Patent Description:
Filters are electronic devices that selectively pass signals based on the frequency of the signal. Various types of filters are used in cellular communications systems. As new generations of cellular communications services have been introduced - typically without phasing out existing cellular communications services - both the number and types of filters that are used has expanded significantly. Filters may be used, for example, to allow radio frequency ("RF") signals in different frequency bands to share selected components of a cellular communications system and/or to separate RF data signals from power and/or control signals. In many applications, filters may be incorporated within a base station antenna. As the number of filters used in a typical cellular communications system has proliferated, the need for smaller, lighter and/or less expensive filters has increased.

The "response" of a filter refers to the amount of energy that passes from a first port (e.g., an input port) of the filter to a second port (e.g., an output port) of the filter as a function of frequency. A filter response will typically include one or more passbands, which are frequency ranges where the filter passes signals with relatively small amounts of attenuation. A filter response also typically includes one or more stopbands. A stopband refers to a frequency range where the filter will substantially not pass signals, usually because the filter is designed to reflect backwards any signals that are incident on the filter in this frequency range. In some applications, it may be desirable that the filter response exhibit a high degree of "local selectivity," meaning that the transition from a passband to an adjacent stopband occurs over a narrow frequency range. Metal resonant cavity filters are typically used in applications where the filter response must exhibit a high degree of local selectivity. One technique for enhancing local selectivity is to add transmission zeros in the filter response. A "transmission zero" refers to a portion of a filter frequency response where the amount of signal energy that passes is very low.

One type of filter that is commonly used in cellular communications applications is the interference mitigation filter. An interference mitigation filter is a two-port device that passes RF energy in a first frequency band (the "passband") while attenuating or "rejecting" RF energy in a second frequency band (the "stopband"). In many applications, the passband and the stopband may be located close to each other, and hence the filter may need to exhibit a high degree of local selectivity. Interference mitigation filters may be used, for example, at base stations that are shared by two different cellular operators. Each cellular operator may mount base station antennas on an antenna tower associated with the base station, and these base station antennas may provide service in the same frequency bands. In order to limit interference between the antennas operated by the different cellular operators, the first cellular operator may transmit and receive RF signals in respective first and second sub-bands of a particular operating frequency band while the second cellular operator may transmit and receive RF signals in respective third and fourth sub-bands of this operating frequency band. For example, the first cellular operator may transmit RF signals in the <NUM>-<NUM> sub-band and may receive RF signals in the <NUM>-<NUM> sub-band of the <NUM>-<NUM> operating frequency band. In this situation, the second (co-located) cellular operator would be assigned different sub-bands and might, for example, transmit RF signals in the <NUM>-<NUM> sub-band and may receive RF signals in the <NUM>-<NUM> sub-band. In such a situation, the first cellular operator may use an interference mitigation filter (or two interference mitigation filters, one for each sub-band) that passes RF signals in the <NUM>-<NUM> and <NUM>-<NUM> sub-bands while rejecting RF signals in the <NUM>-<NUM> and <NUM>-<NUM> sub-bands in order to minimize the impact that the equipment of the second cellular operator has on communications quality. Likewise, the second cellular operator may use an interference mitigation filter (or two interference mitigation filters) that passes RF signals in the <NUM>-<NUM> and <NUM>-<NUM> sub-bands while rejecting RF signals in the <NUM>-<NUM> and <NUM>-<NUM> sub-bands in order to minimize the impact that the equipment of the first cellular operator has on communications quality.

Interference mitigation filters may be implemented using a bandpass filter approach. <FIG> is a top view of a conventional interference mitigation filter <NUM> (with the covers thereof removed) that is implemented as a resonant cavity bandpass filter that creates a stopband (attenuation) above a pair of passbands as shown in <FIG>. As shown in <FIG>, the filter <NUM> includes a metallic housing <NUM> that has a floor <NUM>, exterior walls <NUM> and interior walls <NUM>. The housing <NUM> may be formed, for example, by die casting or machining. The interior walls <NUM> define a plurality of resonant cavities <NUM>. A plurality of coaxial resonating elements or "resonators" <NUM> are provided, with a resonator <NUM> in each of the resonant cavities <NUM>. Each resonator <NUM> extends upwardly from the floor <NUM> of the housing <NUM> and may be implemented, for example, as a metal TEM resonator. Openings <NUM> that are commonly referred to as "windows" are formed in some of the interior walls <NUM>. The windows <NUM> allow the resonators <NUM> in adjacent ones of the resonant cavities <NUM> to couple with each other. An internal metal cover (not shown) is provided that serves as the top of the filter <NUM>. The internal metal cover may have a shape and size substantially similar to the floor <NUM> and may be attached to the upper surface of the exterior and interior walls <NUM>, <NUM> of the housing <NUM> via, for example, screws (the threaded holes <NUM> for the screws are visible in <FIG>). An external cover (not shown) may be attached over the internal cover to cover the tuning screws.

The filter <NUM> further includes an input port <NUM> and an output port <NUM> that are used to couple RF signals into and out of the housing <NUM>. In the depicted embodiment, the input port <NUM> and the output port <NUM> are each formed as a respective coaxial connector that has an outer conductor contact that is physically and electrically connected to the housing <NUM> and a center conductor contact that extends through an exterior wall <NUM> of the housing <NUM> and into the interior of the housing <NUM>. A connectorized coaxial input cable may be coupled to the input port <NUM> and a connectorized coaxial output cable may be coupled to the output port <NUM>. A plurality of resonators <NUM> are also provided that extend between the input port <NUM> and the output port <NUM>. The particular filter <NUM> depicted in <FIG> is a dual bandpass filter that has a transmit passband and a transmit stopband as well as a receive passband (see <FIG>). Tuning screws or other tuning elements (not shown) are provided that extend through the metal cover (not shown). For example, tuning screws may be coaxially aligned with each resonator <NUM>, <NUM> and tuning screws may also be provided between adjacent resonators <NUM>, <NUM>. The tuning screws may be used to tune the center frequencies of the passband and the stopband as well as the size or "bandwidth" of the passband and the stopband.

When an RF signal is input at the input port <NUM>, RF energy within the passband of filter <NUM> passes through the resonant cavities <NUM> (via the windows <NUM> in the interior walls <NUM>) and along the large cavity along the lower edge of the filter that includes the resonators <NUM> and is output through the output port <NUM>. RF energy within the stopband of filter <NUM> is reflected backwardly and hence does not pass to the output port <NUM>. The filter <NUM> has a reciprocal response and hence behaves in the same manner when RF energy is input at port <NUM> and output at port <NUM>. The response of filter <NUM> is shown in <FIG>, which is a plot showing the amount (in dB) that the magnitude of an input RF signal is reduced by the filter as a function of frequency. Two passbands are present at the left hand side of the plot, while a stopband is present at the right hand side of the plot. The lower of the two passbands is primarily generated by the resonators <NUM>, while the upper of the two passbands is primarily generated by the resonators <NUM>. The arrangement of the resonators <NUM>, <NUM> and the coupling windows <NUM> in <FIG> create five transmission zeros immediately above the upper passband to provide the sharp transition and the high level of rejection in the stopband.

Interference mitigation filters that are implemented using the bandpass filter approach of <FIG> may be relatively simple to design and may be easy to tune. However, these filters typically require a relatively large number of resonators and are typically relatively large in size.

Interference mitigation filters may alternatively be implemented using a bandstop or "notch" filter approach. <FIG> is a top view of a conventional interference mitigation filter <NUM> (with the covers thereof removed) that is implemented using a bandstop filter approach. As shown in <FIG>, the filter <NUM> includes a metallic housing <NUM> that has a floor <NUM>, exterior walls <NUM> and interior walls <NUM>. The housing <NUM> may be formed, for example, by die casting or machining. The interior walls <NUM> define a plurality of resonant cavities <NUM>. A plurality of resonators <NUM> are provided, with a resonator <NUM> in each of the resonant cavities <NUM>. Each resonator <NUM> extends upwardly from the floor <NUM> and is implemented as a dielectric TE01 resonator in the depicted embodiment. An internal metal cover (not shown) is provided that serves as the top of the filter <NUM>. The internal metal cover may have a shape and size substantially similar to the floor <NUM> and may be attached to the upper surface of the exterior walls <NUM> via, for example, screws (the threaded holes <NUM> for the screws are visible in <FIG>). An external cover (not shown) may be attached over the internal cover to cover the tuning screws.

Similar to filter <NUM>, filter <NUM> includes an input port <NUM> and an output port <NUM> that are used to couple RF signals into and out of the housing <NUM>. The input and output ports <NUM>, <NUM> may be substantially identical to the input and output ports <NUM>, <NUM>, and hence further description thereof will be omitted. The filter <NUM> further includes an RF transmission line <NUM> that has a first end that is coupled to the center conductor contact of the input port <NUM> and a second end that is coupled to the center conductor contact of the output port <NUM>. The RF transmission line <NUM> may be implemented, for example, as a coaxial transmission line, a stripline transmission line or a microstrip transmission line. In the depicted embodiment, the transmission line is implemented as an air stripline transmission line. Spurs or "stubs" <NUM> extend from the RF transmission line <NUM> into each resonant cavity <NUM>, and each spur <NUM> may extend around a portion of a respective one of the dielectric TE01 resonators <NUM>. Tuning screws or other tuning elements (not shown) are provided that extend through the internal metal cover (not shown). As the tuning elements may be identical to the corresponding tuning elements of filter <NUM>, further description thereof will be omitted.

When an RF signal is input at the input port <NUM>, RF energy within the passband of filter <NUM> passes along the RF transmission line <NUM> to the output port <NUM>. RF energy within the stopband of filter <NUM> passes into the resonant cavities <NUM> and is reflected backwardly and hence does not pass to the output port <NUM>.

<FIG> is a top view of a conventional interference mitigation filter <NUM>' (with the covers removed), where the filter is again implemented as a resonant cavity notch filter with a transmission line extending between the input and output ports. The filter <NUM>' is very similar to filter <NUM> of <FIG>, except that filter <NUM>' includes metal TEM resonators <NUM>' instead of the dielectric TE01 resonators <NUM> of filter <NUM>. <FIG> is a graph of the frequency response of the filter of <FIG>. As shown by the rectangular structures in the plot, the filter design requires two passbands (at about <NUM>-<NUM> and <NUM>-<NUM>) and one stopband (at about <NUM>-<NUM>). The notch structure realizes a single passband that covers both the passband frequency ranges. The resonators <NUM>' generate the stopband response shown in <FIG>. The vertical axis in <FIG> shows the RF signal level at the output of the filter <NUM>' relative to the RF signal level at the input to the filter in dB.

Interference mitigation filters that are implemented using the notch filter approach of <FIG> may include fewer resonators and be smaller than an interference mitigation filter having similar performance that is implemented using the bandpass filter approach of <FIG>. Moreover, the interference mitigation filters that are implemented using the notch filter approach of <FIG> will typically have lower attenuation within the passband (i.e., have better insertion loss performance) and may exhibit higher power handling capabilities. However, interference mitigation filters that are implemented using the notch filter approach of <FIG> tend to be more complex than interference mitigation filters that are implemented using the bandpass approach, may have limited tunability, and may exhibit higher sensitivity to thermal variation. <CIT> discloses a filter comprising bandstop resonators coupled to bandpass resonators with a center frequency far out of the operating band of the filter, wherein the bandpass resonators form a straight transmission line.

There is provided a filter according to claim <NUM>. Pursuant to embodiments of the present invention, filters are provided that include a housing having an input port and an output port and a plurality of resonant cavities within the housing. Each resonant cavity may include a respective notch resonator. The filter may further include a bandpass filter that includes a plurality of bandpass resonators, the bandpass filter extending between the input port and the output port. The filters may be interference mitigation filters in some embodiments.

In some embodiments, the bandpass filter may be configured to directly pass RF signals between the input port and the output port that have frequencies within a passband frequency range of the filter, and/or the resonant cavities may be configured to substantially block RF signals that have frequencies within a stopband frequency range of the filter from passing through the filter.

In some embodiments, the passband frequency range may have a first bandwidth, and a range of frequencies between the passband frequency range and the stopband frequency range may be less than twice the first bandwidth, or less than the first bandwidth.

In some embodiments, the resonant cavities may each include a window that opens to the bandpass filter. In some embodiments, no windows may be provided between resonant cavities.

In some embodiments, the bandpass resonators may be arranged in a staggered line that extends substantially from the input port to the output port.

In some embodiments, the bandpass resonators may be disposed between a first wall and a second wall, and at least some of the resonant cavities may be on one side of the first wall and the bandpass resonators may be on the other side of the first wall. The first wall may include a plurality of first openings that allow RF energy to pass from the bandpass filter into the resonant cavities that are on the first side of the first wall. In some embodiments, additional of the resonant cavities may be on one side of the second wall and the bandpass resonators may be on the other side of the second wall, and the second wall may include a plurality of second openings that allow RF energy to pass from the bandpass filter into the additional of the resonant cavities. In other embodiments, the second wall may be an external wall of the housing.

In some embodiments, the bandpass resonators may be shaped differently than the notch resonators.

Pursuant to further embodiments of the present invention, filters are provided that include a housing having an input port and an output port, a transmission line that extends between the input port and the output port, and a plurality of resonant cavities within the housing, each resonant cavity including a respective first resonator. In these filters, the transmission line is implemented as a bandpass filter that includes a plurality of second resonators.

Pursuant to still further embodiments of the present invention, filters are provided that include a housing having a floor, a first wall and a second wall, a plurality of resonators that are positioned between the first wall and the second wall, the resonators and the first and second walls comprising a bandpass filter, and a first plurality of resonant cavities formed within the housing. The first wall forms a portion of each resonant cavity in the first plurality of resonant cavities.

<FIG> and the associated embodiments are not encompassed by the wording of the claims but are considered useful for understanding the invention.

Note that herein when multiple of the same elements or structures are provided, they may be referred to in some instances using two-part reference numerals, where the two parts are separated by a dash. Herein, such elements may be referred to individually by their full reference numeral (e.g., interior wall xxx-x) and may be referred to collectively by the first part of the applicable reference numeral (e.g., internal walls xxx).

The most delicate part of an interference mitigation filter that is implemented using the notch filter approach is the RF transmission line that extends from the input port to the output port. Aside from the interface between a top internal cover and the exterior walls, the RF transmission line is typically the most common source of passive intermodulation ("PIM") distortion in the filter. As such, the RF transmission line must be manufactured and installed to very precise specifications in order to minimize the risk of PIM distortion. The RF transmission line must be very precisely located with respect to other elements of the filter to guarantee that the filter operates properly. Typically, plastic spacers and/or screws are used to hold the RF transmission line in position. These plastic parts are typically formed of specialized plastic materials in order to minimize their impact on RF performance. Unfortunately, these materials may negatively impact the cost of the filter, and installation of the RF transmission line using numerous plastic screws/spacers complicates the manufacturing process.

Pursuant to embodiments of the present invention, interference mitigation filters are provided that include an integrated bandpass filter that acts as the RF transmission line of the filter. As noted above, conventional interference mitigation filters that are implemented using the notch filter approach include conventional RF transmission line structures such as coaxial transmission lines, stripline transmission lines or microstrip transmission lines. These transmission lines have extremely wide bandwidths, and are capable of passing signals having frequencies from <NUM> to tens of GHz. Many interference mitigation filters, however, need only to pass a relatively narrow bandwidth of frequencies, such as frequencies in a <NUM>-<NUM> range. For example, a typical application might require passing RF signals within a <NUM> frequency band that is centered around <NUM>, meaning that the passband is only <NUM>% of the operating frequency. A bandpass filter can readily pass
RF signals in such a passband with very low insertion loss, and hence the interference mitigation filters according to embodiments of the present invention replace the conventional RF transmission line with a bandpass filter.

As described above, the RF transmission lines used in conventional interference mitigation filters may be sensitive (i.e., small variations in the transmission line may impact performance) and are complex structures that are formed using expensive materials. A bandpass filter-based transmission line may be formed by simply adding additional resonators to the filter, and this can be accomplished, for example, by forming additional resonators during the die casting process, resulting in little extra cost or complexity. The interference mitigation filters according to embodiments of the present invention may be significantly less expensive than conventional interference mitigation filters, and may have improved PIM distortion performance and reduced sensitivity to thermal variations. The filters according to embodiments of the present invention may also be easier to tune and may be tuned over a much wider range. As a result, in some cases, the same filter may be used for different passband and stopband combinations simply by tuning the filter differently.

Pursuant to some embodiments of the present invention, filters are provided that include a housing having an input port and an output port and a plurality of resonant cavities within the housing. Each resonant cavity may include a respective notch resonator. The filter may further include a bandpass filter that includes a plurality of bandpass resonators, the bandpass filter extending between the input port and the output port.

Pursuant to still further embodiments of the present invention, filters are provided that include a housing having a floor, a first wall and a second wall, a plurality of resonators that are positioned between the first wall and the second wall, the resonators and the first and second walls comprising a bandpass filter, and a first plurality of resonant cavities formed within the housing, wherein the first wall forms a portion of each resonant cavity in the first plurality of resonant cavities.

In any of the above filters, the bandpass filter may be configured to directly pass RF signals between the input port and the output port that have frequencies within a passband frequency range of the filter, and the resonant cavities may be configured to substantially block RF signals that have frequencies within a stopband frequency range of the filter from passing through the filter. In some embodiments, the passband frequency range may have a first bandwidth, and a range of frequencies between the passband frequency range and the stopband frequency range may be less than twice the first bandwidth, or less than the first bandwidth. The resonant cavities may each include a window that opens to the bandpass filter. No windows may be provided between resonant cavities. In some embodiments, the bandpass resonators may be arranged in a staggered line that extends substantially from the input port to the output port. In some embodiments, the filter may be an interference mitigation filter.

Embodiments of the present invention will now be discussed in greater detail with reference to <FIG>.

<FIG> is a schematic diagram of a conventional interference mitigation filter <NUM> that is implemented using the notch filter approach. The filter <NUM> may be considered to be a schematic diagram of the filter <NUM> of <FIG> or the filter <NUM>' of <FIG>. As shown in <FIG>, the conventional filter <NUM> includes a housing <NUM>, a plurality of resonant cavities <NUM>, a plurality of resonators <NUM>, an input port <NUM>, an output port <NUM>, and an RF transmission line <NUM>. Each resonant cavity <NUM> may include a respective one of the resonators <NUM>. The RF transmission line <NUM> extends between the input port <NUM> and the output port <NUM>, and includes a plurality of spurs (branches) <NUM> that extend into the resonant cavities <NUM> to couple with the resonators <NUM>.

<FIG> is a schematic diagram of an interference mitigation filter <NUM> according to embodiments of the present invention. As shown, the filter <NUM> may be substantially identical to the filter <NUM> except that the RF transmission line <NUM> of filter <NUM> is replaced with a bandpass filter <NUM> in filter <NUM>.

While the bandpass filter <NUM> is a distinct structure, it will be appreciated by those of skill in the art that the bandpass filter <NUM> typically will not operate independently of the resonant cavities <NUM> and the resonators <NUM> that generate the stopband response of filter <NUM>. Thus, it will be appreciated that when the stopband portion of filter <NUM> is detuned (e.g., the tuning screws are removed or set at locations that do not provide the desired frequency response), then the location of the passband in the response of bandpass filter <NUM> will also be impacted. Thus, when the stopband portion of filter <NUM> is detuned, the bandpass filter <NUM> may not have a passband that corresponds to the desired passband for the filter <NUM> (e.g., it may be moved to another portion of the frequency spectrum) and/or may not even have a conventional bandpass response. However, once the stopband portion of filter <NUM> is properly tuned for operation, the bandpass filter <NUM> will then exhibit a classic bandpass response and the bandpass response will cover the passband for the filter <NUM>. Thus, it will be appreciated that the bandpass filter transmission lines included in the filters according to embodiments of the present invention are not independent structures, but instead will have responses that are impacted by the design and/or tuning of the remainder of the filter. However, once the remainder of the filter is appropriately tuned, the bandpass filter transmission lines according to embodiments of the present invention will exhibit a bandpass response that covers the desired passband for the filter.

<FIG> is a schematic exploded perspective top view of an interference mitigation filter <NUM> according to embodiments of the present invention. As shown in <FIG>, the filter <NUM> includes a metallic housing <NUM> (e.g., a metal housing or a dielectric housing having a metal coating thereon) that has a floor <NUM>, exterior walls <NUM> and interior walls <NUM>. The housing <NUM> may be formed, for example, by die casting or machining. The interior walls <NUM> define a plurality of resonant cavities <NUM>. A plurality of resonators <NUM> are provided, with a resonator <NUM> in each of the resonant cavities <NUM>. Each resonator <NUM> extends upwardly from the floor <NUM>. The resonators <NUM> may be implemented, for example, as dielectric TE01 or TM resonators or as metal TEM resonators. The resonators <NUM> may be referred to herein as "notch" resonators as they are mounted in resonant cavities and configured to form a notch-type (stopband) filter response. The resonant cavities <NUM> and resonators <NUM> may be configured to substantially block RF signals that have frequencies within a stopband frequency range of the filter <NUM> from passing through the filter <NUM>.

The filter <NUM> further includes an input port <NUM> and an output port <NUM> that are used to couple RF signals into and out of the housing <NUM>. The input port <NUM> and the output port <NUM> are each formed as a coaxial connector that has an outer conductor contact that is physically and electrically connected to the housing <NUM> and a center conductor contact that extends through an opening in an exterior wall <NUM> of the housing <NUM> and into the interior thereof. A connectorized coaxial input cable (not shown) may be coupled to the input port <NUM> and a connectorized coaxial output cable (not shown) may be coupled to the output port <NUM>. A plurality of resonators <NUM> are also provided that extend between the input port <NUM> and the output port <NUM>. The resonators <NUM> extend between a pair of interior walls <NUM>-<NUM>, <NUM>-<NUM> and are configured to form a bandpass filter <NUM>. The resonators <NUM> may be referred to herein as "bandpass" resonators as they are configured to form a bandpass filter <NUM> that passes RF signals in a passband of filter <NUM> directly from the input port <NUM> to the output port <NUM>. In other words, RF signals within the passband frequency range of the filter <NUM> only flow along the path of the bandpass filter <NUM> and substantially do not enter the resonant cavities <NUM>. The bandpass resonators <NUM> may be implemented, for example, as metal TEM resonators in some embodiments, although other types of resonators may alternatively be used. The bandpass resonators <NUM> may be arranged in a staggered fashion, as shown in <FIG>, so that each resonator <NUM> may couple with adjacent resonators <NUM> as well as with non-adjacent resonators <NUM>. For example, the bandpass resonators <NUM> may be arranged in a staggered line that extends substantially from the input port <NUM> to the output port <NUM>, as shown in <FIG>. It will also be appreciated that the resonators can be arranged in an interdigital form where the resonators are, for example alternately fixed to the bottom of housing and to the internal cover <NUM> (and the notch resonators <NUM> can be fixed to the internal cover <NUM> as well). The bandpass resonators <NUM> may have shapes and/or sizes that are different from the notch resonators <NUM>, and may or may not be formed using the same general type of resonator (e.g., metal TEM resonators) as the notch resonators <NUM>.

The first interior wall <NUM>-<NUM> and the second interior wall <NUM>-<NUM> each extend the length of the filter <NUM> in the depicted embodiment. The bandpass resonators <NUM> are positioned between interior sides of the first and second interior walls <NUM>-<NUM>, <NUM>-<NUM>. A first subset of the resonant cavities <NUM> are on the exterior side of the first interior wall <NUM>-<NUM>, and a second subset of the resonant cavities <NUM> are on the exterior side of the second interior wall <NUM>-<NUM>. Windows <NUM> are formed in interior walls <NUM>-<NUM>, <NUM>-<NUM>. The windows <NUM> allow RF energy to couple from the bandpass filter <NUM> into the resonant cavities <NUM>. The windows <NUM> may be relatively large. In some embodiments the windows may be open from the floor <NUM> to the internal metal cover <NUM> (described below) of the filter <NUM> in order to allow for sufficient coupling between the bandpass resonators <NUM> and the notch resonators <NUM>. Windows are not provided between any of the resonant cavities <NUM>. In other words, the resonant cavities <NUM> may only open to the bandpass filter <NUM> in some embodiments.

The bandpass filter <NUM>, which acts as a transmission line that extends between the input port <NUM> and the output port <NUM>, is configured to couple with each of the notch resonators <NUM>. In particular, the windows <NUM> in the first wall <NUM>-<NUM> and the second wall <NUM>-<NUM> provide an RF transmission path from the bandpass filter <NUM> into each resonant cavity <NUM>. Each window <NUM> in the first and second walls <NUM>-<NUM>, <NUM>-<NUM> is positioned adjacent a respective one of the bandpass resonators <NUM>. As can be seen in <FIG>, a majority of the bandpass resonators <NUM> (here all but the two on the ends of the transmission line) are configured so that they will each directly couple with a respective one of the notch resonators <NUM>.

An internal metal cover <NUM> is provided that encloses the resonant cavities <NUM> and the bandpass filter <NUM>. The internal cover <NUM> includes a plurality of openings <NUM> that are aligned with the threaded openings <NUM> in housing <NUM>. Set screws (not shown) are threaded into the openings <NUM> and openings <NUM> in order to attach the internal cover <NUM> to the upper surface of the exterior walls <NUM>. A plurality of tuning screws (or other tuning elements) <NUM>, <NUM> are mounted in the internal cover <NUM> (only a few respective tuning screws <NUM>, <NUM> are shown). The tuning screws <NUM> may be coaxially aligned with the resonators <NUM>, <NUM>, and the tuning screws <NUM> may be positioned so that when inserted into the interior of the housing <NUM> they will be between adjacent ones of the resonators <NUM>, <NUM>. The tuning screws <NUM>, <NUM> may be used to tune the center frequencies of the passband and the stopband as well as the size or "bandwidth" of the passband and the stopband. An external cover (not shown) may be attached over the internal cover <NUM> to cover the tuning screws <NUM>, <NUM>.

When an RF signal is input at the input port <NUM>, RF energy within the passband of filter <NUM> passes directly from the input port <NUM> to the output port <NUM> via the bandpass filter <NUM>. RF energy within the stopband of filter <NUM> passes into the resonant cavities <NUM> and is reflected backwardly and hence does not pass to the output port <NUM>.

<FIG> is a graph of the frequency response of the filter of <FIG>. As shown by the rectangular structures in the plot, the filter design requires two passbands (at about <NUM>-<NUM> and <NUM>-<NUM>) and one stopband (at about <NUM>-<NUM>). The notch structure realizes a single passband that covers both the passband frequency ranges, similar to the filter <NUM>' (see <FIG>). The resonators <NUM> generate the bandpass response and the resonators <NUM> generate the stopband response that are shown in <FIG>. The vertical axis in <FIG> shows the RF signal level at the output of the filter relative to the RF signal level at the input to the filter in dB.

As discussed above, interference mitigation filters are commonly used in applications where two cellular operators have base station antennas mounted on the same tower that provide service in the same frequency bands. Each cellular operator may use different sub-bands in these frequency bands to limit interference. Each sub-band may, for example, be a <NUM> or <NUM> sub-band, and each operator will use a first sub-band for the downlink and a second sub-band for the uplink. Unfortunately, the sub-bands used by the two different operators may be very close to each other (e.g., as close as a few MHz). Thus, each operator may include interference mitigation filters along the RF paths through the antenna that pass signals in the operating frequency sub-band of the antenna (i.e., the passband) while attenuating signals in the operating frequency sub-band of the other antenna (i.e., the stopband). In some embodiments, the passband frequency range may have a first bandwidth, and a range of frequencies between the passband frequency range and the stopband frequency range may be less than twice the first bandwidth. In other embodiments, the range of frequencies between the passband frequency range and the stopband frequency range may be less than the first bandwidth, less than ½ the first bandwidth, or less than one-quarter the first bandwidth. In some cases, the interference mitigation filters may be external to the antenna (e.g., implemented as a tower mounted filter).

The filter <NUM> may be both smaller and less expensive to manufacture than a conventional interference mitigation filter having the design of <FIG> that provides comparable performance, and may also exhibit lower attenuation in the passband and have higher power handling capabilities. The filter <NUM> may be cheaper to manufacture than the conventional interference mitigation filters having the designs of <FIG>, and may also exhibit improved PIM performance and be less susceptible to thermal variations than these filters. The filter <NUM> may also have a much wider tuning range and be easier to tune than the filters of <FIG>. For example, in some cases it may be possible to reverse the positions of the stopband and the passband simply by adjusting the tuning screws on filter <NUM>. This means that the same filter could be used by two cellular operators that operate at the same base station.

It will be appreciated that many modifications may be made to the filter <NUM> without departing from the scope of the present invention. For example, the number of resonant cavities <NUM> and resonators <NUM> may be varied based on a desired filter response. As another example, the locations of the resonant cavities <NUM> may be changed. Different types of resonators <NUM>, <NUM> may be used, and the input and output ports <NUM>, <NUM> may have any conventional port design. The internal cover <NUM> may be soldered in place rather than fixed using screws, and any appropriate type of tuning elements may be used. The number and arrangement of bandpass resonators <NUM> may be selected based on a desired response for the filter.

<FIG> is a schematic diagram of an interference mitigation filter <NUM>' according to further embodiments of the present invention. The filter <NUM>' is very similar to the filter <NUM> of <FIG>, except that the filter <NUM>' only includes resonant cavities <NUM> on one side of the bandpass filter <NUM>. Thus, the first sidewall of the bandpass filter <NUM> comprises an internal wall and the second sidewall may comprise an external wall of the housing <NUM> in this embodiment.

It will be appreciated that the techniques disclosed herein may be used in filters other than interference mitigation filters. For example, low-loss combiners include RF transmission lines that could be implemented as bandpass filters according to the teachings of the present invention.

While the example filters <NUM>, <NUM>, <NUM>' according to embodiments of the present invention that are illustrated in <FIG> above include either four or eight resonant cavities, it will be appreciated that any appropriate number of cavities may be provided as necessary to provide a filter having desired filtering characteristics. Likewise, the number of resonators included in the bandpass filter may be varied as appropriate. In a filter according to a first embodiment the notch resonators are configured to generate a plurality of nulls within the stopband, wherein in a filter according to a second embodiment a filter according to the first embodiment is configured such that a first of the nulls is in a bottom half of the stopband and a second of the nulls is within a top half of the stopband, or wherein in a filter according to a third embodiment a filter according to the first embodiment or the second embodiment is configured such that a first of the nulls is in a bottom third of the stopband, a second of the nulls is within middle third of the stopband, and a third of the nulls is within a top third of the stopband, or in a filter according to a fourth embodiment a filter according to any one of the first to third embodiment is configured such that a first of the nulls is in a bottom quarter of the stopband, a second of the nulls is within a quarter of the of the stopband that is between the bottom quarter of the stopband and a center frequency of the stopband, and a third of the nulls is within a top quarter of the stopband, or in a filter according to a fifth embodiment a filter according to any one of the first to fourth embodiment is configured such that none of the nulls is within a quarter of the stopband that is between the top quarter of the stopband and the center frequency of the stopband.

Embodiments of the present invention have been described above with reference to the accompanying drawings, in which embodiments of the invention are shown. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

It will be understood that when an element is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., "between" versus "directly between", "adjacent" versus "directly adjacent", etc.).

Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

Claim 1:
A filter (<NUM>), comprising:
a housing (<NUM>) having an input port (<NUM>) and an output port (<NUM>);
a plurality of resonant cavities (<NUM>) within the housing (<NUM>), each resonant cavity (<NUM>) including a respective notch resonator (<NUM>); and a bandpass filter (<NUM>) that includes a plurality of bandpass resonators (<NUM>), the bandpass filter (<NUM>) extending between the input port (<NUM>) and the output port (<NUM>),
wherein the filter (<NUM>) is an interference mitigation filter that includes a passband and a stopband, and
wherein the notch resonators (<NUM>) are configured to generate a plurality of nulls within the stopband,
wherein the bandpass resonators (<NUM>) are arranged in a staggered line that extends substantially from the input port (<NUM>) to the output port (<NUM>).