Cavity resonator filter structure having improved cavity arrangement

A combined resonator-cavity filter includes a number of cavity structures designed for cooperative arrangement within a housing. The resonator cavities are constructed and arranged to pass energy in an assigned frequency band. The cavities include a first cavity structure having a corresponding cavity volume and constructed to provide a first Q, and a second cavity structure having its corresponding cavity volume and constructed to provide a second Q. The cavity volume corresponding to the second cavity structure is less than the cavity volume corresponding to the first cavity structure. Other aspects are directed to the arrangement and uses of sets of such cavity structures as part of a combined duplexer-receiver having the same housing.

FIELD OF THE INVENTION 
The present invention relates generally to structures and techniques for 
filtering radio waves, and, more particularly, the implementation of such 
filters using resonator cavities. 
BACKGROUND OF THE INVENTION 
Radio frequency (RF) equipment has used a variety of approaches and 
structures for receiving and transmitting radio waves in the selected 
frequency bands. The type of filtering structure used is often dependent 
upon the intended use and the specifications for the radio equipment. For 
example, dielectric filters are often used for filtering electromagnetic 
energy in the ultra-high frequency band, such as those used for cellular 
communications in the 800+ MHz frequency range. Typically, such filter 
structures are implemented by coupling a number of dielectric resonator 
structures together. Coaxial resonators in such filters are coupled 
together via capacitors, strip transmission lines, transformers, or by 
apertures in walls separating the resonator structures. The number of 
resonator structures used for any particular application is also dependent 
upon the system specifications and, typically, added performance is 
realized by increasing the number of intercoupled resonator structures. 
There has been an increasing demand with such intercoupled resonator 
structures, as with almost all electric or electronic devices and 
equipment, to reduce both the size and cost of the equipment. Unlike 
electronic devices that have been significantly miniaturized due to 
advances in semiconductor technology, efforts to downsize and cost-reduce 
RF equipment have been inhibited. This is often due to the inherent size 
of each resonator structure used in an overall RF filter, by specification 
demands which dictate an increasing number of resonator structures per 
filter function and a zero latitude in the number of filters required in 
the RF systems. 
Accordingly, there has been a need for a filter which overcomes the 
above-mentioned and other disadvantages associated with the prior art. 
SUMMARY OF THE INVENTION 
According to one embodiment, the present invention is directed to a 
cavity-resonator filter in a housing. The filter comprises: a set of 
resonator cavities, which are constructed and arranged to pass energy 
through at least one assigned band, including a first cavity structure 
having a corresponding cavity volume and constructed to provide a first Q 
and including a second cavity structure having a corresponding cavity 
volume and constructed to provide second Q. The cavity volume 
corresponding to the second cavity structure is less than the cavity 
volume corresponding to the first cavity structure. 
According to another embodiment, the present invention is directed to a 
combined resonator-cavity filter in a housing structure. The filter 
comprises of three sets of resonator cavities, each set of resonator 
cavities being constructed and arranged to pass energy in one of three 
respectively-assigned bands. Further, each set includes at least one upper 
Q cavity having a corresponding cavity volume and at least one lower Q 
cavity having a corresponding cavity volume that is less than the volume 
corresponding to the upper Q cavity. Two of the three sets of resonator 
cavities are arranged to pass energy in a receive radio mode, and the 
other of the three sets of resonator cavities is arranged to pass energy 
in a radio-transmit mode. 
According to another embodiment, the present invention is directed to a 
combined duplexer-receive filter in a housing structure, as described 
above, and further including a pair of low-noise amplifiers respectively 
coupled to the two sets of resonator cavities. According to more specific 
embodiments, the low-noise amplifiers are arranged in discrete 
compartments within the single housing structure. The first low-noise 
amplifier may be arranged in a first compartment on one side of the 
housing structure, and the second low-noise amplifier may be arranged in a 
compartment opposite the first compartment. 
Another more specific embodiment of an aspect of the present invention is 
directed to a combined duplexer-receive filter in a housing structure 
including three sets of resonator cavities and at least one test coupler 
for testing the operation of the filters. Each set of resonator cavities 
is constructed and arranged to pass energy in one of three respectively 
assigned frequency bands, two of the three sets of resonator cavities 
arranged to pass energy in a receive signal mode, and the other of the 
three sets of resonator cavities arranged to pass energy in a transmit 
signal mode. A first low-noise amplifier is coupled to one of said two 
sets of resonator cavities; and a second low-noise amplifier coupled to 
one of said two sets of resonator cavities. The first and second low-noise 
amplifiers are arranged in discrete compartments opposite one another 
within the housing structure. The housing structure includes a port 
connecting power and status signals to each of the first and second 
low-noise amplifiers, and includes a coupler cavity coupling energy 
between a transmit test port and the other of the three sets of resonator 
cavities arranged to pass energy in a transmit signal mode and between a 
receive test port and the one of the two sets of resonator cavities 
arranged to pass energy in a receive signal mode. 
The above summary is not intended to summarize each aspect or advantage of 
the disclosed embodiments. This is the purpose of the detailed description 
and drawings.

While the invention is susceptible to various modifications and alternative 
forms, specific embodiments thereof have been shown by way of example in 
the drawings and will herein be described in detail. It should be 
understood, however, that the detailed description is not intended to 
limit the invention to the particular forms disclosed. On the contrary, 
the intention is to cover all modifications, equivalents, and alternatives 
falling within the spirit and scope of the invention as defined by the 
appended claims. 
DETAILED DESCRIPTION 
The present invention is believed to be applicable to a variety of radio 
frequency (RF) applications in which achieving low insertion loss in the 
passband with high attenuation in the stopband close to the passband is 
desirable and/or where there is little room for locating radio equipment. 
The present invention has been found to be particularly applicable and 
beneficial for PCS-CDMA base stations, cellular-communication base 
stations, and other duplex-communication applications. While the present 
invention is not so limited, an appreciation of the present invention is 
best presented by way of a particular example application, in this 
instance, in the context of such a communication system. 
Turning now to the drawings, FIG. 1 illustrates a base station 10, 
according to a particular application and embodiment of the present 
invention, including a housing 12 having a receiver 12a for 
diversity-antenna 30 and a duplexer 12b for antenna 32. The radio 10 is 
depicted generally, so as to represent a wide variety of arrangements and 
constructions. The illustrated radio 10 includes a CPU-based central 
control unit 14, audio and data signal processing circuitry 16 and 18 for 
the respective transmit and receive signaling, and a power amplifier 20 
for the transmit signaling. 
According to a general embodiment of the present invention for an 
application requiring low insertion loss, a set of resonator cavities is 
specially constructed to provide a compact filter structure for use, for 
example, in filtering energy in designated passbands for the receiver 12a 
and the duplexer 12b. The set of resonator cavities are constructed and 
arranged to pass energy in at least one assigned frequency band. The set 
includes a first cavity structure having a corresponding cavity volume and 
providing a first Q, and a second cavity structure having a corresponding 
cavity volume and providing a second Q. The cavity volume corresponding to 
the second cavity structure is less than the cavity volume corresponding 
to the first cavity structure. By increasing the volume of at least one of 
the cavities in the filter, the Q of the filter is increased to provide a 
significantly reduced insertion loss. 
Where it is advantageous to include more than one such set of resonator 
cavities in the same housing structure, the present invention can play an 
important role. In the housing 12 of FIG. 1, for instance, one set of 
resonator cavities may be included to implement the receive filter 12a, 
and two other sets may be included to implement the respective transmit 
and receive filter sections of the duplexer 12b. In this manner, the 
various cavity sizes may be arranged with respect to one another to 
optimize the compactness of the housing. 
According to a specific embodiment of the present invention, FIG. 2 
illustrates a perspective view (top plate removed) of a 
duplexer/receive-filter 40 implemented in a relatively compact single 
housing 42. The filter 40 includes three filters, each implemented as a 
set of five intercoupled resonator cavities. The filters are depicted 
generally as 44, 46 and 48, and the individual cavities of each set are 
specifically depicted as 44a-44e, 46a-46e and 48a-48e, respectively. 
As shown in the schematic diagram of FIG. 3, the first filter 44 
corresponds to the transmit filter of the duplexer section of the housing 
42. This filter 44 receives energy from the transmit section of a radio, 
for example, from a power amplifier such as disclosed in FIG. 1, and 
filters the energy according to a designated transmit-frequency passband. 
From the filter 44, filtered energy is coupled to the radio antenna 50 for 
transmission. 
The second filter 46 corresponds to the receive filter of the duplexer 
section of the housing 42. This filter 46 receives energy from the radio 
antenna 50 and filters the energy according to a designated 
receive-frequency passband. From the filter 46, filtered receive energy is 
coupled to a first low-noise amplifier 52 before being processed any 
further by the radio. 
The third filter 48 is for filtering signals received by the 
diversity-antenna 54, according to a designated receive-frequency passband 
associated with the diversity antenna 54. From the filter 48, filtered 
receive energy is coupled to a second low-noise amplifier 56 before being 
processed by the radio. 
The first and second low-noise amplifiers 52 and 56 may be powered and 
monitored, e.g., for status and alarm conditions, using conventional 
wiring coupled to the housing via a suitable connector 58, such as a 
D-connector. In a specific embodiment, each low-noise amplifier includes 
an amplifier and a current-failure alarm circuit for monitoring current to 
the amplifier. A D-connector interconnects to each low-noise amplifier, 
regulated power for powering the amplifier and the output signal of the 
current-failure alarm circuit, which is used to monitor the condition of 
the corresponding low-noise amplifier. 
The housing 42 also contains transmit and receive directional couplers at 
62 and 64 which may be coupled to probes at ports 62a and 64a for 
conventional testing purposes. Similarly, a receive directional coupler at 
68 may be coupled to a test probe at port 68a. In each of these 
illustrated coupler compartments, a conventional microstrip (or other 
suitable) circuit may be secured. 
Another important aspect of this latter embodiment of the present invention 
is arranging and sizing the individual cavities, along with the other 
disclosed structures, so that the housing 42 can provide the necessary 
filtering functions in a relatively compact area. As illustrated in FIG. 
2, some of the cavities are larger than other cavities. For example, the 
filter 44 includes three large-size cavities 44b, 44c and 44d and two 
small-size cavities 44a and 44e. From an electrical vantage point, while 
the order of the relative sizes is not critical, the larger-sized cavities 
provide a higher Q than the smaller-sized cavities. Collectively, the Q's 
of the respective cavities provide a sufficient reduction in insertion 
loss to meet relatively stringent design specifications. From a 
real-estate perspective, by including large-size and small-size cavities, 
the location of the cavities can be important in ensuring that each of the 
illustrated structures fits in the housing without exceeding space 
limitations. 
Another important aspect of the embodiment illustrated in FIG. 2 concerns 
the locations of the low-noise amplifiers 52 and 56 and the test coupler 
64. The low-noise amplifiers 52 and 56 are respectively located as 
conventionally-constructed circuits placed in cavities 52a and 56a. The 
circuit for the low-noise amplifier 52 is secured on the bottom side (not 
shown in FIG. 2) of the housing 42 and located directly opposite and 
arranged substantially in the same manner as the cavity 56a on the top 
side of the housing 42. The housing 42, which may be constructed from 
aluminum, includes a wall separating the two amplifiers. 
Each set of resonator cavities is implemented using conventional bandpass 
filtering techniques, for example, each as coaxial resonator having a 
center conductor projecting upward from the bottom of the housing 42 
toward the top plate. 
The dimensions used to implement the multiple-filter structure can vary and 
largely depend upon the filtering specifications dictated for the 
equipment and type of communication being serviced. In a specific 
embodiment directed to a PCS-CDMA base station, insertion-loss can be 
substantially lessened using larger volumes for cavities 44b and 44c. For 
example, assuming a common cavity depth of 40 millimeters, the cavities 
44b and 44c can be implemented using a diameter of roughly 58 millimeters, 
and the remaining cavities implemented using a diameter of roughly 45 
millimeters. The housing 42, with the above-listed example cavity 
dimensions, can be implemented with dimensions (roughly) as follows: 42 
millimeters thick (excluding the top plate); 317 millimeters long 
(excluding the mounting extensions on each end); and 158 millimeters wide. 
This structure can be used, for example, to provide filtering for a 
PCS-CDMA base station operating in the 1900 MHz range. 
Other aspects and embodiments of the present invention will be apparent to 
those skilled in the art from consideration of the specification and 
practice of the invention disclosed herein. For example, the housing 
illustrated in FIG. 2 may be implemented with fewer or more than the three 
illustrated filters, and the disclosed selection of cavity number and 
cavity size can vary according to design specifications. It is intended 
that the specification and illustrated embodiments be considered as 
exemplary only, with a true scope and spirit of the invention being 
indicated by the following claims.