductors are formed. In another embodiment, at least one further pair of coupled inductors is included to create additional frequency attenuation notches, as well as a wide stop-band.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a low pass filter incorporating coupled inductors to enhance stop band attenuation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Filter circuits that provide stop band attenuation are well-known. On such type of circuit is a notch circuit, that filters frequencies from a certain portion of the frequency spectrum (corresponding to the notch), and allows other frequencies to pass.  FIGS. 1(   a )  1 - 2  and  1 ( b )  1 - 2  illustrate a conventional prior art C-R-C and C-L-C low pass filtering circuits, respectively, as well as their respective performance characteristics. As shown, they provide a stop band at about 800 MHz to 6 GHz, which covers the radio frequency range used for wireless communication. A steep roll off from pass band to stop band frequency is highly desired, especially when high speed signals are incorporated in these devices. 
         [0003]    The present invention a circuit that provides desired stop-band performance in smaller areas, as well as less attenuation of the lower frequencies that are desired. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a low pass filter incorporating coupled inductors to enhance stop band attenuation. 
         [0005]    In one embodiment, the coupled inductors are provided along with various capacitors to provide for superior performance within a smaller surface area of a semiconductor or ceramic integrated device. 
         [0006]    In a further specific embodiment, the capacitors are formed on an integrated device within an area on which entirely intertwined inductors are formed. 
         [0007]    In another embodiment, at least one further pair of coupled inductors is included to create additional frequency attenuation notches, as well as a wide stop-band. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0009]      FIGS. 1(   a )  1 - 1  and  1 ( b )  1 - 1  illustrate a conventional prior art circuit, and  FIGS. 1(   a )- 2  and  1 ( b )- 2  provide the frequency domain filter performance. 
           [0010]      FIG. 2(   a ) illustrates an embodiment of a coupled inductor notch circuit according to the present invention; 
           [0011]    FIG.  2 (B) 1 - 2  illustrates a layout of the coupled inductor circuit according to  FIG. 2(   a ) of the present invention. 
           [0012]      FIG. 2(   c ) illustrates performance characteristics at different coupling ratios, respectively, according to the present invention; 
           [0013]      FIG. 2(   d ) illustrates a performance comparison between an embodiment of the present invention and a conventional C-L-C filter. 
           [0014]      FIG. 2(   e ) illustrates performance characteristics at a different notch frequency using the same circuit topology as in  FIG. 2(   a ), according to the present invention; 
           [0015]      FIGS. 3(   a )-( c ) illustrates a circuit that uses inductors that are uncoupled, a performance curve for the circuit, and a physical layout of the circuit. 
           [0016]      FIGS. 4(   a )-( b ) illustrate another embodiment of a coupled notch circuit according tot the present invention that has multiple notches and the performance characteristics associated therewith. 
           [0017]      FIG. 5(   a ) illustrates an embodiment of a coupled inductor notch circuit  500  according to the present invention, and  FIG. 5(   b ) the performance curve. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The present invention is preferably implemented on a single integrated circuit, as is described herein. The invention is used as a filter in order filter various different frequencies, including noise as well as upper harmonics of clock frequencies, particularly in frequency bands of interest such as 850 MHz. Wide passband is important for signal integrity as it is preferred to have as many harmonics as possible to pass through to preserve the waveform. In order to filter out any un-wanted signal, a wide stopband is important. 
         [0019]    What is the frequency range of un-wanted signal highly relies on application. In cell phone applications, for instance, other than the audio filter a stopband from 0.8 GHz to 6 GHz is desired, which corresponding to mobile frequency range (GSM and CDMA 0.8 GHz-0.9 GHz, 1.8 GHz-2 GHz, Bluetooth 2.4 GHz-2.5 GHz, wireless LAN 2.4 GHz-2.5 GHz, 5.15 GHz-5.350 GHz, 5.725 GHz-5.825 GHz). The WiMax band of 2-11 GHz is also of interest. 
         [0020]    In a typical application, the data that is being preserved is that digital data with a data rate which increases. Typically it is up to 70 Mbit/s—and the 5th harmonic frequency of such a data rate is 350 MHz, which signal needs to pass through. 
         [0021]      FIG. 2(   a ) illustrates an embodiment of a coupled inductor notch circuit  200  according to the present invention. As shown, the circuit includes an input  210 , an output  220 , and a ground  230 , each of which, as shown, have associated therewith a parasitic resistance and a parasitic inductance. Coupled inductors  240  and  250 , each having a parasitic resistance associated therewith as shown, are connected between nodes  1 ,  2  and  3  as shown, with inductor  240  being in series with resistor  260  between the input  210  and the output  220 , and inductor  250  being in series with capacitor  272  between node  2  and ground  230 , with node  2  being the node between inductor  250  and capacitor  272 . Capacitors  270  and  276  are connected between node  1  and ground and output  230  and ground, respectively, as shown. An example of values for the circuit elements is provided in the Table 1 below for the  FIG. 2(   a ) column. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 FIG. 2(a) 
                 FIG. 2(d) 
                 FIG. 3(a) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 input 210 
                 0.3 ohm 
                 0.3 ohm 
                 0.3 ohm 
               
               
                   
                 resistance 
               
               
                   
                 input 210 
                 1 nH 
                 1 nH 
                 1 nH 
               
               
                   
                 inductance 
               
               
                   
                 output 220 
                 0.3 ohm 
                 0.3 ohm 
                 0.3 ohm 
               
               
                   
                 resistance 
               
               
                   
                 output 220 
                 1 nH 
                 1 nH 
                 1 nH 
               
               
                   
                 inductance 
               
               
                   
                 ground 230 
                 0.05 ohm 
                 0.05 ohm 
                 0.05 ohm 
               
               
                   
                 resistance 
               
               
                   
                 ground 230 
                 15 pH 
                 15 pH 
                 15 pH 
               
               
                   
                 inductance 
               
               
                   
                 inductor 240 
                 24 nH; 
                 240 nH; 
                 37 nH; 
               
               
                   
                   
                 15 ohm 
                 50 ohm 
                 20 ohm 
               
               
                   
                 inductor 250 
                 3.5 nH; 
                 34 nH; 
                 9.0 nH; 
               
               
                   
                   
                 6 ohm 
                 15 ohm 
                 4 ohm 
               
               
                   
                 resistor 260 
                 26 ohm 
                   
                 10 ohm 
               
               
                   
                 capacitors 270 
                 15 pF 
                 145 pF 
                 17 pF 
               
               
                   
                 capacitor 272 
                 2.5 pF 
                 20 pF 
                 4 pF 
               
               
                   
                 capacitor 274 
                 10 pF 
                 140 pF 
                 6 pF 
               
               
                   
                   
               
             
          
         
       
     
         [0022]      FIG. 2(   b ) 1  illustrates a layout of the coupled inductor circuit according to  FIG. 2(   a ) of the present invention. As is shown inductors  240  and  250  are coupled, such that the coils of inductor  240  overlap with the coils of inductor  250 . The number and size of the coils in inductors  240  and  250  will depend on the application requirements, such as filter pass-band and stop-band frequencies, current handling capability and resistance requirements.  FIG. 2(   b ) 2  shows an example configuration of a spiral inductor that can be used according to the present invention in a multi-layer structure. 
         [0023]      FIG. 2(   c ) illustrates performance characteristics at different coupling ratios, respectively, according to the present invention. As is shown, depending upon the degree of overlap (coupling coefficient K) the performance of the circuit changes with fixed inductor size. Degree of overlap is most significant with respect to the surface area of the coils that overlap, with coils that are intertwined but on separate planes having the most overlap, and the amount of the separation between the planes of the coils having an effect, but a secondary effect, on the coupling coefficient K. The coupling coefficient K, as is known, will depend on the degree of shared magnetic field of the two inductor coils. In a preferred embodiment, in addition to being entirely intertwined, the coils of inductors  240  and  250  are preferably positioned to that there the coils of one inductor are positioned in gaps between the coils of the other inductor, thereby avoiding surface alignment of the coils and minimizing parasitic capacitance between them. When entirely intertwined, the typical coefficient coupling K is in the range of 0.3-0.9. 
         [0024]      FIG. 2(   d ) illustrates a performance comparison between an embodiment of the present invention and a conventional C-L-C filter. The steep attenuation and the initial notch that has greater attenuation than the conventional C-L-C circuit are noticeable and advantageous characteristics. 
         [0025]      FIG. 2(   e ) illustrates performance characteristics at a different notch frequency using the same circuit topology as in  FIG. 2(   a ), according to the present invention. The circuit values for this stop band frequency are provided in the Table 1 above. 
         [0026]      FIGS. 3(   a )-( b ) illustrates the same circuit as in  FIG. 2(   a ), other than that the inductors  240  and  250  are uncoupled, and as such  FIG. 3(   a ) shows the performance curve for the circuit, and a physical layout of the circuit is shown in  FIG. 3(   b ). The circuit values for this comparison circuit are provided at Table 1 above as well. This circuit occupies a significantly larger area, as there is a non-overlapping inductor layout, and there is also a larger self-inductance value for each of the inductors  240  and  250 . 
         [0027]      FIGS. 4(   a )-( b ) illustrate another embodiment of a coupled notch circuit according to the present invention that has multiple notches and the performance characteristics associated therewith. Circuit elements are identified in  FIG. 4(   a ), with an input  410 , an output  420 , and a ground  430 , each of which, as shown, have associated therewith a parasitic resistance and a parasitic inductance. One of the inductors  440   a  and  450   a  within the coupled inductors pairs  440   a/b  and  450   a/b , each having a parasitic resistance associated therewith, are connected between the input  410  and the output  420 , and connected together at Node  1 . The other inductor  440   b  is connected between node  1  and the ground, in series with capacitors  470 ; the inductor  450   b  is connected between output and the ground, in series with capacitors  474 ; while capacitor  476  is directly connected between Node  1  and ground  430 . An example of values for the circuit elements is provided in the Table 2. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 FIG. 4(a) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 input 410 
                 0.3 ohm 
               
               
                   
                 resistance 
               
               
                   
                 input 410 
                 1 nH 
               
               
                   
                 inductance 
               
               
                   
                 output 420 
                 0.3 ohm 
               
               
                   
                 resistance 
               
               
                   
                 output 420 
                 1 nH 
               
               
                   
                 inductance 
               
               
                   
                 ground 430 
                 0.05 ohm 
               
               
                   
                 resistance 
               
               
                   
                 ground 430 
                 15 pH 
               
               
                   
                 inductance 
               
               
                   
                 inductor 440a 
                 20 nH; 
               
               
                   
                   
                 10 ohm 
               
               
                   
                 inductor 440b 
                 8 nH; 3 ohm 
               
               
                   
                 inductor 450a 
                 50 nH; 
               
               
                   
                   
                 30 ohm 
               
               
                   
                 inductor 450b 
                 1 nH; 0.5 ohm 
               
               
                   
                 capacitor 270 
                 2 pF 
               
               
                   
                 capacitor 272 
                 10 pF 
               
               
                   
                 capacitor 274 
                 1 pF 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    In this embodiment, which is also referred to as a 5-pole circuit, rather than the 3 pole circuit of  FIG. 2(   a ), different filtering characteristics are achieved. In addition to more notches to substantially attenuate the frequencies corresponding to the notches, as well as creates a wide stop-band corresponding to the line  490  that shows attenuation of at least the corresponding predetermined dB amount as shown by the use of additional notches. 
         [0029]      FIG. 5(   a ) illustrates an embodiment of a coupled inductor notch circuit  500  according to the present invention, and  FIG. 5(   b ) the performance curve. As shown, the modification is an a L-C filter (instead of C-L-C) using the same concept of coupled inductors to enhance stopband performance. Although the performance of this circuit is not as good as C-L-C filter of  FIG. 2(   a ) with notch, it can be used within the context and scope of the present invention. As shown, the circuit includes an input  510 , an output  520 , and a ground  530 , each of which, as shown, have associated therewith a parasitic resistance and a parasitic inductance. Coupled inductors  540  and  550 , each having a parasitic resistance associated therewith as shown, with inductor  540  being in series with resistor  560  between the input  510  and the output  520 , and inductor  550  being in series with capacitor  572 . Capacitors  576  is also connected between output  520  and ground, respectively, as shown. An example of values for the circuit elements is provided in the Figure. 
         [0030]    The present invention allows for significant space savings in discrete semiconductor filter circuits, which may or may not also include ESD protection, as well as in circuits formed on multi-layer technologies, which include not only semiconductor technologies, but also ceramic technologies and others. The inductor, in addition to being formed in a planar manner as shown, can also be spiral or other configurations. 
         [0031]    Although the present invention has been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that various changes, modifications and substitutes are intended within the form and details thereof, without departing from the spirit and scope of the invention. Accordingly, it will be appreciated that in numerous instances some features of the invention will be employed without a corresponding use of other features. Further, those skilled in the art will understand that variations can be made in the number and arrangement of components illustrated in the above figures. It is intended that the scope of the appended claims include such changes and modifications.