Resonator and high-frequency filter

Details Resonator and high-frequency filter Resonator and high-frequency filter

2001-06-14

2004-06-15

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S219100

Reexamination Certificate

active

06750739

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a resonator constituting a radio frequency filter and the like, used for a radio frequency circuit device of a mobile communication system and the like.
Conventionally, a radio frequency communication system indispensably requires a radio frequency circuit element basically constructed of a resonator, such as a radio frequency filter. As a resonator for a low-loss radio frequency filter, often used is a dielectric resonator including a dielectric secured in a conductor shield.
FIGS. 19A and 19B
are a perspective view and a cross-sectional view, respectively, of a conventional dielectric resonator
503
often used for a low-loss dielectric filter, which operates in a TE
01&dgr;
mode as the base mode. The dielectric resonator
503
includes a cylindrical dielectric
501
and a cylindrical case
502
surrounding the dielectric
501
with a space therebetween. The dielectric
501
is mounted on a support and connected to the bottom portion of the case
502
via the support. The ceiling of the case
502
is apart from the top surface of the dielectric
501
by a given distance, and the sidewall (cylindrical portion) of the case
502
is apart from the cylindrical face of the dielectric
501
by a given distance.
Note that the case
502
is actually constructed of a case body and a lid as shown in
FIG. 20
although it is shown in a simplified form in
FIGS. 19A and 19B
.
The above resonator using a TE mode (hereinafter, referred to as a “TE-mode resonator”) is superior to resonators using other modes in that it is small in loss and exhibits a good Q value, but has a disadvantage of being large in volume. Therefore, when a small resonator is desired, a resonator using a mode other than the TE mode as the base mode is used in some cases at the expense of the Q value characteristic to some extent.
FIG. 20
is a cross-sectional view of a radio frequency filter
530
having a resonator using a TM mode (hereinafter, referred to as a “TM-mode resonator”) that is considered a promising candidate for downsizing implementation. The resonator shown in
FIG. 20
uses a TM mode called a TM
010
mode among the other TM modes.
Referring to
FIG. 20
, the radio frequency filter
530
includes a cylindrical dielectric
540
and a case
531
composed of a case body
532
for housing the dielectric
540
and a lid
533
. The case body
532
and the lid
533
are tightened together with bolts
535
so that the bottom surface of the lid
533
is in contact with the top face of the sidewall of the case body
532
. The bottom surface of the lid
533
and the top surface of the bottom portion of the case body
532
are in contact with the top and bottom surfaces of the dielectric
540
, respectively. In other words, the dielectric
540
is sandwiched between the lid
533
and the case body
532
. The sidewall (cylindrical portion) of the case body
532
concentrically surrounds the dielectric
540
with a space therebetween. An input coupling probe
536
for input coupling with the dielectric
540
and an output coupling probe
537
for output coupling with the dielectric
540
are formed at the bottom portion of the case body
532
.
However, it was found that the TM
010
mode resonator shown in
FIG. 20
failed to provide expected filter characteristics when it was actually prototyped. The present inventors consider the reason for this failure is as follows.
In the TE mode (TE
01&dgr;
mode) resonator shown in
FIGS. 19A and 19B
, most of electromagnetic energy is confined within the dielectric, and only a small amount of radio frequency current flows to the side portion of the case
502
. However, in the TM mode resonator shown in
FIG. 20
, a radio frequency induced current flows in the side portion of the case body
532
in a direction parallel to the axial direction. Therefore, conductor loss comparatively largely influences the TM mode resonator. In particular, a large current flows across the corner at which the sidewall of the case body
532
and the lid
533
meet forming a connection Rcnct. If contact failure occurs at the connection Rcnct during the actual assembly of the resonator
530
, this will presumably cause large deterioration in Q value and instability of operation. In addition, it has been found that if a gap exists between the top or bottom surface of the dielectric
540
and the lid
533
or the case body
532
due to size errors of components during the manufacture and the like, the resonant frequency sharply increases, and this possibly causes instability of operation. In particular, in the case of assembling a plurality of resonators to construct a filter, it is required to accurately fix the resonant frequency of the plurality of resonators. Therefore, in order to obtain desired filter characteristics while being free from instability of operation, considerably complicated work is presumably required.
In construction of a radio frequency filter using either type of resonator, the TE mode resonator or the TM mode resonator, the following three functions are important: that is,
(1) securing intense input/output coupling having a desired fractional bandwidth;
(2) having a resonant frequency adjusting mechanism that can reduce deterioration in the Q value of the resonator and also easily secure a wide frequency adjustable range; and
(3) having an inter-stage coupling degree adjusting mechanism that can easily secure a wide coupling degree adjustable range in the case of constructing a multi-stage radio frequency filter having a plurality of resonators. It is desired to implement a radio frequency filter having these functions.
SUMMARY OF THE INVENTION
A first object of the present invention is providing a dielectric resonator and a radio frequency filter that are small in size, have a simple structure, and operate stably.
A second object of the present invention is providing a radio frequency filter having the functions (1) to (3) described above.
The first resonator of the present invention includes: a columnar dielectric; and a shielding conductor surrounding the dielectric, the resonator using a resonant mode causing generation of a current crossing a corner of the columnar dielectric, wherein the shielding conductor is formed in direct contact with the surface of the dielectric.
With the above construction, the corner of the resonator is constructed of the continuous shielding conductor. Therefore, even in the resonator using a TM mode in which a radio frequency induced current flows over the side face of the column parallel to the axial direction of the column and the end face thereof orthogonal to the axial direction, good conduction is secured, and stability against vibration and the like is secured. Thus, deterioration in Q value and instability of operation are suppressed, and the characterbility of operation are suppressed, and the characteristics of the TM mode resonators of being able to be downsized and having a good Q value can be provided.
The dielectric may include a center portion and an outer portion covering at least part of the center portion, and the dielectric constant of the center portion is higher than the dielectric constant of the outer portion. This reduces conductor loss particularly at the cylindrical portion, and thus improves the unloaded Q value.
The columnar dielectric may be in a shape of a cylinder or a square pole. This facilitates the manufacture.
The shielding conductor may be a metallized layer formed on the surface of the dielectric. This provides high adhesion to the dielectric, and thus the effect is significant.
The second resonator of the present invention includes: a dielectric; and a case for housing the dielectric, wherein part of the case is constructed of conductive foil, and the conductive foil partly shields the dielectric electromagnetically.
With the above construction, the conductive foil is formed at a position such as a seam of the case in which electromagnetic shielding is unstable, to secure the electromagnetic shielding function. This stabilizes the operation c

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