Abstract:
A notched SAW image frequency rejection filter system includes a SAW filter having an input, an output and a ground output and an impedance matching network including a first matching inductance connected to the SAW filter output and a second matching inductance connected to the ground output of the SAW filter; the SAW filter having an inherent internal capacitance that produces a predetermined capacitive leakage current at the image frequency; an inherent internal inductance that produces an inductance leakage current at the image frequency; and a boosted inherent parasitic ground inductance at the ground output of the SAW filter for generating a voltage across the second matching inductance to produce a compensation current which is substantially opposite in phase and substantially matched in magnitude with the capacitive leakage current for reducing the capacitive leakage current and increasing the image frequency rejection.

Description:
FIELD OF THE INVENTION 
     This invention relates to a notched SAW image frequency rejection filter system. 
     BACKGROUND OF THE INVENTION 
     Superheterodyne radio receivers achieve good sensitivity and selectivity by mixing the signal received by the antenna (which has a frequency f RF ) with a local oscillator (which has a frequency f LO ). What is meant by mixing is that the two are multiplied together. Mathematically, this results in the production of signals at additional frequencies: for example at the sum of the two frequencies and at the difference between the two frequencies. It is the signal at the difference of the two frequencies f IF  which is important as it is a much lower frequency and thus can be filtered more easily. The problem with this method is that there are two frequencies at the antenna which produce the same difference frequency: one is higher in frequency than f LO  and one is lower in frequency than f LO . Since only one of these is desired to be received, the other (called the image) must be filtered out prior to the mixing operation. It is convenient for the difference frequency f IF  to be as low as possible. For this reason, it has become common to use SAW filters to filter out the image because of their sharp filtering properties. However, the ultimate attenuation which SAW filters provide is usually not enough and other filtering must be added to supplement them, which forces f IF  to be higher than desirable. One possible solution is to use multiple stages of SAW filtering (and amplification to overcome the loss in the SAW filter) but this increases size and cost and degrades the intermodulation rejection properties of the receiver. Another possible solution is to use an image reject mixer, but this also increases size (unless it is integrated into a chip), cost and current consumption. 
     SUMMARY OF THE INVENTION 
     In accordance with various aspects of the subject invention in at least one embodiment the invention presents an improved notched SAW image frequency rejection filter system which may use only a single stage filter preserving smaller size and lower cost while attaining a substantial increase in filter response and low IF. 
     The subject invention results from the realization, in part that, an improved notched SAW image frequency rejection filter system in various aspects can be achieved by adding external coupling between the input and output pins of the SAW filter to counteract the internal coupling such that at the image frequency the external coupling is the same level but opposite in phase to the internal coupling, thus resulting in cancellation of coupling at the image frequency, and thus a much higher ultimate attenuation at the image frequency. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features a notched SAW image frequency rejection filter system including a SAW filter having an input, an output and a ground output, and an impedance matching network including a first matching inductance connected to the SAW filter output and a second matching inductance connected to the ground output of the SAW filter. The SAW filter has an inherent internal capacitance that produces a predetermined capacitive leakage current at the image frequency; an inherent internal inductance that produces an inductance leakage current at the image frequency; and a boosted inherent parasitic ground inductance at the ground output of the SAW filter for generating a voltage across the second matching inductance to produce a compensation current which is substantially opposite in phase and substantially matched in magnitude with the capacitive leakage current for reducing the capacitive leakage current and increasing the image frequency rejection. 
     In preferred embodiments the compensation current may be greater than the capacitive leakage current and the system further may include a boost leakage capacitance connected across the SAW filter input and output for increasing the capacitive leakage current to match the compensating current. The SAW input and output may include input and output terminal pins, respectively, and the spacing, length and diameter of the terminal pins may determine the value of the boost leakage capacitance. The boost inductance value may be determined by the length and widths of the conductor connecting the boost ground inductance to ground. The SAW filter may be implemented in a surface mount construction and the boost ground inductance value may be determined by the length and width of the conductor connecting the boost ground inductance to ground. The SAW filter may be implemented in a surface mount construction and the boost capacitance may be determined by boost capacitance etched conductors length and spacing. 
     This invention also features a notched SAW image frequency rejection filter system including a SAW filter having an input, an output and a ground output and an impedance matching network including a first matching inductance connected to the SAW filter output and a second matching inductance connected to the ground output of the SAW filter. The SAW filter has an inherent internal capacitance that produces a predetermined capacitive leakage current at the image frequency, an inherent internal inductance that produces an inductance leakage current at the image frequency, and an inherent parasitic ground inductance at the ground output of the SAW filter for generating a voltage across the second matching inductance which has a reduced inductance value for producing a compensation current which is substantially opposite in phase and substantially matched in magnitude with the capacitive leakage current for reducing the capacitive leakage current and increasing the image frequency rejection 
     In preferred embodiments the compensation current may be greater than the capacitive leakage current and the system may further include a boost leakage capacitance connected across the SAW filter input and output for increasing the capacitive leakage current to match the compensating current. The SAW input and output may include input and output terminal pins, respectively, and the spacing length and diameter of the terminal pins may determine the value of the boost leakage capacitance. The parasitic ground inductance value may be determined by the length and width of the conductor connecting the parasitic ground inductance to ground. The SAW filter may be implemented in a surface mount construction and the parasitic ground inductance value may be determined by the length and width of the conductor connecting the parasitic ground inductance to ground. The SAW filter may be implemented in a surface mount construction and the boost capacitance may be determined by boost capacitance etched conductors lengths and spacings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a prior art SAW filter system including a matching circuit; 
         FIG. 2  is the band pass filter response for the system of  FIG. 1  and showing the improved notched response achievable with a notched SAW image frequency rejection filter system according to this invention; 
         FIG. 3  is a schematic diagram of a notched SAW image frequency rejection filter system according to this invention; 
         FIG. 4  illustrates the waveforms of input voltage, capacitive leakage current and compensation current achieved according to this invention; 
         FIG. 5  is a schematic, diagrammatic, side, sectional view of a through hole or leaded package implementation of a notched SAW image frequency rejection filter system according to this invention; and 
         FIG. 6  is a schematic, diagrammatic plan view of a surface mount implementation of a notched SAW image frequency rejection filter system according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     This invention describes a technique for improving the attenuation of a SAW filter at the image frequency allowing the use of a low f IF  with only one SAW filter required to filter out the image. The SAW filter is a very well known filter device using surface acoustic wave technology for the filtering. Assuming that the SAW filter does not have any spurious responses at the image frequency the ultimate attenuation of the SAW is determined by the coupling which occurs within the package of the SAW filter. In accordance with this invention external coupling between the input and the output of the SAW filter is added to counteract the internal coupling at the image frequency. The external coupling is at substantially the same level and is substantially opposite in phase to the internal coupling. This results in a substantial offset or cancellation of the coupling at the image frequency and a much higher ultimate attenuation at the image frequency. 
     SAW filters typically require a high inductive impedance at their input and output in order to have minimum loss and good filtering characteristics. 
     There is shown in  FIG. 1  a conventional SAW filter  10  whose input  12  impedance is typically 225 ohms-j375 ohms and whose output  14  is connected to a matching circuit  16  including, for example, matching inductances  18  and  20 . Inductance  20  is connected to ground at  22 . SAW filter  10  has a ground connection  24  as well. 
     A band pass filter response  30 ,  FIG. 2 , for a conventional SAW filter peaks at  32  at the RF frequency, f RF    34 , with only a small insertion loss of typically 60 dB. The local oscillator frequency, f LO , appears at  36  and the image frequency, f image , appears at  38 . The local oscillator frequency  36  is separated from the RF frequency  34  by a specific difference frequency indicated at  40 . At any given time the signals on the antenna are likely to include a frequency which is the same distance  40 ′ below the local oscillator frequency f LO    36  as the RF, frequency f RF    34  is above it. This second frequency is known as the image frequency or f image    38 . The presence of this second, image, frequency  38  interferes with proper filtering and processing and for this reason it is sought to be suppressed as much as possible. As can be seen in  FIG. 2  at  42  the suppression is significant, in the prior art, approximately 50 dB down. However, in accordance with this invention, a further suppression of the image frequency can be effected as indicated at  44  where, with the notch afforded by this invention, the attenuation of the image frequency is down approximately another 20 dB or more. 
     One embodiment of the notched SAW image frequency rejection filter system  50 ,  FIG. 3 , according to invention includes a SAW filter  52  and matching circuit  54  which feed a load  56 , typically a 50 ohm impedance load. SAW filter  52  is not shown in detail as it is a well known device, a surface acoustic wave filter. What is shown inside the box is simply the equivalent circuit indicating the inherent internal capacitance  56  and the inherent internal leakage inductance  58 . Also associated with SAW filter  52  is an inherent parasitic ground inductance  60 . Impedance matching circuit  54  includes matching inductances  62  and  64 . SAW filter  52  receives voltage v from some source  66  at the SAW input  68  referenced to common ground reference  73  and provides a signal at its output  70 . Also associated with SAW filter  52  is a ground output  72 . Inherent internal capacitance  56  provides a leakage current i s    74  through inductance  62  which limits the ultimate attenuation in prior art devices. However, in system  50  according to an embodiment of this invention the leakage current from inherent internal inductance  58  occurring at ground output  72  develops a voltage across inherent parasitic ground inductance  60 . This voltage appears across matching inductance  64  and develops a compensation current i c    76 . In one embodiment ground inductance  60  would be sought to be decreased as much as possible. However, in this embodiment ground inductance  60  is increased in order to increase the voltage across matching inductance  64  to increase current i c    76  so that it generally matches capacitive leakage current i s    74 . This results in the increased drop  44 ,  FIG. 2 , at the notch providing the additional attenuation of 20 dB or more. 
     Alternatively, instead of increasing ground inductance  60 , matching inductance  64  may be decreased. However, there are limitations to how far matching inductance  64  can be decreased. For it is a part of impedance matching circuit  54  and it is limited by the necessity to perform the matching function required of it. For if it does not, the band pass response will be compromised. If compensation current i c    76  is less than required then either approach can be increased, that is, matching inductance  64  can be decreased if permissible, or ground inductance  60  may be increased. However if the matching is over compensating, that is the compensation current i c    76  is greater than the capacitive leakage current i s    74  then a boost capacitance  80  can be added to more nearly balance the current. The current from inherent internal inductance  58  is typically much greater than that required to compensate for capacitive leakage current i s    74 , but that current is applied as indicated to ground inductance  60  which develops a voltage sufficient to develop the offsetting compensation current i c    76  through matching inductance  64 . This compensation current i c    76  nicely offsets the capacitive leakage current i s    74  with the values of capacitance and inductance shown in  FIG. 3  at the image frequency, e.g. 172 MHz. 
     Compensation current i c    76 ,  FIG. 4 , typically is substantially out of phase with capacitive leakage current  74 . Since a SAW filter is a narrow band device there is no need for the matching networks to be wideband. The problem is to get the magnitude of compensation current i c    76  high enough to offset the capacitive leakage current i s    74 . As can be seen the capacitive leakage current i s    74  is a leading current with respect to the input voltage v whereas the compensation current i c    76  lags voltage v. 
     The invention may be embodied in a through hole or leaded package as shown in  FIG. 5  where similar parts have been given like numbers accompanied by a lower case a There it can be seen that the inherent internal parasitic inductance is embodied in the leads  58   a  as they extend further through conductors  58   aa  to output  72   a . Boost ground inductance  60   a  is implemented by a conductor that connects from output  72   a  to ground output  73   a . The boost capacitance  80   a  is implemented by the input  68   a  terminal pin and output terminal pin  70   a . It is the length and the diameter of terminal pins  68   a  and  70   a  which determines the area of the capacitor “plates” and thus the capacitance. Making them longer or larger will increase the capacitance of the boost capacitance  80   a . In this way, with only minor changes the system can be tuned to provide the notched increased attenuation illustrated by  44  in  FIG. 2 . By simply lengthening or shortening or enlarging or slimming terminal pins  68   a  and  70   a  boost capacitance  80   a  can be controlled. Likewise by tailoring the length and width of conductors  58   a  and  58   aa  the inherent internal parasitic conductance can be varied. The ground inductance  60   a  can be varied by varying the length and width of conductor  60   a . The inductance increases as the length of the conductor  60   a  is increased and decreases as the width of the conductor is increased. 
     The invention is not limited to a through hole and lead implementation or any particular implementation. For another example, in a surface mount application,  FIG. 6 , a notched SAW image frequency rejection filter system according to this invention can be implemented in a surface mount application. As shown by system  50   b ,  FIG. 6 , where similar parts have been given like numbers accompanied by a lower case b. Again, the inherent internal inductance can be implemented by leads  58   b  as well as  58   bb  and ground inductance  60  can be implemented by leads  60   b . Boost capacitance  80   b  may be implemented using leads  80   bb  and  80   bb ′ whose length, width and spacing are used to tailor the value of boost capacitance  80   b.    
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.