COUPLING MECHANISM FOR A PCB MOUNTED MICROWAVE
RE-ENTRANT RESONANT CAVITY
The present invention relates to a coupling mechanism to couple
microwave signals from a transmission line embedded in a Printed Circuit Board
PCB to a resonant cavity mounted on said PCB.
The RF front-end filtering/duplexing device constitutes one of the
most critical devices for the performance and compliance of modern cellular,
high-power, base-stations (BTS). Due to the requirement for high overall BTS
power efficiency and the strict compliance rules imposed by the regulatory
authorities, the transfer function of these filtering/duplexing devices should meet
several stringent specifications such as minimal in-band insertion loss, maximal
out-of-band rejection, and high close-to-band selectivity. The implementation of
such transfer functions, together with the high-power handling capability that is
usually required, result into filtering devices that are bulky in volume and
expensive in their fabrication.
In terms of the underlying RF technology, these filters are usually
composed of waveguide/cavity resonators, coupled through irises or other
defects on the walls that form the cavities. Given that the exact dimensions of
the resonating cavities and the employed coupling mechanisms determine the
filters' RF characteristics (operation band, insertion loss, return loss) high
mechanical accuracy is required during their fabrication. Nevertheless, the
required accuracy is almost never achieved during the production process and,
therefore, post-production manual tuning is required for the optimization of the
filters' transfer function.
Future cellular networks, able to support much higher data rates and
heavier traffic, are envisioned to be composed of smaller cells (smaller radiated
power per BTS) or to employ BTS composed of several modular radios,
radiating medium power levels per element (e.g. Active Antenna Arrays). In
such cases, the reduced power radiated by each BTS RF front-end could allow
for relaxed filter requirements (e.g. relaxed requirements for the in-band
insertion losses or the close-to-band selectivity), but the architecture of these
BTS would impose some extra requirements related with the filters' volume and
weight, and their integrability with the remaining RF front-end.
Given these new requirements, the filtering/duplexing devices of
future, small cell or modular BTS could resemble more those currently
employed in mobile terminals than the traditional high-power filtering/duplexing
devices of modern BTS. In reality, filtering technologies that position themselves
between these two extreme cases (in terms of their quality performance and
their size properties) would be the most suitable for such applications.
Ceramic filters are one of the technologies that could provide niche
solutions for such applications. Nevertheless, the design of such filters meeting
medium power-handling specifications (e.g. average power greater than 4 W) or
strict isolation conditions (e.g. Tx/Rx isolation for the FDD LTE 2.6 GHz band) is
not always possible. Besides, the cost of this technology is very much
dependant on the production volume, and unless such filters are produced in
multi-million quantities, the cost per filter remains relatively high.
Another filtering technology that could be employed in applications
that require simultaneously high quality filtering performance and relatively
small-size properties or integrability features is the surface-mount filtering
technology. In this approach, 3-D resonant cavities (able to deliver high-Q
values), such as re-entrant (coaxial) resonators, are mounted on conventional
Printed Circuit Boards PCBs. These cavities are interconnected through
transmission lines embedded in the PCB. The same transmission lines are also
used for the implementation of the required filtering function. In that way, the
filtering devices can be integrated with the remaining of the RF front-end on the
same PCB.
A cross-sectional representation of a conventional microwave
re-entrant (coaxial) resonator mounted on a Printed Circuit Board PCB 14 is
depicted in Fig. 1. In this configuration, the 3-D part of a resonator 10 is
soldered (through a soldering layer 13) on the external metalized surface of the
Printed Circuit Board PCB 14. In this case, both the 3-D part of the resonator 10
and the external surface of the PCB that is bounded by the 3-D component are
forming a resonant cavity 15. As far as the 3-D part of the resonator 10 is
concerned, it is composed of an outer wall 11, an internal re-entrant stub/rod 12
and may be of either cylindrical or rectangular shape (in a coaxial
configuration). This part can be formed by either milling in or casting from a
metallic volume, or by metal-plating plastic 3-D forms (for weight reduction).
The electromagnetic properties of the resonant, air-filled cavity 15
are dependent on the exact dimensions of the effective coaxial configuration
(i.e. the length of the inner rod 12 and its distance from the external wall 11 of
the cavity) and the capacitive gap 16 formed between the inner rod 12 and the
external metalized surface of the PCB that comprises part of the resonant
cavity.
For the synthesis of microwave filtering structures using resonant
cavities similar with that of Fig. 1, microwave signals have to be guided to and
away from the cavity. This can be done through the employed PCB and by
embedding on it different types of transmission lines. This is also schematically
represented in Fig. 1, where an embedded on the PCB input
waveguide/transmission line 17 guides the microwave signal to the cavity, feeds
the signal into the cavity through a coupling mechanism 18 and then the
resonating signal is fed through another coupling mechanism 19 to an output
waveguide/transmission line 20.
Then, microwave filters can be synthesized based on conventional
filter synthesis models such as that of Fig. 2, where the employed resonators 30
are interconnected through admittance inverters 3 1, properly synthesized to
implement specific transfer functions. In the case of cavities such as that of Fig.
1, the inverters can be designed using also PCB-embedded transmission lines.
For the implementation of resonant cavities such as that of Fig. 1, the
design of the coupling mechanisms 18, 19 in a way that the comparative
advantages of this configuration remain untouched (fully printed interconnecting
lines and coupling mechanisms) constitutes the most challenging part.
Solutions are proposed, e.g., in documents of Jan Hesselbarth such
as the Patent Application WO 2008/036180 A2 entitled "Re-entrant resonant
cavities, filters including such cavities and method of manufacture", the Patent
Application WO 2008/036179 A1 entitled "Resonant cavities and method of
manufacturing such cavities", the Patent Application WO 2008/036178 A1
entitled "Re-entrant resonant cavities, filters including such cavities and method
of manufacture", and the publication "Surface-mount cavity filter technology", in
Proc. European Microwave Conference 2007, pp. 442-445, Oct. 2007.
Therein, the above challenge was tackled by splitting the 3-D
resonant cavity in two halves and locating the first half of the cavity on the upper
external surface of the PCB and other half of the cavity on the lower external
surface of the PCB, as shown in Fig. 3 . In this configuration, the two parts of the
cavity were electrically connected through the PCB, using the via posts 44
embedded in the PCB, and the microwave signals were coupled electrically to
and from the inner stub of the cavity through transmission lines 45 that were
penetrating the cavity. This approach was experimentally validated, but, given
that the PCB itself and the interconnecting via posts were part of the resonant
cavities, the operation of the cavities in this configuration were usually
accompanied with relatively high losses (decreased quality factors).
The major challenge in the design of coupling mechanisms for the
feeding of microwave signals to and from PCB mounted 3-D resonant cavities is
to retain the major comparative advantages of such configurations (high - Q
resonators interconnected through fully-printed PCB-embedded networks),
while achieving the desired functionality of low loss coupling mechanism
providing a large range of coupling coefficients and some tunability for the
cavity resonances.
An object of the present invention is to provide a coupling
mechanism to feed microwave signals to a 3-D PCB mounted resonant cavity,
similar with that of Fig. 1 described above, but which implements an
easy-to-fabricate mechanism leading to high-quality filtering.
According to a characterizing embodiment of the invention, this
object is achieved due to the fact that the end of said transmission line is
provided with a metalized feeding pad located at the external layer of said PCB
inside said resonant cavity.
In this way, the size and position of the metalized feeding pad
defines and allows adjusting the coupling mechanism while providing
high-quality filtering with reproducible characteristics.
Another characterizing embodiment of the present invention is that
said resonant cavity is provided with a re-entrant inner stub orthogonal to said
PCB and separated from said PCB by a capacitive gap, and that said metalized
feeding pad is facing said inner stub in the area of said capacitive gap and is
offset from the axial direction of said inner stub.
The proposed filter technology exhibits considerably improved
performance as compared to the filter technology of the known prior art. The
present technology is more robust to manufacturing errors and the operation of
the resulting RF filters is associated with significantly reduced insertion losses
(easier to maintain the high-Q values of the 3-D resonant cavities).
Also another characterizing embodiment of the present invention is
that said resonant cavity is mounted on an external metalized surface of said
PCB, and that said metalized feeding pad is separated from said external
metalized surface by a surface capacitive gap.
The capacitive gap formed by relative position of the metalized
feeding pad in offset with respect to the axial direction of the re-entrant inner
stub and the surface capacitive gap separating the metalized feeding pad from
the external metalized surface of the PCB define the characteristics of the
coupling mechanism.
In a preferred characterizing embodiment of the present invention,
said metalized feeding pad has the shape of a disk surrounded by said surface
capacitive gap and whereof the centre is offset from the axial direction of said
inner stub.
Although the metalized feeding pad may have most any shape, it has
been proved that a disk form leads to optimal results.
In a variant embodiment of the present invention, said PCB is
provided with an input embedded transmission line of which an input end is
provided with an input metalized feeding pad, and with an output embedded
transmission line of which an output end is provided with an output metalized
feeding pad, and said input and output metalized feeding pads are located at
the external layer of said PCB inside said resonant cavity and are separated by
an electric wall.
The proposed filter technology provides a niche solution for
applications (RF front-ends of modern BTS/ nodeB/ e-nodeB/ etc) that require
high-quality filtering performance and relatively high-power handling capabilities
together with low-volume properties and a high degree of integration (filter
integrated with the other components of the RF front-end). These features
together with its low-cost and fully-automated fabrication process (fully-printed
PCB/ soldered top-mounted metalized-plastic cavities) make the present
coupling mechanism a very promising technology for future PCB mounted
microwave re-entrant resonant cavities.
Further characterizing embodiments of the present coupling
mechanism are mentioned in the appended claims.
It is to be noticed that the terms "comprising" or "including", used in the
claims, should not be interpreted as being restricted to the means listed thereafter.
Thus, the scope of an expression such as "a device comprising means A and B"
should not be limited to an embodiment of a device consisting only of the means A
and B. It means that, with respect to embodiments of the present invention, A and
B are essential means of the device.
Similarly, it is to be noticed that the term "coupled", also used in the
claims, should not be interpreted as being restricted to direct connections only.
Thus, the scope of the expression such as "a device A coupled to a device B"
should not be limited to embodiments of a device wherein an output of device A is
directly connected to an input of device B. It means that there may exist a path
between an output of A and an input of B, which path may include other devices
or means.
The above and other objects and features of the invention will
become more apparent and the invention itself will be best understood by
referring to the following description of an embodiment taken in conjunction with
the accompanying drawings wherein:
Fig. 1 shows a classical resonant cavity formed by a 3-D re-entrant
(coaxial) resonator mounted on a PCB;
Fig. 2 represents a filter synthesis model of the resonant cavity of
Fig. 1 employing admittance inverters;
Fig. 3 shows another PCB-mounted resonant cavity as known from
prior art;
Fig. 4 shows a coupling mechanism employed to couple signals from
a PCB-embedded waveguide (stripline) to a single PCB-mounted resonant
cavity according to an embodiment of the invention;
Fig. 5 is a top-view of the coupling mechanism of Fig. 4 at the level of
the external surface of the PCB, with respect to the location of the mounted 3-D
resonant cavity;
Fig. 6 represents an equivalent circuit of the coupling mechanism of
Figs. 4 and 5;
Fig. 7 shows a coupling mechanism employed to couple signals from
a PCB-embedded waveguide (stripline) to a single resonant cavity and reverse
according to another embodiment of the invention;
Fig. 8 shows a coupling mechanism employed to couple signals from
a PCB-embedded waveguide (stripline) to a pair of inductively coupled resonant
cavities and reverse according to another embodiment of the invention;
Fig. 9 represents a filter synthesis model according to the
configuration of Fig. 8;
Fig. 10 shows a 3-D model of a 4-pole Chebyshev filter that employs
a coupling mechanism according to an embodiment of the present invention to
couple the microwave signal from a PCB-embedded waveguide (stripline) to the
3-D resonant cavities and reverse;
Fig. 11 is a bottom-view of a single pair of inductively coupled
resonant cavities employed in the 3-D model of Fig. 10;
Fig. 12 is a cross-section view of the 3-D model of Fig. 10 along a
single pair of inductively coupled resonant cavities;
Fig. 13 shows a coupling mechanism of the microwave signal from
the PCB-embedded striplines ( 13 1) to the inductively coupled pair of resonant
cavities; and
Fig. 14 represents a simulated response of the 4-pole filter of Fig. 10 .
An embodiment of a coupling mechanism, that fulfills all the
requirements of the present invention, is presented in Fig. 4 and Fig. 5 .
A cross-sectional representation of a Printed Circuit Board PCB
mounted microwave re-entrant resonant cavity is shown at Fig. 4, while Fig. 5
shows a top-view of the coupling mechanism at the level of the external surface
of the PCB, with respect to the location of the mounted 3-D resonant cavity.
As in a conventional microwave re-entrant (coaxial) resonator
mounted on a PCB, the 3-D part of the present resonator is soldered on an
external metalized surface of the PCB. Both the 3-D part of the resonator and
the external surface of the PCB that is bounded by the 3-D component are
forming a resonant cavity 60. The 3-D part of the resonator is composed of an
outer wall, an internal re-entrant stub or rod 64 and may be of either cylindrical
or rectangular shape (in a coaxial configuration). This part can be formed by
either milling in or casting from a metallic volume, or by metal-plating plastic 3-D
forms, mainly for weight reduction.
The re-entrant inner rod 64 is orthogonal to the PCB with one end
fixed to the outer wall and the other end facing the PCB and separated thereof
by a capacitive gap 66.
The electromagnetic properties of the resonant, air-filled resonant
cavity 60 are dependent on the exact dimensions of the effective coaxial
configuration (i.e. the length of the inner rod 64 and its distance from the
external wall of the cavity) and the capacitive gap 66 formed between the inner
rod 64 and the external metalized surface of the PCB that comprises part of the
resonant cavity.
The microwave signal is considered to be guided to the resonator
through an embedded waveguide/transmission line 6 1 that employs the external
metalized surface of the PCB, on which the 3-D cavity is mounted, as a ground
plane. This line can be implemented, for example, in either microstrip or stripline
technology. When the microwave signal reaches the end of the feeding
transmission line, it is guided through a vertical via post (or an array of via
posts) 62 to a metalized feeding pad 63 located, inside the cavity, at the
external layer of the PCB on which the 3-D resonator is mounted.
In Fig 5, the top view of this metalized feeding pad 7 1, with respect to
the location of the coaxial configuration 72 of the resonant cavity mounted on
the external surface of the PCB 73, is clearly depicted. Given that between the
feeding pad 7 1 and the external surface 73 of the PCB there is no electrical
connection, a displacement current will be supported across a surface
capacitive gap 74 formed between them.
The metalized feeding pad 63/71 preferably has the shape of a disk
which is surrounded by the surface capacitive gap 74 and whereof the centre is
offset from the axial direction of the internal re-entrant stub/rod 64.
This is the first mechanism of electromagnetic coupling of the feeding
microwave signal to the resonant cavity, given that the part of the external
surface of the PCB around the metalized feeding pad comprises part of the
resonant cavity, as was shown in Fig. 4 .
The electromagnetic properties of this coupling (i.e. magnitude) are
depended on the radius of the pad and the width of the surface capacitive gap
74. Both these features can be adjusted when designing the PCB, irrespectively
of the 3-D part of the resonant cavity, and constitute significant design
parameters while synthesizing a specific filtering transfer function and the
corresponding PCB layout.
The second mechanism of electromagnetic coupling between the
metalized feeding pad 63/71 and the resonant cavity is the capacitance
supported between the inner stub 64 of the 3-D coaxial configuration and the
feeding pad itself.
As shown in Fig. 5, the feeding pad and the inner stub of the
resonator overlap over a surface 75 that is dependent on the radius of the
feeding pad and its position (offset) with respect to the centre of the coaxial
configuration of the 3-D part of the resonator. These two parameters constitute
another two major design parameters that should be properly set during the
filter synthesis and PCB layout design. The importance of this second coupling
mechanism is attributed to the fact that apart from providing electromagnetic
coupling between the feeding pad and the resonant cavity, it provides a means
of slightly adjusting and tuning the resonance of the resonant cavity through the
design/layout of the PCB.
Specifically, with the introduction of the feeding pad, the total
capacitance supported between the external layer of the PCB on which the 3-D
cavity is mounted and the inner stub of the coaxial configuration, that originally
played a key role in the estimation of the total capacitance of the resonating
cavity and its resonant frequency, is divided into two components. The first is
the capacitance supported between the feeding pad and the inner stub of the
3-D part of the cavity and the second is the surface capacitance supported by
the external PCB surface around the feeding pad overlaying with the inner stub
of the 3-D part of the cavity. The ratio between these two capacitances should
be equal with the ratio of the feeding pad surface and the external PCB surface
overlaying with inner rod of the coaxial configuration and, hence, can be
adjusted by adjusting the position of the feeding pad. Although the sum of these
two capacitances should be approximately equal with the total capacitance of
the first case, in the latter case the capacitance between the feeding pad and
the inner stub of the coaxial configuration does not influence the capacitive
characteristics and the resonant frequency of the resonant cavity.
Therefore, by altering (increasing/decreasing) this capacitance, the
total capacitance of the resonant cavity can be inversely altered
(decreased/increased). By this means, the effective resonant frequency of the
resonant cavity can be adjusted through the design of the layout of the external
PCB surface.
In order to schematically represent the major properties of the
proposed coupling mechanism, the equivalent circuit of a configuration similar
with that of Fig. 4 is shown in Fig. 6 . In this equivalent circuit, the resonant
cavity is represented through a shunt LC circuit that is composed of an
inductance L¥ ax and a capacitance C¥ ax that are attributed to the 3-D coaxial
configuration, and a capacitance Cgap that is attributed to the capacitance
supported between the inner stub of the coaxial configuration and the PCB
surface on which the 3-D cavity is mounted and comprises part of the resonant
cavity.
In the absence of the feeding pad, this capacitance is depended
exclusively on the geometrical characteristics of the gap (gap width d and total
area S over which the capacitance is supported Cgap = e0 S/d ) and directly
loads the coaxial resonator (C0 = Cgap according to the notation of Fig. 6 ) .
In the presence of the feeding pad, this capacitance is split in two
components: one of them, that is supported between the inner stub of the
resonator and the ground plane, loads the coaxial resonator similarly as before,
and the second, that is supported between the inner stub of the resonator and
the feeding pad, corresponds to a coupling capacitance that is serially
connected to the resonator. The ratio between these two capacitances is
defined by the fraction of the inner stub area overlapping with the feeding disk.
Therefore, if the overlapping ratio is considered to be k, then Co = (1-k) Cgap
and CSer = k Cgap . Finally, the coupling capacitance between the feeding disk
and the external surface of the PCB that comprises part of the resonant cavity
can be considered to be in parallel with the resonator (CS in Fig. 6).
In a conventional filter configuration, electromagnetic signals have to
be guided to and from each of the resonators that comprises the filter, as shown
in Fig. 1. Therefore, two coupling mechanisms, similar with those presented in
the previous text, are required to be implemented within each of the cavities.
This is depicted in Fig. 7, where an input transmission line 8 1 guides the signal
to an input metalized feeding pad in the cavity through the coupling mechanism
82, while the coupling mechanism 83 couples the signal from an output
metalized feeding pad in the cavity to an output transmission line 84.
The configuration of Fig. 7 is prone to several parasitic phenomena
that may influence, deteriorate or limit the operation of the cavity resonator as
part of a microwave filter. Given its compact size, the coupling mechanisms 82
and 83 are located close to each other.
This may result in some direct coupling between them, either through
the resonant cavity itself or through the substrate on which the cavity is
mounted, deteriorating the electromagnetic performance of the resonator.
Even though the latter case can be addressed with the use of an
electric wall 85 inserted between them (this wall can be implemented using
closely spaced coppers vias), there is very little to be done to avoid direct
electromagnetic coupling between the two coupling mechanism through the
resonant cavity. In fact, a simple solution to this problem would be to keep them
as far as possible from each other.
Nevertheless, this approach would reduce the overlapping area
between each of the coupling disks and the inner stub of the resonator (75 in
Fig. 5) and therefore the total achievable coupling coefficient implemented by
each of those mechanisms. Therefore, a configuration similar with that of Fig. 7
may not be the preferred implementation of filters composed of cavity
resonators mounted on a PCB.
A preferred alternative implementation of such filters is shown in
Fig. 8 . In the context of this implementation, the modular elements for the
design of PCB-mounted resonant cavity filters are considered to be pairs of
inductively coupled 3-D resonant coaxial cavities, similar with that shown in Fig.
8 . In this approach, any two of the 3-D coaxial resonators have to be built within
one block 9 1. On the common electric wall 95 separating the input resonant
cavity from the output resonant cavity, an iris open window 92 secures the
inductive coupling between the two cavities built on the same block. The
input/output resonant cavity of the pair is provided with a distinct input/output
inner stub orthogonal to the PCB and separated thereof by an input/output
capacitive gap, respectively. Moreover, an input metalized feeding pad is
implemented in the input resonant cavity of the pair, whilst an output metalized
feeding pad is implemented in the output resonant cavity of the pair. The input
metalized feeding pad is facing the end face of the input inner stub in the area
of the input capacitive gap and is offset from the axial direction of this input
inner stub, whilst the output metalized feeding pad is facing the end face of the
output inner stub in the area of the output capacitive gap and is offset from the
axial direction of this output inner stub.
The exact dimensions of the coupling window 92 determine the
magnitude of the corresponding inductive coupling.
The advantage of a configuration similar with that of Fig. 8 is that it
allows to feed the microwave signal from the PCB to the input cavity through the
input coupling mechanism 93 and then couple out the filtered signal from the
output cavity to the PCB through the output coupling mechanism 94.
In that way, the two PCB-cavity coupling mechanisms are
implemented in two different cavities and therefore no significant parasitic direct
coupling between them (through the cavities) is present.
Furthermore, the design parameters associated with each of the two
coupling mechanisms (i.e. feeding disk diameter, position of the feeding disk
etc) are free to be chosen according to the requirements of the filter design
procedure without having to satisfy any major restrictions (i.e. size, relative
position of the two coupling mechanisms etc).
When the configuration of Fig. 8 is employed for the synthesis of
filtering devices, the filter functions should be synthesized according to the
model of Fig. 9 . In this model, the input and output coupling to and from the first
and last resonator of the filter is implemented through transmission line based
admittance inverters (J0 i and JNN+I ) , while the coupling between the resonators
are implemented interchangeably using inductive coupling irises (My) and
transmission line based impedance inverters (Jy).
In order to verify the possibility of designing PCB-mounted filters
employing 3-D re-entrant (coaxial) resonators excited and interconnected by
means of the coupling mechanisms proposed in this invention, a 4 h-order
Chebyshev filter has been designed and simulated. The targeted operating
band for this filter has been the downlink Tx band of the WCDMA air interface
(21 10 MHz - 2170MHz).
The model employed for the simulation of this filter is depicted in Fig.
10 . For this filter implementation, the configuration of Fig. 8 was employed.
Specifically, the four resonators of the 4th order filter were built in two pairs of
inductively coupled resonators. Then these two pairs were interconnected
through admittance inverters implemented on the PCB and the filter function
was synthesized according to the model of Fig 9 .
Referring to the 3-D model of Fig. 10, the two pairs of inductively
coupled cavities are milled in two metallic volumes 100. These volumes are
considered to be soldered on the top surface of a PCB 101 that is metal-plated
on both its upper and lower sides. Inside the PCB, striplines are employed to
synthesize the interconnecting admittance inverters. Furthermore, copper vias
102 have been embedded within the PCB, shorting the two sides of the PCB
(ground planes of the employed striplines), to enhance electromagnetic isolation
between the coupling mechanisms and reduce the parasitic effects associated
with the operation of the striplines.
The bottom view of each of the two inductively coupled pairs of the
re-entrant resonators milled in metallic volumes (100 in Fig. 10) is depicted in
Fig. 11. In this implementation, the two resonant cavities 110 are composed of
cylindrical inner rods 111 and rectangular outer walls 112. In the general case,
the shapes of the inner and outer contactors can be any that support a similar
coaxial configuration. Finally, the two cavities are coupled through an iris 113
that is formed by removing material from the walls separating the two cavities.
In Fig. 12, the cross-section of the 3-D filter model along one pair of
inductively coupled cavities is depicted. As shown, the signal is guided to and
from the pair of cavities through the striplines 123 that have been embedded in
the PCB that bares the cavities. The aforementioned striplines use metallic
surfaces 12 1 and 122 as upper and lower ground planes, while the surface 12 1
is also used to attach on the 3-D parts of the resonant cavities. These striplines
are connected with the feed disks of the upper ground plane layer 12 1 of the
PCB using copper vias 124.
A better representation of the mechanisms that couple the signals
from the striplines to the resonant cavities through the vias and the feeding
disks is shown in Fig. 13 . Specifically, in Fig. 13, input/output striplines 13 1 feed
the RF signal to coupling disks 132 that have been formed on a metallic surface
130 that, apart from operating as an upper ground plane for the stripline, also is
used to attach the 3-D resonant cavity.
Finally, the full-wave simulated response of the 4-pole Chebyshev
filter of Fig. 10 is depicted in Fig. 14, which represents the S-parameters in dB
(vertical axis) of the aforementioned filter structure against the frequency bands
in GHz of interest (horizontal axis). According to these results, low insertion loss
is achieved across the passband of the filter (<0.6 dB) while a better than 50 dB
isolation for the downlink Rx band of the WCDMA air interface is also achieved.
It is to be noted that the present coupling mechanism can be used to
couple signals not only to/from single or double cavities but in a more general
case to structures composed of an arbitrary large number of cavities. To this
end, the general coupling mechanism comprises several resonant cavities
mounted on the PCB, which is provided with a same amount of embedded
transmission lines each having an end provided with a metalized feeding pad
located inside a distinct one of the resonant cavities. Each resonant cavity is
provided with an inner stub orthogonal to the PCB and separated thereof by a
capacitive gap. Each metalized feeding pad is facing the end face of a
corresponding inner stub in the area of the capacitive gap and is offset from the
axial direction of the corresponding inner stub. The metalized feeding pads are
further separated by electric walls.
A final remark is that embodiments of the present invention are
described above in terms of functional blocks. From the functional description of
these blocks, given above, it will be apparent for a person skilled in the art of
designing electronic devices how embodiments of these blocks can be
manufactured with well-known electronic components. A detailed architecture of
the contents of the functional blocks hence is not given.
While the principles of the invention have been described above in
connection with specific apparatus, it is to be clearly understood that this
description is merely made by way of example and not as a limitation on the
scope of the invention, as defined in the appended claims.

CLAIMS
1. A coupling mechanism to couple microwave signals from a
transmission line (61 ) embedded in a Printed Circuit Board PCB (67) to a
resonant cavity (60) mounted on said PCB,
characterized in that the end of said transmission line (61 ) is
provided with a metalized feeding pad (63) located at the external layer of said
PCB (67) inside said resonant cavity (60).
2. The coupling mechanism according to claim 1,
characterized in that said resonant cavity (60) is provided with a re
entrant inner stub (64) orthogonal to said PCB and separated from said PCB by
a capacitive gap (66),
and in that said metalized feeding pad (63) is facing said inner stub
in the area of said capacitive gap and is offset from the axial direction of said
inner stub (64).
3. The coupling mechanism according to claim 1,
characterized in that said resonant cavity (60) is mounted on an
external metalized surface (73) of said PCB (67),
and in that said metalized feeding pad (63, 7 1) is separated from
said external metalized surface by a surface capacitive gap (74).
4. The coupling mechanism according to the claims 2 and 3,
characterized in that said metalized feeding pad (63, 7 1) has the shape of a
disk surrounded by said surface capacitive gap (74) and whereof the centre is
offset from the axial direction of said inner stub (64).
5. The coupling mechanism according to claim 1, characterized in
that the end of said transmission line (61 ) is coupled to said metalized feeding
pad (63) through at least one via post (62).
6. The coupling mechanism according to claim 1, characterized in
that said transmission line (61 ) embedded in said PCB (73) is a waveguide
implemented in microstrip or stripline technology.
7. The coupling mechanism according to claim 1,
characterized in that said PCB is provided with an input embedded
transmission line (81 ) of which an input end is provided with an input metalized
feeding pad, and with an output embedded transmission line (84) of which an
output end is provided with an output metalized feeding pad,
and in that said input and output metalized feeding pads are located
at the external layer of said PCB inside said resonant cavity and are separated
by an electric wall (85).
8. The coupling mechanism according to claim 7,
characterized in that said resonant cavity is provided with a
re-entrant inner stub orthogonal to said PCB which is separated from said PCB
by a capacitive gap,
and in that said input and output metalized feeding pads are both
facing an end face of the inner stub in the area of said capacitive gap, and are
each offset from the axial direction of said inner stub.
9. The coupling mechanism according to claim 7, characterized in
that said electric wall (85) is implemented by closely spaced coppers vias in
said PCB.
10. The coupling mechanism according to claim 7,
characterized in that said resonant cavity mounted on said PCB is
constituted by a pair of inductively coupled 3-D resonant cavities built within a
same block (91 ) ,
in that an input resonant cavity of said pair is provided with an input
inner stub orthogonal to said PCB and separated from said PCB by an input
capacitive gap, and an output resonant cavity of said pair is provided with an
output inner stub orthogonal to said PCB and separated from said PCB by an
output capacitive gap,
in that said input and output resonant cavities are separated by a
common electric wall (95),
in that an iris open window (92) is provided between said input and
output resonant cavity of said pair,
and in that said input metalized feeding pad is implemented in said
input resonant cavity of said pair, and said output metalized feeding pad is
implemented in said output resonant cavity of said pair.
11. The coupling mechanism according to claim 10,
characterized in that said input metalized feeding pad is facing the
end face of said input inner stub in the area of said input capacitive gap and is
offset from the axial direction of said input inner stub, and said output metalized
feeding pad is facing said end face of the output inner stub in the area of said
output capacitive gap and is offset from the axial direction of said output inner
stub.
12. The coupling mechanism according to claim 1,
characterized in that said coupling mechanism comprises a
plurality of resonant cavities mounted on said PCB,
in that said PCB is provided with a plurality of embedded
transmission lines each having an end provided with a metalized feeding pad
located inside a distinct resonant cavity of said plurality of resonant cavities,
in that each of said resonant cavities is provided with an inner stub
orthogonal to said PCB and separated from said PCB by a capacitive gap,
in that each of said metalized feeding pad is facing an end face of a
corresponding inner stub in the area of said capacitive gap and is offset from
the axial direction of said corresponding inner stub,
and in that said metalized feeding pads are separated by electric walls.