Wave transmission lines and networks – Resonators
Reexamination Certificate
2003-04-09
2004-08-31
Callahan, Timothy P. (Department: 2816)
Wave transmission lines and networks
Resonators
C333S222000
Reexamination Certificate
active
06784768
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to dielectric resonators circuits. More particularly, the invention pertains to techniques for coupling energy to and from dielectric resonator circuits.
BACKGROUND OF THE INVENTION
Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers and other circuits. The higher the dielectric constant of the dielectric material of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a mu (magnetic constant) of 1, i.e., they are transparent to magnetic fields.
FIG. 1
is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator
10
is formed as a cylinder
12
of dielectric material with a circular, longitudinal through hole
14
. Individual resonators are commonly called “pucks” in the relevant trades. While dielectric resonators have many uses, their primary use is in connection with microwave communication systems.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE
01&dgr;
(or TE hereafter). Typically, it is the fundamental TE mode that is the desired mode of the circuit or system in which the resonator is incorporated. The second mode is commonly termed the hybrid mode, H
11&dgr;
(or H
11
hereafter). The H
11
mode is excited from the dielectric resonator, but a considerable amount of electric field lies outside of the resonator and, therefore, is strongly affected by the cavity. The H
11
mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned and has two polarizations. The H
11
mode field is orthogonal to the TE mode field. There are additional higher order modes.
Typically, all of the modes other than the TE mode, are undesired and constitute interference. The H
11
mode, however, often is the only interference mode of significant concern because it tends to be rather close in frequency to the TE mode. However, the TM
01&dgr;
or TM
01
(Transverse Magnetic) mode also can be of concern. The longitudinal through hole
14
in the resonator helps to push the frequency of the Transverse Magnetic mode upwards. However, during the tuning of a filter, the frequency of the Transverse Magnetic mode could be brought downward and close to the operating band of the filter. The remaining higher order modes usually have substantial frequency separation from the TE mode and thus do not cause significant interference with operation of the system.
FIG. 2
is a perspective view of a microwave dielectric resonator filter
20
of the prior art employing a plurality of dielectric resonators
10
. The resonators
10
are arranged in the cavity
22
of a conductive enclosure
24
. The conductive enclosure
24
typically is rectangular, as shown in FIG.
2
. Microwave energy is introduced into the cavity by a coupler
28
coupled to a cable, such as a coaxial cable. Conductive separating walls
32
separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators
10
. Particularly, irises
30
in walls
32
control the coupling between adjacent resonators
10
. Walls without irises generally prevent any coupling between adjacent resonators separated by those walls. Walls with irises allow some coupling between adjacent resonators separated by those walls. By way of example, the field of resonator
10
a
couples to the field of resonator
10
b
through iris
30
a
, the field of resonator
10
b
further couples to the field of resonator
10
c
through iris
30
b
, and the field of resonator
10
c
further couples to the field of resonator
10
d
through iris
30
c
. Wall
32
a
, which does not have an iris, prevents the field of resonator
10
a
from coupling with physically adjacent resonator
10
d
on the other side of the wall
32
a.
One or more metal plates
42
are attached to a top cover plate (the top cover plate is not shown) generally coaxially with a corresponding resonator
10
to affect the field of the resonator to set the center frequency of the filter. Particularly, plate
42
may be mounted on a screw
43
passing through a threaded hole in the top cover plate (not shown) of enclosure
24
. The screw may be rotated to vary the spacing between the plate
42
and the resonator
10
to adjust the center frequency of the resonator. The sizes of the resonator pucks
10
, their relative spacing, the number of pucks, the size of the cavity
22
, and the size of the irises
30
all need to be precisely controlled to set the desired center wavelength of the filter and the bandwidth of the filter.
An output coupler
40
is positioned adjacent the last resonator
10
d
to couple the microwave energy out of the filter
20
and into a coaxial connector (not shown). Signals also may be coupled into and out of a dielectric resonator circuit by other techniques, such as microstrips positioned on the bottom surface
44
of the enclosure
24
adjacent the resonators.
FIG. 3
shows one typical coupling element design that can be used as the input coupler
28
or output coupler
40
in the dielectric resonator circuit of FIG.
2
. The resonator is shown at
31
. The coupler
38
is mounted through the wall
32
of the resonator circuit and couples, for instance, to a coaxial cable
33
that carries a signal to or from the resonator circuit. The coupler
38
comprises a conductive loop
35
that is generally coaxial with and surrounds the dielectric resonator
31
. The coupling loop can be an electric coupling loop or a magnetic coupling loop. Despite the terminology (which is conventional), coupling is predominantly magnetic in either case. Also, the coupling loop can be open or closed. If the loop is closed, the loop is fully coupled to the magnetic flux of the resonator. If the loop is open, it is only partially coupled to the magnetic flux of the resonator. For exemplary purposes,
FIG. 3
shows an open, magnetic coupling loop that extends around the resonator
31
approximately 270°. An electric coupling loop, on the other hand, operates on the principal of capacitive coupling through a conductive plate positioned near the resonator.
Achieving a particular coupling strength between the loop and the resonator is crucial to meeting the desired filter specifications, especially return loss. Hence, selection of an appropriate type of coupling loop and appropriate selection of its other attributes, such as radius, position relative to the resonator and length of the wire, are essential to achieving such goals. One particularly significant attribute is the distance between the loop and the resonator
31
. An adjusting screw
36
is mounted on the far side of the enclosure
37
opposite from the wall. In this particular design, there is another wall
39
of the enclosure
37
at that position and, thus, the adjusting screw
36
passes through and threadingly engages a hole
38
in the far wall
37
. The adjusting screw
36
is nonconductive and can contact the loop
35
as shown in FIG.
3
. By rotating the screw
36
so as to screw it into the cavity (to the left in FIG.
3
), the distal end of the screw can contact the loop
35
and push it closer to the resonator, thus, increasing coupling. Likewise, by rotating the screw outwardly (to the right in FIG.
3
), the loop can resiliently spring back out, thus moving further away from the
Channabasappa Eswarappa
Khalil Adil
Pance Kristi Dhimiter
Callahan Timothy P.
M/A - Com, Inc.
Nguyen Linh M.
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