PATENT ABSTRACT
A hybrid diplex bandstop filter includes a lowpass filter circuit which passes a first range of frequencies and a highpass filter circuit passes a third range of frequencies. The filter blocks a second range of frequencies. The third range of frequencies is higher than the second range and the first range. A tuning circuit tunes at least one re-resonance of a frequency inside the first range of frequencies to either within the second range of frequencies or outside an industry specified upper limit which is in the third range of frequencies.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of diplex filters used in cable television systems, and more particularly to a low pass diplex filter which reduces high pass resonance. 
       BACKGROUND OF THE INVENTION 
       [0002]    A diplex circuit, or more simply a “diplexer,” is a device which separates or combines RF signals. Diplexers are used in connection with CATV equipment in a number of situations, some of which use two diplexers back-to-back. These include step attenuators, power bypass circuits, cable simulators and equalizer circuits. Many of the prior art CATV diplex circuits are used to act on signals traveling in so-called forward and return paths, e.g., relatively high frequency RF signals pass from a source of such signals to a television set at subscriber premises in the forward direction over one leg of the circuit while lower frequency (DC) signals pass from the premises in the return direction. 
         [0003]    Bandstop or “notch” filters are commonly employed in the CATV industry to block transmission of signals in a specified frequency range. For example, certain channels may be designated as premium channels, requiring payment of a fee from the subscriber in order to receive the signals carrying information representing such channels. If the service is not ordered, i.e., the fee is not paid, an appropriate filter is installed in the cable line coming into the non-paying premises. This is but one of the more traditional uses of bandstop filters, i.e., as a so-called trap. A more recent example is the aforementioned use in the handling of forward and return path signals between a head end and subscriber facilities. 
         [0004]    Using typical minimum inductor or capacitor design techniques in the lowpass leg of a diplex bandstop filter creates a problem in achieving a flat upper passband, thus causing return loss and increased delay. The capacitor and coil combination that is used to make up the low frequency lowpass filter contains capacitors and coils that are extremely large in value. In fact, the values are large enough to create multiple re-resonances in the upper passband of the highpass leg of the filter, which are undesirable. See, for example,  FIG. 1 . 
       SUMMARY OF THE INVENTION 
       [0005]    Briefly stated, a hybrid diplex bandstop filter includes a lowpass filter circuit which passes a first range of frequencies and a highpass filter circuit passes a third range of frequencies. The filter blocks a second range of frequencies. The third range of frequencies is higher than the second range and the first range. A tuning circuit times at least one re-resonance of a frequency inside the first range of frequencies to either within the second range of frequencies or outside an industry specified upper limit which is in the third range of frequencies. 
         [0006]    According to an embodiment of the invention, a hybrid diplex bandstop filter includes means for allowing a first range of frequencies to pass through the filter from an input to an output; means for blocking a second range of frequencies from passing through the filter from the input to the output, wherein the second range of frequencies is higher than the first range of frequencies; means for allowing a third range of frequencies to pass through the filter from the input to the output; wherein the third range of frequencies is higher than the second range of frequencies; and tuning means for tuning at least one re-resonance of a frequency within the first range of frequencies to either within the second range of frequencies or outside an industry specified upper limit within the third range of frequencies. 
         [0007]    According to an embodiment of the invention, a hybrid diplex bandstop filter includes a lowpass filter circuit which allows a first range of frequencies to pass through the filter from an input to an output; a bandstop circuit which blocks a second range of frequencies from passing through the filter from the input to the output, wherein the second range of frequencies is higher than the first range of frequencies; a highpass filter circuit which allows a third range of frequencies to pass through the filter from the input to the output, wherein the third range of frequencies is higher than the second range of frequencies; and a tuning circuit which tunes at least one re-resonance of a frequency within the first range of frequencies to either within the second range of frequencies or outside an industry specified upper limit within the third range of frequencies. 
         [0008]    According to an embodiment of the invention, a method of manufacturing a hybrid bandstop filter includes the steps of: (a) making a lowpass filter circuit which allows a first range of frequencies to pass through the filter from an input to an output; (b) making a bandstop filter circuit which prevents a second range of frequencies from passing through the filter from the input to the output; wherein the second range of frequencies is higher than the first range of frequencies; (c) making a highpass filter circuit which allows a third range of frequencies to pass through the filter from the input to the output, wherein the third range of frequencies is higher than the second range of frequencies; and (d) making a tuning circuit which tunes at least one re-resonance of a frequency within the first range of frequencies to either within the second range of frequencies or outside an industry specified upper limit within the third range of frequencies. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a frequency performance chart of a typical diplex filter. 
           [0010]      FIG. 2  shows a hybrid low pass diplex filter according to a first embodiment of the invention. 
           [0011]      FIG. 3  shows a hybrid low pass diplex filter according to a second embodiment of the invention. 
           [0012]      FIG. 4  shows a hybrid low pass diplex filter according to a third embodiment of the invention. 
           [0013]      FIG. 5  shows a hybrid low pass diplex filter according to a fourth embodiment of the invention. 
           [0014]      FIG. 6  shows a hybrid low pass diplex filter according to a fifth embodiment of the invention. 
           [0015]      FIG. 7  shows a hybrid low pass diplex filter according to a sixth embodiment of the invention. 
           [0016]      FIG. 8  shows a hybrid low pass diplex filter according to a seventh embodiment of the invention. 
           [0017]      FIG. 9  shows a hybrid low pass diplex filter according to a eighth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring to  FIG. 1 , the typical prior art minimum inductor or minimum capacitor design in the lowpass leg of a diplex bandstop filter creates a problem in achieving a flat upper passband, thus causing poor return loss and increased delay. This problem arises because the capacitors and coils used in combination to make the low frequency lowpass leg are extremely large in value, causing multiple re-resonances in the upper passband of the highpass leg of the filter. For example, the primary resonance at around 55.25 MHZ, as identified by reference numeral  12 , causes a re-resonance at around 550 MHz as identified by reference numeral  14 . 
         [0019]    As known in the art, the lowpass leg of a diplex bandstop filter is one of four types: (1) minimum capacitor filter, (2) minimum inductor filter, (3) minimum inductor elliptic function filter, and (4) minimum capacitor elliptic function filter. In the present invention, a hybrid filter is defined as a filter which is a hybrid of at least two of these four filter types. 
         [0020]    Referring to  FIG. 2 , an embodiment of the present invention includes a hybrid diplex filter  20  with an upper passband leg  30  and a lower passband leg  40 . Resonance tanks  42  and  44  in series are indicative of a minimum inductance elliptic function filter, whereas an LC combination  46  (which consists of an inductor  45  and a capacitor  47 ) is indicative of a minimum inductance filter. The combination of a capacitance shunt  48  with the LC combination  46  forms a typical lowpass Pi filter. In the present invention, inductors  50   a,    50   b,  and  50   c  are added in series with capacitance shunts  48   a,    48   b,  and  48   c,  respectively, to locate the re-resonance of filter  20  higher than any one of the four types of typical filters (minimum capacitance, minimum inductance, minimum inductance elliptic, and minimum capacitance elliptic) can by itself. The goal is to locate the re-resonances outside whatever the current industry specified upper limit of usable bandwidth in the cable industry is. For example, the first industry specified upper limit was 216 MHz; at the present time the industry specified upper limit is 860 MHz in the United States, but some systems go to 1.0 Ghz. The industry specified upper limit for Japan is 2.185 GHz because of the manner in which satellite and cable are combined. As far as is known, the techniques of the present invention can be used to tune at least one re-resonance from a frequency within the lowpass filter range to outside the industry specified upper limit, no matter what the upper limit is. 
         [0021]    The circuit of this embodiment is useful when the lowpass filter has a cutoff below 200 MHz. The circuit of this embodiment has fewer parasitics than the prior art designs, because it breaks up the circuit so that the loading is less. 
         [0022]    Note that the circuit of lower passband leg  40  is symmetrical about a middle capacitance shunt  52 , so description concerning the right half of lower passband leg  40  is omitted. Additional sections can be added in pairs, i.e., on the right side and on the left side of lower passband leg  40  to make the filter sharper. For example, the section on the left side would consist of another inductor in series with a shunt capacitor similar to the capacitance shunt  48   a  and inductor  50   a  combination but connected at a point  49 , with another inductor similar to inductor  45  connected between point  49  and a point  51 . A symmetrical section would also be added on the right side of the circuit. 
         [0023]    Referring to  FIG. 3 , a hybrid diplex filter  60  includes upper passband leg  30  and a lower passband leg  70 . Lower passband leg  70  is similar to lower passband leg  40  of  FIG. 2 , but has tuning capacitors  62   a,    62   b,  and  62   c  across inductors  50   a,    50   b,  and  50   c,  respectively. Adding tuning capacitors  62   a,    62   b,  and  62   c  to lower passband leg  70  in this manner adds capacitance to inductors  50   a,    50   b,  and  50   c  and lowers the resonance of inductors  50   a,    50   b,  and  50   c.  This embodiment moves the re-resonance lower into the stop band of filter  60 . 
         [0024]    Referring to  FIG. 4 , a hybrid diplex filter  80  includes upper passband leg  30  and a lower passband leg  90 . Lower passband leg  90  is similar to lower passband leg  70  of  FIG. 3 , but includes tuning inductors  82   a  and  82   b  which tune tank circuits  84   a  and  84   b,  respectively, down into the stop band by increasing the loading inductance. 
         [0025]    Referring to  FIG. 5 , a hybrid diplex filter  100  includes upper passband leg  30  and a lower passband leg  110 . Lower passband leg  110  is similar to lower passband leg  90  in  FIG. 4  in that it includes tuning inductors  82   a  and  82   b  which tune tank circuits  84   a  and  84   b,  respectively, as well as tuning capacitors  62   a,    62   b,  and  62   c,  but differs from the previous embodiments in that it includes additional tuning capacitors  92   a,    92   b  which have the effect of further tuning the waveform produced by lower passband leg  110  and making the waveform sharper. 
         [0026]    Referring to  FIG. 6 , a hybrid diplex filter  120  includes upper passband leg  30  and a lower passband leg  130 . Lower passband leg  130  is similar to lower passband leg  70  of  FIG. 3  but with capacitors  94   a,    94   b  added in parallel with inductors  45   a,    45   b,  which makes inductors  45   a,    45   b  a notch filter section instead of a lowpass filter section. This embodiment makes the passband to stopband transition produced by the embodiment of  FIG. 3  sharper. 
         [0027]    Referring to  FIG. 7 , a hybrid diplex filter  140  includes upper passband leg  30  and a lower passband leg  150 . Lower passband leg  150  is similar to lower passband leg  130  of  FIG. 6  but with more tanks to ground, i.e., by adding tanks  96   a,    96   b,  which makes the passband to stopband transition sharper by tuning the re-resonance into the stop band. 
         [0028]    Referring to  FIG. 8 , a hybrid diplex filter  160  includes upper passband leg  30  and a lower passband leg  170 . Lower passband leg  170  is similar to lower passband leg  90  of  FIG. 4  but with tuning inductors  98   a,    98   b  added to tank circuits  102   a,    102   b  respectively to further sharpen the passband to stopband transition produced by the embodiment of  FIG. 4 . 
         [0029]    Referring to  FIG. 9 , a hybrid diplex filter  180  includes upper passband leg  30  and a lower passband leg  190 . Lower passband leg  190  is similar to lower passband leg  170  of  FIG. 8  but with additional tank circuits  104   a,    104   b  to ground, which has the effect of tuning more re-resonances down into the stop band. 
         [0030]    While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.