WO2019115072A1

WO2019115072A1 - Wideband rf filter, multiband rf filter and rf filter component

Info

Publication number: WO2019115072A1
Application number: PCT/EP2018/078913
Authority: WO
Inventors: Oleksandr GAVRYLIUK; Petro KOMAKHA; Wai San WONG; Alexander Chernyakov
Original assignee: RF360 Europe GmbH
Priority date: 2017-12-11
Filing date: 2018-10-22
Publication date: 2019-06-20
Also published as: DE102017129473A1

Abstract

A wideband and/or multiband RF filter is provided. The filter comprises two or more electromagnetically coupled resonators.

Description

Wideband RF filter, multiband RF filter and RF filter

component

The present invention refers to wideband RF filters and multiband RF filters, e.g. for wireless communication

systems .

RF filters in devices for wireless communication systems can be used to separate wanted RF signals from unwanted RF signals. Such filters can be band pass filters or band reject filters having a low insertion loss within a pass band and a high out-of-band rejection for frequencies outside one or more pass bands.

The ongoing trend towards higher signal transmission rates makes requirements concerning filter characteristics more stringent .

RF filters are known from US 8,031,035 B2.

However, the performance of current filter topologies and filter technologies is limited. What is wanted is an RF filter that allows an increased width of pass bands, a reduced insertion loss, steeper pass band skirts, reduce overall filter solution and preferably an improvement in more than one of such characteristics simultaneously.

To that end, RF filters and RF components according to the independent claims are provided. Dependent claims provide preferred embodiments. The wideband RF filter comprises a first port and a second port. Further, the filter comprises a first electroacoustic resonator coupled to the first port and a second

electroacoustic resonator coupled to the second port. The first electroacoustic resonator is electromagnetically coupled to the second electroacoustic resonator.

Additionally or as an alternative, it is possible that the wideband RF filter comprises a first port and a second port. Further, the filter comprises a first electroacoustic

resonator coupled to the first port and a second

electroacoustic resonator coupled to the second port.

Additionally, the resonator comprises a first coupling element electrically connected to the first resonator and a second coupling element electrically connected to the second resonator. The first coupling element is coupled to the second coupling element.

Thus, RF filters are provided in which electroacoustic resonators are coupled. The coupling between the resonators can be a direct coupling between the resonators or an

indirect coupling via additional coupling elements. An electromagnetic coupling is a preferred type of coupling.

Electroacoustic resonators can comprise electrode structures connected to a piezoelectric material. Via the piezoelectric material effect a resonator converts between RF signals and acoustic waves. The resonator can be a BAW resonator (BAW = bulk acoustic wave) , an SAW resonator (SAW = surface acoustic wave) or a GBAW resonator (GBAW = guided bulk acoustic wave) . In BAW resonators a piezoelectric material is sandwiched between a bottom electrode and a top electrode. In SAW resonators the electrodes have a comb-like shape and electrode fingers arranged one next to another and electrically connected to opposite electrodes excite acoustic waves that can propagate at the top side of the piezoelectric material .

Due to the intrinsic connection between RF signals and acoustic waves in electroacoustic resonators it is possible to couple such resonators acoustically to achieve

improvements of the filter characteristic of an RF filter comprising such resonators.

However, it was found that such resonators can also be coupled electromagnetically to extend the range of beneficial effects of such couplings. Establishing an electromagnetical coupling between electroacoustic resonators, however, demands for the provision of special filter elements. For the sake of clarity some of the possible coupling elements are described below .

Correspondingly, it is possible that the first coupling element is coupled electromagnetically to the second coupling element. The first element and the second coupling element couple the first electroacoustic resonator to the second electroacoustic resonator.

It is possible that the first electroacoustic resonator and the second electroacoustic resonator are coupled via a coupling circuit comprising one or more LC circuits or one or more transmission lines.

LC elements and transmission lines can be realized as

electromagnetically active elements when subjected to an RF signal. It is possible to construct and arrange such elements in the vicinity of one or the other such that RF signals emitted by one element can be received by another element.

For example, electrodes of a capacitive element can be arranged in the vicinity of an electrode of another

capacitive element to electromagnetically couple capacitive elements. Also, it is possible to realize an inductive element in the form of a coil-like structure where at least one conductor segment of the coil or one winding or a

plurality of windings of the coil is arranged in the vicinity of and arranged relative to another inductive element such that an RF signal can be transmitted from the first inductive element to the second inductive element. In particular, it is possible to realize an electromagnetic coupling by allowing a coupling between electrical field components associated with two capacitive elements or allow an electromagnetic coupling by utilizing a magnetic interference between inductive elements .

To that end, it is possible to arrange one or several

capacitive elements and one or several inductive elements in a multi-layer structure comprising dielectric layers and metallized structures. The capacitive elements (C) and inductive elements (L) are then realized as metallization structures in the metallization layers between the dielectric layers. Metallization structures of different layers can be electrically connected via through-holes through the

dielectric layers.

Also, it is possible to establish an electromagnetic coupling utilizing transmission lines. Transmission lines can be realized as strip-shaped conductor segments. The length of the conductor segments can correspond to a quarter wavelength or multiples of a quarter wavelength of the corresponding RF signal. The strip-shaped conductor segments can also be realized as structured metallizations on a dielectric layer or between dielectric layers in a multi-layer component.

When transmission lines are used it is preferred that the strip-shaped conductor segments of different transmission lines are arranged mainly parallel but galvanically separated from one another.

The trend towards miniaturization demands for electric components such as electric filter components with smaller spatial dimensions. To reduce the length of a transmission line it is possible to electrically connect the corresponding transmission line in series with a capacitive element.

It is possible that the RF filter comprises a plurality of two or more basic coupling sections coupled, e.g.

electromagnetically, between the first port and the second port, e.g. in series to other circuit elements.

It is possible that each basic coupling section comprises an electroacoustic resonator and a coupling element.

Thus, the number of electroacoustic resonators and the number of coupling elements are not limited to two. RF filters with three, four, five, six, seven and more basic coupling

sections, each having an electroacoustic resonator, are possible .

It is possible to electromagnetically couple the

electroacoustic resonator of one basic coupling section to one, to two or to more electroacoustic resonators. To that end, it is possible to arrange the corresponding coupling element that can be electrically connected with the

electroacoustic resonator in the vicinity of the

corresponding coupling element of the respective associated basic coupling section. Correspondingly, it is possible that the first port is electrically connected to a first basic coupling section comprising the first electroacoustic

resonator. The first basic coupling section is

electromagnetically or electrically connected to one other basic coupling section, i.e. a second basic coupling section comprising the second electroacoustic resonator. The second basic coupling section can also be electromagnetically coupled to a third electroacoustic resonator. Thus, the second basic coupling element is electromagnetically coupled to two other basic coupling sections.

The basic coupling sections can be arranged iteratively similar to the rungs of a ladder until the last basic

coupling section is electromagnetically coupled only to its predecessor and electrically connected to the second port.

When transmission lines are used as electromagnetic coupling elements to couple different electroacoustic resonators then one electrode of the corresponding electroacoustic resonator can be electrically connected between one distal end of the conductor patch of the transmission line while the respective other electrode of the resonator is coupled to ground.

The respective other distal end of each conductor segment of a transmission line can also be electrically connected to or coupled to ground.

The electric connection of the transmission line of the first basic coupling section and the first port can be arranged such that the first port connects the corresponding conductor patch at one distal end or in-between distal ends.

Correspondingly, the second port can be connected to the last basic coupling section such that the port is directly

connected to one distal end or in-between distal ends of the corresponding conductor patch.

The configuration comprising electromagnetically active elements and electroacoustically active elements increases the number of degrees of freedom for a designer to utilize acoustic resonances and electromagnetic resonances to enhance the frequency characteristics of a filter. The lengths and the positions of the conductor segments of transmission lines, for example, can be chosen such that a wideband band pass filter structure is obtained. Additionally,

characteristic frequencies of the electroacoustic resonators, e.g. resonance frequencies or anti-resonance frequencies can be used to increase the steepness of filter skirts.

A configuration in which an electroacoustic resonator

electrically connects an electromagnetic coupling element to ground establishes the possibility of conducting RF power of an unwanted frequency range directly to ground, in particular when the unwanted frequency range coincides with a resonance frequency of the electroacoustic resonator. Thus, it is preferred that the resonator is tuned such that its resonance frequency coincides with RF frequencies for which the high suppression level is wanted or at which a very steep filter skirt is wanted.

Original transmission line filter design consists of

electromagnetically coupled transmission lines. One distal end of each transmission line is connected to a grounding capacitor Csh that are shortening the electrical length of the respective transmission line. By replacing Csh with an electroacoustic resonator that has static capacitance Cstatic equal to Csh (usually 0.5 Csh < Cstatic < 1.5 Csh) the same filter bandwidth can be achieved. By tuning resonance

frequency of the electroacoustic resonator inside the pass band, a notch is created within the pass band. The width of the notch is determined by number of the basic sections.

It is further possible that the filter has a signal path arranged between the first port and the second port. The signal path is provided to transmit an RF signal from one of the ports to the other respective port. The signal path comprises galvanically separated sections. Thus, it is possible to conduct an RF signal from the first port to the second port or from the second port to the first port. Thus, the first port can be an input port and the second port can be an output port or the second port can be an input port and the first port can be an output port.

The presence of the signal path does not necessarily mean that a galvanic connection between the first port and the second port is present. The signal path can have path

segments in which the RF signal propagates utilizing electric currents in a conductor material and other segments in which the RF signal propagates as electromagnetic signals outside an electrical conductor.

It is possible that the first electroacoustic resonator and the second electroacoustic resonator are coupled to ground.

Of course, when more than two electroacoustic resonators are present, e.g. in further basic coupling sections, then the additional electroacoustic resonators of additional basic coupling sections can also be coupled to ground.

Also, it is possible that the electroacoustic resonators are directly connected with one of their electrodes to a ground potential. The respective other electrode can be coupled or directly connected to a distal end of a transmission line or to a corresponding electromagnetic coupling element, e.g. an LC circuit.

It is possible that the filter further comprises a series resonator or a series capacitor electrically connected between the first port and the second port.

The electroacoustic resonator discussed above may be used to conduct unwanted RF power to ground and can be regarded as shunt resonators. In contrast thereto, a series resonator or capacitor is connected between the first port and the second port and establishes a part of the signal path or establishes a shunt path parallel to the signal path or parallel to a main section of the signal path.

Such a series resonator or capacitor can electrically connect one circuit element or one part of a circuit element of one basic coupling section to one circuit element or a part of a circuit element of the corresponding basic coupling section associated to the second port. It is possible that the series resonator electrically connects one distal end of a

transmission line associated with the first port to one distal end of a transmission line associated with the second port. It is possible that the series resonator electrically connects the corresponding distal ends that are connected to the corresponding first and second port. However, it is possible that the series resonator or capacitor electrically connects the one basic section associated with first port to another basic section associated with second port in-between the distal ends of each respective transmission line.

Further, it is possible that the RF filter comprises a second or more than one further series resonator or capacitor electrically connected between selected basic coupling sections. Thus, it is possible that a series resonator or capacitor connects a basic coupling section selected from the first, the second, the third, the fourth, the fifth, the sixth, the seventh, the eighth basic coupling section with a coupling section selected from the first, the second, the third, the fourth, the fifth, the sixth, the seventh, the eighth basic coupling section.

It is possible that the resonance frequency of the first and/or the second electroacoustic resonator is provided to create one or more notches in a transfer function of the filter .

As already described above, it is preferred to use a

resonance of an electroacoustic resonator to create a notch or to enhance the steepness of a pass band skirt and to use a static capacitance of an electroacoustic resonator to reduce the electrical length of transmission line.

The above-described topology allows pass band filters with an increased pass band width while simultaneously having ultra steep filter skirts. By varying the number of basic coupling sections the width of the pass band can be adjusted to a specific requirement. The provision of electroacoustic resonators with the corresponding characteristic frequencies can be used to provide steep pass band skirts or notches at preferred positions within a pass band. Thus, it is possible to use notches to divide a pass band into two or more

frequency sections. Notch width depends on number of basic sections and electromagnetic coupling between the basic sections .

One example is to provide a pass band filter that is

compatible with the requirements of frequency blocks of a sub-six 5G spectrum. In particular, it is possible to provide a pass band that has a low insertion loss between 3.3 GHz and 3.8 GHz and between 4.4 GHz and 4.99 GHz and between 5.15 GHz and 5.925 GHz which is not possible with current filter technology and design methodology.

It is possible that the filter is multi-band RF filter. That is the provided passband is so wide that more than one usable frequency bands, e.g. of one or more communication system, can be placed in it.

Within the band none or one or more artificial notches can separate such bands .

It is possible that the above-described topology is realized as an RF filter component. The component can have a multi layer carrier substrate. The multi-layer carrier substrate can have two or more dielectric layers. The two or more electroacoustic resonators are arranged at the top side of the carrier substrate and/or in a cavity embedded within the carrier substrate. The one or more coupling elements for electromagnetically coupling the electroacoustic resonators are realized as metallized structures in metallization layers between dielectric layers. It is possible that the RF filter component further comprises an integrated passive device (IPD) and a piezoelectric material. The electroacoustic resonators and the coupling elements are arranged on the same piezoelectric substrate.

The piezoelectric substrate may comprise or consist of the piezoelectric material. The piezoelectric material may be, e.g., LiTaCy (lithium tantalate) , LiNbCy (lithium niobate) , or another typical material for BAW or GBAW components.

Correspondingly, the resonators may be of the SAW-type, the BAW-type and/or the GBAW-type.

It is possible that the carrier substrate is a LTCC substrate (low temperature co-fired ceramics) or a HTCC substrate (HTCC = high temperature co-fired ceramics) . Further, it is

possible that the carrier substrate comprises laminated material that may comprise a polymer material, IPD

(integrated passive device) or can be implemented on

piezoelectric material, e.g. LiTa03, LiNb03, BAW or GBAW.

Selected aspects and details of selected embodiments are explained utilizing the accompanying schematic drawings. In the drawings :

Fig. 1 shows a possible configuration of the RF filter.

Fig. 2 shows an RF filter utilizing coupling elements.

Fig. 3 shows the possibility of utilizing LC elements.

Fig. 4 shows the possibility of utilizing transmission lines. Fig. 5 shows the possibility of providing a plurality of basic coupling sections.

Fig. 6 shows the possibility of series resonators.

Fig. 7 shows the possibility of series resonators or series capacitor .

Fig. 8 shows the possibility of providing a pass band.

Fig. 9 shows the possibility of providing two pass bands separated by a frequency region of high out-of-band

suppression .

Fig. 10 shows the possibility of providing two pass bands separated by a frequency region of high out-of-band

suppression and very steep left skirt in the first passband and very steep right skirt in the second passband.

Figure 1 shows a possible arrangement of a wideband RF filter F. The filter F comprises a first port PI and a second port P2. Further, the filter F comprises a first resonator R1 and a second resonator R2. The first resonator R1 and the second resonator R2 are electroacoustic resonators working with acoustic waves. An electromagnetic coupling EMC exists between the two electroacoustic resonators.

The first port PI can be utilized to receive RF signals from an external circuit environment and the second port P2 can be utilized to provide filtered RF signals to an additional external circuit environment. The use of both: electroacoustic effects and electromagnetic effects provide an increased number of possibilities and an increased number of degrees of freedom to enhance frequency characteristics of RF filters.

Figure 2 shows a wideband RF filter configuration F utilizing a first coupling element CE1 and a second coupling element CE2 to establish the electromagnetic coupling EMC between the first electroacoustic resonator R1 and the second

electroacoustic resonator R2.

The first coupling element CE1 and the second coupling element CE2 are not limited to specific types of coupling elements. The two coupling elements can be of any type that allow electromagnetic interaction between one another. The first coupling element CE1 can be of the same type as the second coupling element CE2. However, it is possible that the types of coupling elements for the first coupling element CE1 and the second coupling element CE2 are different.

Figure 3 shows the possibility of using capacitive (C) elements and/or inductive (L) elements to establish the electromagnetic interaction between the resonators. Each of the LC elements can act as a radiating "antenna" element for the respective associated element of the corresponding coupling circuit CC . Capacitive elements and inductive elements can be electrically connected in parallel to

establish a parallel LC circuit or can be electrically connected in series to establish a series LC circuit.

Figure 4 shows the possibility of using a transmission line TL in a coupled circuit CC . The transmission lines TL

comprise conductor patches that may be arranged in parallel and one next to another to enhance the electromagnetic coupling between the transmission lines. The first port PI and the second port P2, respectively, can be electrically connected to one distal end or in-between of distal ends of the transmission lines TL. The respective other distal ends of the transmission lines TL can be electrically connected to ground via the acoustic resonators Rl, R2.

One property of an electroacoustic resonator is that it has a static capacitance which allows to reduce the geometrical length of the transmission line TL to which the resonator is connected. Thus, a combination of a transmission line and an electroacoustic resonator does not only provide the

possibility of using the acoustic resonance and of providing the electromagnetic resonance but also helps to reduce the spatial dimensions of the corresponding filter component.

Figure 5 illustrates the possibility of having more than two basic coupling sections. Every basic section is coupled to the rest of the basic sections of the filter with the

coupling factor that is determined by the line length, thickness and dielectric constant of the dielectric

substrate, distance between each line, width of the line, thickness of the line, connection to ground and electrical property of the ground (such as inductance of the ground) .

The filter F can have a first basic coupling section BCS1, a second basic coupling section BCS2 and an nth basic coupling section BCSn. The number n can be 2, 3, 5, 6, 7, 8, 9, 10 or more. The first basic coupling section BCS1 and the nth basic coupling section BCSn are electromagnetically coupled only to one further basic coupling section: the first basic coupling section BCS1 is electromagnetically coupled only to the second basic coupling section BCS2. The nth basic coupling section BCSn is electromagnetically only coupled to the n-lth basic coupling section. However, the other basic coupling sections are electromagnetically coupled to two further basic coupling sections: the basic coupling section of the next lower number and of the next higher number.

The number of basic coupling sections mainly determines the bandwidth of the corresponding transfer function. The number of resonators determines the number of possible notch

frequencies .

It is possible that the filter has further transmission lines that are not directly electrically connected to a resonator or further resonators that are not directly electrically connected to a coupling circuit such as an LC circuit or a transmission line.

It is to be noted that the connection point of the first basic coupling section BCS1 to the first port and of the nth basic coupling section BCSn to the second port are not limited to the shown position. An RF signal can be fed into the filter structure F and extracted from the filter

structure at different positions, too.

Figure 6 illustrates the possibility of electrically

connecting a series resonator along the signal path. It is possible to electrically connect a first series resonator SRI between the first basic coupling section and the last basic coupling section. In particular, it is possible to

electrically connect the first series resonator SRI at a location near the connectin point of the first port PI and of the second port P2, respectively. As an alternative or additionally to the first series resonator SRI it is possible to provide a second series resonator SR2 that can electrically connect the opposite distal ends of the transmission lines, respectively.

Figure 7 illustrates the possibility of electrically

connecting a series resonator or a series capacitor along the signal path. It is possible to electrically connect a first series resonator SRI or the first series capacitor SCI between the first basic coupling section and the last basic coupling section. In particular, it is possible to

electrically connect the first series resonator SRI or the first series capacitor SCI at a location near the connection point of the first port PI and of the second port P2, respectively .

As an alternative or additionally to the first series resonator SRI or to the first series capacitor SCI it is possible to provide a second series resonator SR2 or a second series capacitor SC2 that can electrically connect the opposite distal ends or in-between distal ends of the

transmission lines, respectively. The opposite distal ends can be electrically connected to a grounding coupling element GCE . Grounding coupling element can be an inductor that is greater than or equal to 0.1 nH, a transmission line or a combination of the inductor and the transmission line.

Figure 8 illustrates the transmission (matrix element S21) and reflection (matrix element Sll) characteristics of a band pass filter comprising five basic coupling sections. Matrix element Sll shows the presence of four notches in the reflectivity characteristic corresponding with good

transmission values.

The number of notches in matrix element Sll is dependent on the number of basic coupling sections.

Further, Figure 9 illustrate filter characteristics of an improved embodiment comprising five basic coupling sections with a transmission line and an electroacoustic resonator in each basic coupling section. The transmission characteristic (matrix element S21) is provided. Electroacoustic resonances are used to create notches within the passband resulting multiple passbands (PB1 and PB2) . Furthermore, the combined width of the resulting passband is not significantly impacted by introduction of the notch. PB1+PB2 is approximately PB .

Figure 10 illustrate filter characteristics of an improved embodiment comprising five basic coupling sections with a transmission line, an electroacoustic resonator in each basic coupling section and a series capacitor connected between the first port and the second port. The transmission

characteristic (matrix element S21) provides a first pass band PB1 with very steep left skirt and a second pass band PB2 with very steep right skirt. Between the two pass bands a frequency range of a high reflectivity (matrix element Sll) is provided. Electroacoustic resonances are used to create notches within the passband resulting multiple passbands (PB1 and PB2) . Furthermore, the combined width of the resulting passband is not significantly impacted by introduction of the notch .

The wideband RF filter and the filter component are not limited to the subject-matter described above and to the technical features shown in the accompanying figures. Filter topologies comprising further filter elements such as resonators, transmission lines, LC circuits or shunt or additional parallel paths are also comprised. The filter component can also comprise further dielectric or

metallization layers and elements integrated in or arranged at an outer side of the substrate.

List of Reference Signs

BCS1 , 2...: first, second, basic coupling section

CC: coupling circuit comprising coupling element CE1 , CE2 : first, second coupling element

Csh : grounding capacitor

Cstatic : static capacitance of an electroacoustic resonator

EMC: electromagnetic coupling

F: wideband RF filter

GCE : grounding coupling element

PI, P2 : first, second port

PB: pass band

Rl, R2 : first, second electroacoustic resonator Sll, S21: matrix elements

SR, SR2 : first, second series resonator

TL: transmission line

Claims

1. A wideband RF filter, comprising

- a first port and a second port,

- a first electroacoustic resonator coupled to the first port,

- a second electroacoustic resonator coupled to the second port, wherein

- the first electroacoustic resonator is electromagnetically coupled to the second electroacoustic resonator.

2. A wideband RF filter, comprising

- a first port and a second port,

- a first electroacoustic resonator coupled to the first port,

- a second electroacoustic resonator coupled to the second port,

- a first coupling element electrically connected to the first resonator,

- a second coupling element electrically connected to the second resonator,

wherein

- the first coupling element is coupled to the second coupling element.

3. The filter of the previous claim, wherein

- the first coupling element is coupled electromagnetically to the second coupling element and

- the first coupling element and the second coupling element couple the first electroacoustic resonator to the second electroacoustic resonator.

4. The filter of one of the previous claims, wherein the first electroacoustic resonator and the second

electroacoustic resonator are coupled via a coupling circuit comprising one or more LC circuits or one or more

transmission lines.

5. The filter of one of the previous claims, comprising a plurality of two or more basic coupling sections coupled in series between the first port and the second port.

6. The filter of the previous claim, wherein each basic coupling section comprises an electroacoustic resonator and a coupling element.

7. The filter of one of the previous claims, comprising

- arranged between the first port and the second port, a signal path provided to transmit an RF signal from the first port to the second port,

wherein

- the signal path comprises galvanically separated sections.

8. The filter of one of the previous claims, wherein the first electroacoustic resonator and the second

electroacoustic resonator are coupled to ground.

9. The filter of one of the previous claims, further

comprising a series resonator or a series capacitor

electrically connected between the first port and the second port .

10. The filter of one of the previous claims, where a

resonance frequency of the first and/or the second electroacoustic resonator is provided to create one or more notches in a transfer function of the filter.

11. The filter of one of the previous claims, being a multi band RF filter.

12. An RF filter component comprising the filter topology of one of the previous claims,

- comprising a multi layer carrier substrate with dielectric layers ,

wherein

- the electroacoustic resonators are arranged at the top side of the carrier substrate and/or in a cavity embedded within the carrier substrate,

- coupling elements for electromagnetically coupling the resonators are realized as metallized structures in

metallization layers between dielectric layers.

13. The RF filter component of the previous claim, further comprising

- an integrated passive device (IPD);

- a piezoelectric material,

wherein

- the electroacoustic resonators and the coupling elements are arranged on the same piezoelectric substrate.