Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-11-09
2003-12-16
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S203000
Reexamination Certificate
active
06664872
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronic filters and, more particularly, to an iris-less combline filter with improved performance characteristics and low manufacturing costs.
BACKGROUND OF THE INVENTION
Conventional combline filters typically are used in front-end transmit/receive filters and diplexers of communication systems such as Personal Communication System (PCS), and Global System for Mobile communications (GSM). The combline filters are configured to pass only certain frequency bands of electromagnetic waves as needed by the communication systems. The combline filters can include uniform resonators rods or stepped resonator rods having steps.
FIG. 1
is a perspective view of a conventional combline filter
10
(with a cover removed therefrom) having uniform resonator rods. As shown, the combline filter
10
includes a plurality of uniform resonator rods
6
disposed within a metal housing
2
, input and output terminals
12
and
14
disposed on the outer surface of the metal housing
2
, and loops
16
a
and
16
b
for inductively coupling electromagnetic signals to and from the input and output terminals
12
and
14
. The metal housing
2
is provided with a plurality of cavities
4
separated by dividing walls
4
a
. Certain dividing walls
4
a
have a well-known structure called a decoupling “iris”
8
having an opening
8
a
. The dividing wall
4
a
having the iris
8
is used to control the amount of coupling between two adjacent resonator rods
6
, which controls the bandwidth of the filter. The resonator rods
6
resonate at particular frequencies to filter or selectively pass certain frequencies of signals inductively applied thereto. Particularly, input signals from the input terminal
12
of the combline filter
10
are inductively transmitted to the first resonator rod
6
through the loop
16
a
and are filtered through the resonance of the resonator rods
6
. The filtered signals are then outputted at the output terminal
14
of the combline filter
10
through the loop
16
b.
A combline filter having stepped resonator rods is also known in the art. In such a filter, resonator rods having steps are used in lieu of the uniform resonator rods. The structure of this filter would be identical to that of the filter
10
shown in
FIG. 1
, except that the uniform resonator rods
6
are replaced with stepped resonator rods and different dimensions may be used. This type of filter also has the decoupling irises and multiple dividing walls to control the coupling coefficients between the stepped resonator rods.
In all these conventional combline filters, the passing frequency range of the filter is selectively varied by changing the lengths or dimensions of the resonator rods whether they be uniform rods or stepped rods. The operational bandwidth of the filter is selectively varied by changing the electromagnetic (EM) coupling coefficients between the resonator rods. The EM coupling coefficient represents the strength of EM coupling between two adjacent resonator rods and equals the difference between the magnetic coupling coefficient and the electric coupling coefficient between the two resonator rods. The magnetic coupling coefficient represents the magnetic coupling strength between the two resonator rods, whereas the electric coupling coefficient represents the electric coupling strength between the two resonator rods. Usually, the magnetic coupling coefficient is larger than the electric coupling coefficient.
To vary the EM coupling (i.e., EM coupling coefficient) between two resonator rods, the size of the iris opening disposed between the two resonator rods is varied. The larger the iris between the two resonator rods, the higher the EM coupling between the two resonator rods. This results in a wide bandwidth operation of the filter. In contrast, if the iris
8
has a smaller opening, a lower EM coupling between the resonator rods is effected, resulting in a narrow bandwidth operation of the filter.
Although effective, conventional combline filters with decoupling irises have a number of problems or drawbacks. For instance, the cavities, dividing walls and decoupling irises in the metal housing must be formed very precisely. Thus, the conventional combline filters require sophisticated milling, which increases costs and decreases throughput. Further, the plurality of dividing walls erected between the resonator rods of the filter significantly increases the signal loss known as “insertion loss”. Moreover, if different bandwidth characteristics are desired for the combline filter, the metal housing of the filter must be re-machined to change the size of the iris openings. In this respect, the milling of the metal housing only allows the iris openings to be enlarged (e.g., by removing a portion of the dividing wall), but does not allow a reduction in the size of the iris openings. Thus, if a decrease in the coupling coefficient between the resonator rods is desired, the metal housing cannot be re-machined and the entire filter housing must be replaced to provide the desired coupling coefficient. Conventional combline filters are therefore restricted in applicability and adaptability.
Accordingly, there is a need for an improved combline filter which overcomes the above-described problems and other problems that are associated with conventional combline filters.
SUMMARY OF THE INVENTION
The present invention presents an innovative approach for controlling the EM coupling between resonators (resonator rods) which overcomes problems that are associated with conventional combline filters. Particularly, the present invention eliminates the use of decoupling irises and instead utilizes a capacitive coupling element to enhance electric coupling between resonators to control the overall EM coupling between the resonators. In one embodiment, the capacitive coupling element is a conductive rod supported by a non-conductive support member and disposed between two adjacent resonators. The capacitive coupling element is placed between the resonators, without contacting the resonators, where the electrical field is dominant, which improves the electric coupling between the resonators. In another embodiment, the capacitive coupling element is a conductive rod attached to one of two adjacent resonators, and is placed between the two resonators where the electrical field is dominant, which improves the electric coupling between the resonators. An increase in the electric coupling decreases the overall EM coupling between the resonators. Then, by selectively varying the dimensions of the capacitive coupling element which varies the amount of electric coupling present between the two resonators, the present invention controls the overall EM coupling between the two resonators without the use of decoupling irises. The use of capacitive coupling elements according to the present invention provides many advantages over conventional combline filters having decoupling irises. For example, a capacitive coupling element is more configurable than a decoupling iris. To modify the size of the iris openings to vary the EM coupling between the resonators, the entire metal housing needs to be re-machined. In contrast, in the present invention, only the capacitive coupling element needs to be reconfigured. Reconfiguration of the capacitive coupling element may involve trimming the ends of the capacitive coupling element, which can be easily accomplished, or replacing the capacitive coupling element with a new capacitive coupling element having different dimensions and/or configurations, which also can be easily accomplished. For instance, If less EM coupling is desired between two resonators, the existing capacitive coupling rod can be replaced with a longer capacitive coupling rod or a thicker capacitive coupling rod, or the height of the coupling rod can be increased. Thus, by merely varying the length, thickness, diameter, and/or height of the capacitive coupling elements and without requiring re-machining or replacement of the metal housing as in the conven
Channabasappa Eswarappa
McNamara Chris
Jones Stephen E.
Pascal Robert
Tyco Electronics Corporation
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