Abstract:
A phase cancellation circuit for a cavity filter including a sampler loop assembly arranged to receive an input signal, a variable loop assembly connected to the sampler loop assembly by a cable, wherein the variable loop assembly is arranged to transmit an output signal from cavity filter, wherein the sampler loop assembly samples a cancellation signal at an isolation frequency from the input signal and transmits the cancellation signal to the variable loop assembly via the cable, and, wherein the cable has a length equal to a multiple of a half-wavelength at the desired isolation frequency, wherein the cancellation signal undergoes a 180° phase shift by traveling through the cable, wherein the variable loop assembly combines the cancellation signal with the input signal to cancel the input signal at the isolation frequency due to the 180° phase shift for creating the output signal with a notch at the isolation frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/003,024, filed Nov. 14, 2007, which application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to filters used in the field of RF communication, more specifically to cavity filters, and even more specifically to phase cancellation circuits for bandpass cavity filters. 
     BACKGROUND OF THE INVENTION 
     A band pass cavity filter preferably passes one narrow band of frequencies and attenuates all others with increasing attenuation above and below the pass frequency. Band pass filters are ideal when the interfering frequencies are not known to any degree of accuracy or when very broadband filtering is needed. Collectively, the frequencies that are allowed to pass are called the pass band. 
     Bandpass filters are well known in the art. An example of a bandpass cavity filter is shown in  FIG. 1 . Cavity filter  2  includes a hollow cylindrical body  3  which has a coarse tuning rod  4  and a fine tuning rod  5 . Apertures  6  are arranged with screws  7  for securing loop assemblies  8  to the cavity filter. When properly installed, the loops of loop assemblies  8  are contained in the cavity filter, while connectors, jacks, or receptacles  9  protrude out from the cavity filter. Connectors  9  are generally bi-directional, and connect to coaxial cables for either receiving or transmitting signals to or from the filter. 
     However, communications equipment, receivers particularly, may require additional isolation at specific frequencies. One solution to solve this problem is to connect separate notch filters to the bandpass filter to provide isolation at each of the desired frequencies. However, this is not always ideal as it requires multiple filters to be used, which can be costly, and requires a substantial amount of additional space to install extra cavity filters. 
     Thus, what is needed is a device to be used with a bandpass cavity filter to provide isolation at desired frequencies without the need for multiple cavity filters. What is also needed is such a device that is tunable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention broadly comprises a phase cancellation circuit for a cavity filter including a sampler loop assembly arranged to receive an input signal, a variable signal insertion loop assembly connected to the sampler loop assembly by a cable, wherein the variable signal insertion loop assembly is arranged to transmit an output signal from cavity filter, wherein the sampler loop assembly samples a cancellation signal at an isolation frequency from the input signal and transmits the cancellation signal to the variable signal insertion loop assembly via the cable, and, wherein the cable has a length equal to a multiple of a half-wavelength at the desired isolation frequency, wherein the cancellation signal undergoes a 180° phase shift by traveling through the cable, wherein the variable signal insertion loop assembly combines the cancellation signal with the input signal to cancel the input signal at the isolation frequency due to the 180° phase shift for creating the output signal with a notch at the isolation frequency. 
     In one embodiment, the cavity filter is a bandpass cavity filter. In another embodiment, the sampler loop assembly comprises a bulkhead connector for receiving the input signal, a connector block secured to the bulkhead connector, wherein the connector block is hollow and includes an aperture for enabling insertion of the cable into the connector block, a loop ring secured to the connector block opposite from the bulkhead connector, and a loop extending out of the connector block, wherein a first end of the loop is affixed to a center pin of the bulkhead connector and a second end of the loop is grounded to the loop ring. In a further embodiment, a first end of the cable is stripped to a center conductor wire of the cable, and the center conductor wire is affixed to the center pin of the bulkhead connector and to the loop. In yet a further embodiment, a rigid sleeve is affixed to the cable proximate to the first end of the cable, and wherein the rigid sleeve is positioned in the aperture and clamped to the connector block by a set screw. 
     In one embodiment, the variable signal insertion loop assembly comprises a bulkhead connector for transmitting an output signal, a connector block secured to the bulkhead connector, wherein the connector block is hollow and includes an aperture for enabling insertion of the cable into the connector block, a loop ring secured to the connector block opposite from the bulkhead connector, a coupling tube housed within the connector block, wherein the coupling tube includes a hole for partial insertion of a central pin of the bulkhead connector into the coupling tube, wherein the couple tube rests on a non-conductive spacer that separates the bulkhead connector from contacting the coupling tube, a dielectric ring concentrically arranged within the coupling tube, and a loop extending out of the connector block, wherein a first end of the loop is affixed to a the coupling tube and a second end of the loop is grounded to the loop ring. 
     In a further embodiment a second end of the cable is stripped down to a center conductor wire of the cable, wherein the center conductor wire is fitted with a voltage probe, and wherein the voltage probe is inserted through the aperture in the block connector and concentrically positioned within the dielectric tube for creating a field coupling between the coupling tube and the voltage probe. In yet a further embodiment a rigid sleeve is affixed to the cable proximate to the second end of the cable, and wherein the rigid sleeve is positioned in the aperture and clamped in place by a shaft lock arranged about the aperture. 
     The current invention also broadly comprises a method of creating a notch in a signal at a desired isolation frequency transmitted through a cavity filter including the steps of (a) receiving an input signal with a cavity filter, (b) sampling a cancellation signal from the input signal at the desired isolation frequency, (c) shifting the cancellation signal 180° by transmitting the cancellation signal through a cable having a length equal to a multiple of a half-wavelength at the desired isolation frequency, (d) combining the cancellation signal with the input signal for cancelling the input signal at the desired isolation frequency, thereby creating an output signal that substantially resembles the input signal, but with a notch at the desired isolation frequency, and (e) transmitting the output signal from the cavity filter. 
     In a further embodiment a sampler loop assembly of the cavity filter receives the signal in step (a), and the sampler loop assembly samples the cancellation signal in step (b), a variable signal insertion loop assembly combines the cancellation signal with the signal in step (d), and the sampler loop assembly, the variable signal insertion loop assembly and the cable comprise a phase cancellation circuit. In another embodiment the method further includes the step of: (f) tuning the phase cancellation circuit by altering a position of a voltage probe with respect to a coupling tube housed within a connector block of the variable signal insertion loop assembly, wherein the voltage probe is affixed to one end of the cable. In one embodiment step (f) occurs before step (a). 
     The current invention also broadly comprises a cavity filter including a phase cancellation circuit for a cavity filter comprising a sampler loop assembly operatively arranged to receive an input signal to the cavity filter, a variable signal insertion loop assembly connected to the sampler loop assembly by a cable, wherein the variable signal insertion loop assembly is operatively arranged to transmit an output signal from cavity filter, wherein the sampler loop assembly samples a cancellation signal at a desired isolation frequency from the input signal and transmits the cancellation signal to the variable signal insertion loop assembly via the cable, and wherein the cable has a length, and the length is equal to a multiple of a half-wavelength at the desired isolation frequency, wherein the cancellation signal undergoes a 180° phase shift by traveling over the length of the cable, wherein the variable signal insertion loop assembly combines the cancellation signal with the input signal to cancel the input signal at the isolation frequency due to the 180° phase shift for creating the output signal, and wherein the output signal substantially resembles the input signal but with a notch at the desired isolation frequency. In one embodiment, the cavity filter is a bandpass filter. 
     These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
         FIG. 1  is a perspective view of a typical bandpass filter; 
         FIG. 2  is a side view of a phase cancellation circuit according to the current invention; 
         FIG. 3  is an exploded perspective view of the phase cancellation circuit shown in  FIG. 2 ; 
         FIGS. 4A and 4B  are side views of a sampler loop assembly including showing two alternative positions for grounding a loop of the loop assembly to a loop ring of the loop assembly; 
         FIG. 5  is a diagram of the performance of a theoretical band pass cavity filter including the current invention phase cancellation circuit and grounded as shown in  FIG. 4B ; and, 
         FIGS. 6-8  are diagrams of the performance of a theoretical cavity filter including the current invention phase cancellation circuit and grounded as shown in  FIG. 4A , as a position of a voltage probe of the phase cancellation circuit is incrementally moved. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. Also, the adjectives, “top,” “bottom,” “right,” “left,” and their derivatives, in the description herebelow, refer to the perspective of one facing the invention as shown in the figure under discussion. 
     Furthermore, it should be understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It should also be understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
       FIG. 2  shows a side view of phase cancellation circuit  10 , while  FIG. 3  shows an exploded perspective view of the phase cancellation circuit. The following description should be read in view of  FIGS. 2 and 3 . Phase cancellation circuit  10  is used in conjunction with a cavity filter, such as cavity filter  2  shown in  FIG. 1 . Phase cancellation circuit  10  includes sampler loop assembly  14  and variable signal injection loop assembly  16  connected by coaxial cable  18 . As will be discussed in more detail infra, the length of cable  18  is equal to a half-wavelength of a signal at the frequency that isolation of the signal is desired. The loop assemblies in  FIG. 2  are shown so that the inner components of the assemblies are illustrated in dashed or hidden lines. 
     In one embodiment, the cavity filter is cylindrical, such as cavity filter  2 , but it should be understood that the cavity filter could be rectangular or any other configuration known in the art. Taking cavity filter  2  in  FIG. 1 , for example, phase cancellation circuit  10  would be installed by replacing loop assemblies  8  with one of each of loop assemblies  14  and  16 . Alternatively stated, a cavity filter according to the current invention would almost exactly resemble cavity filter  2 , with the exception of loop assembles  14  and  16  being installed in apertures  6  instead of loop assemblies  9 . Loop assemblies  14  and  16  would be connected via cable  18 . 
     In a preferred embodiment, cable  18  connects variable signal loop assembly  16  to sampler loop assembly  14 . Cable  18  is preferably a 50-ohm coaxial cable, as is common in RF communication. In a preferred embodiment the cable is a readily purchasable, standard cable, made from a silver plated steel center conductor  18 A surrounded respectively by a dielectric layer, double-braided shield layer  18 B, and then jacketed in polytetrafluoroethylene. Cable  18  and its corresponding layers, particularly layers  18 A and  18 B, are illustrated in  FIGS. 2 and 3 . It should be apparent that other types of cables are known in the art, and this particular cable merely represents a preferred choice of cable. 
     In a preferred embodiment, connector block  20  is a hollow box. Aperture  21  enables cable  18  to access the interior of the connector block so the cable can be soldered to loop  26  and bulkhead connector  22 . The connector block also has four thru-holes  25 ′ on both the top and bottom of the block which are correspondingly threaded to receive screws  25 . 
     Sampler loop assembly  14  includes connector block  20  which is connected to bulkhead connector  22  and loop ring  24 . In a preferred embodiment the connector block is secured to each of bulkhead connector  22  and loop ring  24  by four screws  25 . However, it should be appreciated that screws are not necessary, and that in an alternate embodiment, a different securing means known in the art, such as welding or soldering, could be substituted. 
     Loop  26  is shown extending out of hole  27  in loop ring  24  and affixed to grounding pin  28 . In a preferred embodiment, grounding pin  28  is soldered to both loop ring  24  and loop  26 . Loop ring  24  includes four apertures  25 ″ for aligning with apertures  25 ′ and engaging with screws  25  to secure the loop ring to the connector block. Loop ring  24  also includes hole  43 ′ which is aligned with threaded bore  43  in connector block  20  for set screw  41 . In a preferred embodiment loop ring  24  and grounding pin  28  are silver plated brass. 
     In a preferred embodiment, sleeve  40  is soldered to double braided shield  18 B of cable  18  and secured to block  20  by set screw  41 . In a preferred embodiment set screw  41  is a 4-40 set screw. It should be appreciated that other varieties of cabling which do not include a double braided shield layer could be substituted for cable  18 , but that a double-braided cable is preferred. Furthermore, sleeve  40  could be secured to cable  18  by any other suitable method know in the art. 
     The end of loop  26  which extends inside of connector block  20  is soldered to connector pin  23  of bulkhead connector  22 . In a preferred embodiment, loop  26  is secured by soldering. Also, cable  18  is partially stripped so that a portion of the center conductor  18 A of cable  18  protrudes out of sleeve  40  into block connector  20 . This portion of center conductor  18 A of cable  18  is illustrated partially hooked around center pin  23 . In a preferred embodiment, the center conductor  18 A is soldered to loop  26  and connecting pin  23  of bulkhead connector  22 . 
     Variable signal injection loop assembly  16  includes connector block  30  which is connected to bulkhead connector  32  and loop ring  34  by a total of eight screws  35 . As discussed supra with respect to screws  25 , an alternate embodiment may include a different means for securing the connector block to the loop ring and bulkhead connector. Loop  36  is shown extending out of hole  37  in loop ring  34  connecting to grounding pin  38 . In a preferred embodiment, grounding pin  38  is soldered to loop ring  34  and loop  36 . It should be appreciated that loop ring  34  and grounding pin  38  are substantially identical to loop ring  24  and grounding pin  28 , with the exception of set screw hole  43  located in loop ring  24 . Likewise, loop ring  34  includes four apertures  35 ″ for aligning with apertures  35 ′ and engaging screws  25  to secure the loop ring to the connector block. 
     In a preferred embodiment, connector block  30  is a hollow rectangular box similar to connector block  20 . Aperture  31  is included to enable cable  18  and voltage probe  42  to access the interior of connector block  30  so the voltage probe can rest within dielectric tube  52  and create a field coupling with coupling tube  48 . The connector block also has four thru-holes  35 ′ on both the top and bottom of the block which are correspondingly threaded to receive screws  35 . 
     In a preferred embodiment, loop  36  is a piece of copper strap which must be bent into the loop shape and twisted 90 degrees so that the loop can be affixed to coupling tube  48 . In a preferred embodiment, loop  36  is soldered to the coupling tube, but it should be appreciated that other methods known in the art, such as bolting or screwing the loops to the coupling tube may also suffice. Coupling tube  48  has aperture  50  to provide access into the coupling tube from the central pin  33  on bulkhead connector  32 . Coupling tube  48  rests on spacer  52 . Spacer  52  is a non-conductive ring that surrounds central pin  33 , and in a preferred embodiment is made of polytetrafluoroethylene. Dielectric tube  54  is also non-conductive and fits inside coupling ring  48 . Voltage probe  42  is housed within dielectric tube  54  when voltage probe  42  is inserted into variable signal injection loop assembly  16  and held in place by sleeve  44  being clamped in shaft lock  46 . 
     Voltage probe  42  includes a longitudinal bore which allows the probe to fit over and center conductor  18 A of cable  18 . In a preferred embodiment the voltage probe is a portion of brass rod with copper and silver plating. In a preferred embodiment, probe coupling tube  48  is a hollow brass tube. As discussed supra, dielectric tube  54  fits snugly inside probe coupling tube  48 . The dielectric tube aids in the creation of a field coupling between the voltage probe and the coupling tube when a signal is passed through the filter. 
     One end of cable  18  is stripped of the jacketing material and double-braided shield  18 B is soldered directly to sleeve  40 . In a preferred embodiment sleeve  40  is partially inserted into aperture  21  in the side of block connector  20  and secured inside of connector block  20  with set screw  41 . The opposite end of cable  18  terminates in voltage probe  42 . Voltage probe  42  inserts inside of connector block  30 . Similar to sleeve  40 , sleeve  44  is soldered to the double-braided shield of cable  18 . Sleeve  44  in combination with shaft lock  46  enables voltage probe  42  to be variably inserted into connector block  30 , so that the voltage probe can create a field coupling with coupling tube  48  and dielectric tube  54 . The sleeves are fabricated from a rigid material, such as a metal, and are included to enable the cable to be tightly clamped in place by set screw  41  and shaft lock  46 . 
     Shaft lock  46  is operatively arranged so that it can selectively secure cable  18 , and therefore voltage probe  42 , in place. In a preferred embodiment, shaft lock  46  is operatively arranged to clamp down on sleeve  44  when a threaded hex nut  46 A included on the shaft lock is tightened. The threaded hex nut can also be loosened to release sleeve  44 . It should be appreciated that other methods of variably positioning the end of cable  18  connector block  30  could be used, and shaft lock  46  represents only a preferred means for selectively securing the voltage probe within block connector  30 . 
     Bulkhead connectors  22  and  32  are bi-directional and can be used as inputs into the cavity filter, or outputs out of the cavity filter. In a preferred embodiment, the bulkhead connectors are standard electronic components which are readily purchasable and known in the art. In a preferred embodiment, connector  22  receives an input signal to the cavity filter while connector  32  transmits an output signal from the cavity filter. However, it should be appreciated that there are many styles of receptacles and connectors, and any other input or output connection means known in the art could be used. 
     In a preferred embodiment the loops are soldered to their respective grounding pins. However, it should be appreciated that some embodiments do not include grounding pins, but instead the loops are directly bolted or screwed to the loop rings, or use some other means known in the art to affix the loops to the loop rings for grounding purposes. It should be appreciated in a preferred embodiment, soldering is the method of affixing all electrical components in the phase cancellation circuit, but that this only represents a single preferred method. 
     As discussed supra, cable  18  is a standard cable known in the art, and has several components, including center conductor  18 A, a dielectric layer, double braided shield  18 B, and a jacketing. In a preferred embodiment, portions of the cable are stripped or exposed so the cable can be soldered to different components. For example, as discussed supra, the double braided shield is soldered to sleeves  40  and  44 , and the center conductor is soldered to pin  23  of bulkhead connector  22 . 
     The purpose of cable  18  is to transfer a sampled portion of the input signal at a desired isolation frequency to the output of the cavity filter 180 degrees out of phase. By desired isolation frequency we mean the frequency at which a user desires to position a notch, or achieve isolation. When combined with the input signal, the 180 degree phase shifted sample signal, or cancellation signal, cancels the input signal at the desired isolation frequency. Effectively, this results in a notch at the desired isolation frequency. 
     In order to achieve the 180 degree phase shift, the length of cable  18  is determined such that it is equal to a half-wavelength, or a multiple of the half-wave length of the desired isolation frequency. At low frequencies, the half wavelength is sufficiently long to enable the cable to connect the variable signal injection and sampler loop assemblies. At higher frequencies, however, the half-wave length shortens, and it becomes physically impossible to connect the sampler loop assembly to the variable signal injection loop assembly with a length of cable equal to only one half-wave length. Therefore, multiples of the half-wave must be used to provide the proper length of the cable. 
       FIGS. 4A and 4B  are front views of variable signal injection loop assemblies  16  which include grounding pins located at two alternate, opposite locations.  FIG. 4A  shows grounding pin  28  on the opposite side from where cable  18  is inserted into the connector block, while  FIG. 4B  shows grounding pin  28  on the same side as the insertion of cable  18 . Varying the position of the grounding pin will effect the performance of the phase cancellation circuit, as discussed below. 
       FIGS. 5-8  are diagrams of the theoretical performance of a band pass cavity filter including to phase cancellation circuit  10 , illustrating notch  70  at a frequency above pass band  72 . In the shown example, pass band  72  occurs a range of frequencies surrounding 460 MHz. The notch will flip to the opposite side of the pass band for each incremental half-wavelength that is added to the length of the cable. For example, if the cavity filter is arranged so that the notch is above the pass band, as shown in  FIG. 5 , increasing the cable by another half-wavelength will produce a notch below the pass band, such as shown in  FIG. 6 . By below the pass band we mean at a lower frequency than the pass band frequency, and similarly, by above we mean at a higher frequency than the pass band frequency. It should be appreciated that the diagrams of  FIGS. 5-8  are included for explanation purposes only and should in no way limit the current invention. 
     By varying the position of the grounding pin from the arrangement in  FIG. 4A  to the arrangement in  FIG. 4B , notch  70  will alternate between the low and high sides of the pass band, respectively. It should therefore be appreciated that switching the position of the grounding pin, such as from the arrangement in  FIG. 4A  to  FIG. 4B , will change the position of the notch, without requiring a change in the length of cable  18 . That is, alternating the position of grounding pin  28  from the arrangement of  FIG. 4B  to the arrangement of  FIG. 4A  will produce notches on opposite sides of the pass band. This behavior is generally depicted by  FIGS. 5 and 6  which show notch  70  on the high side and low side of pass band  72 , respectively. 
       FIGS. 6-8  are diagrams showing a theoretical performance of a bandpass filter including phase cancellation circuit  10 , that results from moving the position of voltage probe  42  incrementally out from connector block  30 . That is, the relative position of the voltage probe to the coupling tube affects the resulting field coupling, which in turn affects the performance of the phase cancellation circuit.  FIG. 6  shows the theoretical performance of the cavity filter with phase cancellation circuit  10  when the voltage probe is deeply inserted into connector block  30 , and therefore the coupling tube. This produces notch  70  close to pass band  72 , and the notch is inherently narrow in bandwidth exhibits limited attenuation. 
     As the cable and voltage probe are incrementally pulled out of the connector block, the performance of which is shown progressively in  FIGS. 6-8 , it can be seen that the notch  70  moves away from pass band  72 , and the notch becomes wider in bandwidth, with increasing attenuation. 
     Therefore, it should be appreciated that by altering the length of the cable, shifting the grounding pins, and changing the position of the voltage probe with respect to the coupling tube, the phase cancellation can be achieved over a broad frequency range, above or below the cavity filter pass band. Alternatively stated, the notch created by the phase cancellation circuit is tunable. 
     Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention.