Electroacoustic lattice filter and RF filter module compris ing a lattice filter

The present invention refers to RF filters having an in creased bandwidth and which utilize a lattice filter topology and to RF filter modules comprising corresponding filters.

In mobile communication devices signals of wanted frequency ranges must be separated from signals of unwanted frequency ranges. To that end, filters, e.g. bandpass filters, are em ployed. Bandpass filters can comprise electroacoustic resona tors. Electroacoustic resonators employ acoustic waves and have a piezoelectric material together with electrode struc tures. Electroacoustic resonators can be electrically config ured in a ladder-type like topology. Ladder-like topologies are known, e.g. from DE 10 2008 023 374.

However, ladder-like filter topologies having electroacoustic resonators having a limited bandwidth. The bandwidth of lad der-like filter topologies is limited by the effective elec troacoustic coupling coefficient K ² of the piezoelectric ma terial. The piezoelectric coupling coefficient K ² is typi cally about 6.4 to 7% for BAW resonators (BAW= bulk acoustic wave) and 9% to 10% for SAW resonators (SAW = surface acous tic wave) . With such coefficients bandwidths up to 4% to 5% fractional bandwidth can be obtained.

Dopants can be added to the piezoelectric material in order to increase the piezoelectric coupling coefficient K ² . How ever, this increases manufacturing costs and the increase in bandwidth may not be sufficient for future applications. Further, corresponding filter modules should have a small spatial size. The filter modules should be manufacturable with low production costs and the filters should provide a flexible use. Further, corresponding filter topologies should provide a low insertion loss and a high out-of-band rejec tion.

To increase the out-of-band rejection level conventional RF filters can have cascaded stages of ladder-type like ele ments. Further, the effective coupling coefficient of a reso nator can be increased by electrically connecting the resona tor and an inductance element in parallel.

However, to obtain a wideband filter structure having a high out-of-band rejection, a plurality of ladder-type filter stages leading to a high number of additional inductance ele ments is needed. However, such a filter topology intrinsi cally has a higher insertion loss within the passband.

Thus, modern requirements lead to contradictions in conven tional bandpass filter topologies and it is impossible to im plement such filter topologies, especially with low produc tion costs.

Thus, an RF filter is desired that provides a low insertion loss within a passband, a high out-of-band rejection level, a large bandwidth and small manufacturing costs. Further, it should be possible to implement such a filter in a module with small spatial dimensions. To that end, an RF filter and an RF filter module according to the independent claims are provided. Dependent claims pro vide preferred embodiments.

The RF filter can be established utilizing electroacoustic resonators in a lattice filter topology. Thus, an electroa coustic lattice filter is provided. The electroacoustic lat tice filter comprises a first port and a second port. The first port has a first terminal and a second terminal. The second port has a first terminal and a second terminal. Fur ther, the filter has a first signal line between the first terminal of the first port and the first terminal of the sec ond port. Further, the filter has a connection between the second terminal of the first port and the second terminal of the second port. Additionally, the filter comprises a first electroacoustic resonator electrically connected in the sig nal line between the first terminal of the first port and the first terminal of the second port. Further, the filter has a second electroacoustic resonator electrically connected be tween the first terminal of the first port and the second terminal of the second port. Further, the filter has a diago nally-crossed shunt element electrically connected between the second terminal of the second port and the second termi nal of the first port.

The diagonally-crossed shunt element is the characteristic element of a lattice topology and is not present in a ladder- type like configuration.

Thus, the characteristic difference between a lattice filter and a ladder-type like filter is the presence of a diago nally-crossed shunt element that is electrically connected between a terminal of one port and the corresponding opposite terminal of the corresponding opposite terminal.

In this context, the term "port" denotes one pair of termi nals according to the general understanding in the field of two-port networks. It is possible and preferred that the electroacoustic lattice filter is a bandpass filter.

The electroacoustic resonators can be selected from resona tors working with bulk acoustic waves (BAW resonator) and electroacoustic resonators working with surface acoustic waves (SAW resonator) .

In BAW resonators the piezoelectric material is sandwiched between a bottom electrode and a top electrode. This sandwich construction employs a longitudinal wave mode. The sandwich construction is isolated from its environment by an acoustic mirror below the sandwich construction or a cavity below the sandwich construction.

In SAW resonators comb-like interdigitated electrode struc tures are arranged on the surface of a piezoelectric material and electrode fingers are electrically connected to one of two busbars. Acoustic energy is confined to the resonating area via reflector fingers at the distal ends of the acoustic track .

It is possible that the electroacoustic lattice filter has its diagonally-crossed shunt element selected from a first inductance element and a third electroacoustic resonator.

Further, it is possible that the filter further comprises a second inductance element. The second inductance element is electrically connected between the second electroacoustic resonator and the second terminal of the second port.

It is possible that the diagonally-crossed shunt element and the second inductance element are electromagnetically cou pled. In particular, it is possible that the diagonally- crossed shunt element is the first inductance element. Then, the first inductance element and the second inductance ele ment are electromagnetically coupled. In particular, it is possible that the coupling between the two inductance ele ments is a magnetic coupling. The magnetic coupling can be obtained by providing at least one winding of the first in ductance element in the vicinity of the winding of the second inductance element.

It is possible to realize inductance elements as SMD elements at the top side of a module. Further, it is possible realize inductance elements as structured conductor segments at the surface or in intermediate layers of a carrier substrate. The carrier substrate may be a multilayer substrate. Then, con ductor segments can be realized as structured elements in metallization layers.

It is possible that the filter further comprises a fourth electroacoustic resonator electrically connected between the second terminal of the first port and the second terminal of the second port.

It is possible that the filter further comprises an induct ance element electrically connected in parallel to an elec troacoustic resonator. The number of electroacoustic resona tors can be two, three, four or more. Correspondingly, it is possible that - in addition to the above-mentioned inductance elements, the filter has two, three, four or more such addi tional inductance elements. The first electroacoustic resona tor can have an inductance element electrically connected in parallel thereto. Additionally or as an alternative, the sec ond electroacoustic resonator can have an inductance element electrically connected in parallel thereto. Additionally or as an alternative, the third electroacoustic resonator can have an inductance element electrically connected in parallel thereto. Alternatively or in addition, the fourth electroa coustic resonator can have an inductance element connected in parallel thereto.

Further, it is possible that the filter has a fifth induct ance element electrically connected between the first termi nal of the first terminal of the first port and the second terminal of the first port. Additionally or as an alternative it is possible that a sixth inductance element is electri cally connected between the first terminal of the second port and the second terminal of the second port.

It is possible that the filter is a bandpass filter. It is possible that fractional bandwidth of the bandpass filter is 10% or more.

In this context, the term "fractional bandwidth" denotes the difference between the upper frequency interval limit and the lower frequency interval limit divided by the center fre quency of the passband. The center frequency is located in the center within the interval.

It is possible that the bandpass filter has a center fre quency between 3.3 GHz and 3.8 GHz, between 3.8 GHz and 4.2 GHz or between 4.4 GHz and 4.9 GHz . In particular, it is possible that the filter can be used in the LTE sub-6 GHz frequency range which ranges from 3 GHz to 6 GHz. The frequency range of the filter can be used for LTE advanced pro signals and for 5G NR (NR = new radio) signals.

The filter can be used in TDD systems (TDD = time division duplexing) or FDD systems (FDD = frequency division duplex ing) . Correspondingly, it is possible that the filter is a filter of a duplexer or a multiplexer of the higher degree.

It is possible that the first port and the second port are selected from a balanced port and an unbalanced port.

A balanced port receives or provides balanced RF signals. An unbalanced port receives or provides unbalanced RF signals.

In balanced RF signals two parallel signal lines exist. Each signal line carries the same information of the respective other signal line. In particular, an amplitude and a fre quency of corresponding signals are equal. However, a phase difference between the signals of two signal lines of a bal anced port is 180°.

In contrast, an unbalanced port has one terminal that pro vides or receives an RF signal while the corresponding other terminal has a fixed electric potential, e.g. a ground poten tial. Balanced signal conduction has the advantage that a common-mode signal disturbance applied to both signal lines can be cancelled out by subtracting the signal of one of the two signal lines from the respective other signal. Correspondingly, it is possible that the above filter is uti lized in an RF filter module. In this case, an electroacous tic resonator can be realized on a first carrier and an in ductance element can be realized on the first carrier or on a second carrier.

The first carrier can comprise a piezoelectric material, e.g. in the case of an SAW resonator. However, it is possible that the first carrier is a carrier for a BAW resonator arranged on the first carrier.

The inductance element can be arranged on the same carrier on which an SAW resonator is arranged, e.g. a piezoelectric ma terial and it is possible that the inductance element is ar ranged on the carrier, e.g. a silicon chip, for the corre sponding BAW resonator.

However, it is possible to separate the acoustic components from electric components and to arrange resonator structures on a first carrier and inductance elements on a second car rier. Both carriers can be arranged on a common carrier sub strate .

Central aspects of the present RF filter and details of pre ferred embodiments are shown in the accompanying schematic figures. In the figures:

Fig. 1 shows a basic lattice filter topology;

Fig. 2 shows a basic lattice filter topology where the diago nally-crossed shunt element is realized by an inductance ele ment ; Fig. 3 shows a lattice topology comprising two magnetically coupled inductance elements;

Fig. 4 shows a lattice topology comprising two negatively coupled inductance elements;

Fig. 5 shows the use of inductance elements electrically con nected in parallel to the electroacoustic resonators;

Fig. 6 shows the use of an electroacoustic resonator as the diagonally-crossed shunt element;

Fig. 7 shows a lattice topology comprising four electroacous tic resonators;

Fig. 8 shows the use of inductance elements connected between terminals of a port;

Fig. 9 shows a lattice topology comprising four electroacous tic resonators and four inductance elements;

Fig. 10 shows the increase in bandwidth that can be obtained via the presented lattice topology;

Fig. 11 shows a possible arrangement of structures on a car rier; and

Fig. 12 shows the use of a common carrier for a carrier for the electroacoustic components and for an inductance element.

Figure 1 illustrates a possible basic equivalent circuit dia gram of an electroacoustic lattice filter ELF. The filter ELF has a first port PI and a second port P2. The first port has a first terminal T1 and a second terminal T2. The second port P2 has a first terminal T1 and a second terminal T2. Between the first terminal T1 of the first port PI and the first ter minal T1 of the second port P2 a first electroacoustic reso nator R1 is connected.

A second electroacoustic resonator R1 is electrically con nected between the first terminal T1 of the first port PI and the second terminal T2 of the second port P2.

Further, the filter ELF comprises a diagonally-crossed shunt element DCSE electrically connected between the first termi nal T1 of the second port P2 and the second terminal T1 of the first port PI. Thus, a conductor segment comprising the diagonally-crossed shunt element DCSE crosses a conductor segment in which the second resonator R2 is configured and the filter ELF establishes a lattice filter having the char acteristic diagonal crossing DC.

Figure 2 illustrates the possibility of using an inductance element, i.e. the first inductance element LI, as the diago nally-crossed shunt element DCSE between the first terminal T1 of the second port P2 and the second terminal T2 of the first port PI .

Figure 3 illustrates the possibility of arranging a second inductance element L2 between the second resonator R2 and the second terminal T1 of the second port P2.

The first inductance element LI and the second inductance el ement L2 can be electromagnetically coupled. In particular, it is possible that the electromagnetic coupling between the inductance elements is a magnetic coupling between the in ductance elements, in particular between at least one conduc tor segment of the first inductance element and one conductor segment of the second inductance element L2.

Figure 4 shows one possibility of coupling inductive

elements. The first inductive element LI and the second inductive element L2 can inductors that establish a 1:1 transformer, e.g. by having a similar or the same windings number and a similar, comparable or identical geometry. The first inductive element LI and the second inductive element L2 can be negatively coupled inductors, e.g. with respect to their voltage, current or magnetic field direction.

Inductive coupling between inductive elements in schematic circuits shown in other figures, e.g. of figure 5, can also be realized as negative coupled inductive elements, e.g.

shown in figure 4.

Figure 5 illustrates the possibility of having a third in ductance element L3 and/or a fourth inductance element L4.

The third inductance element L3 can be electrically connected in parallel to first electroacoustic resonator R1. The fourth inductance element L4 can be electrically connected in paral lel to the second electroacoustic resonator R2.

It is possible to increase the effective electromagnetic cou pling coefficient of a resonator via a corresponding parallel inductance element as shown for the first electroacoustic resonator R1 and the second electroacoustic resonator R2 in Figure 5. Figure 6 illustrates the possibility of using a third elec troacoustic resonator R3 as the diagonally-crossed shunt ele ment DCSE. Thus, a lattice filter comprising three electroa coustic resonators is provided.

It is to be noted that a single stage of such a lattice fil ter topology can simultaneously fulfil requirements concern ing a low insertion loss within a passband and a high out-of- band rejection. Thus, even in the case that additional in ductance elements parallel to a resonator are needed, the number of inductance elements - which usually needs a large volume to be integrated - can be reduced to a ladder-type like filter topology where a plurality of filter stages, causing the need for a plurality of parallel inductance ele ments, is required.

The lattice filters shown in Figures 1 to 7 can be used for unbalanced RF signals. The signal line between the first ter minal T1 of the first port PI and the first terminal T1 of the second port P2 can be used to conduct an RF signal. The connection between the second terminal T2 of the first port and the second terminal T2 of the second port can be kept at a constant electric potential, e.g. a ground potential.

Figure 7 illustrates the possibility of electrically connect ing a fourth electroacoustic resonator R4 between the second terminal T2 of the first port PI and the second terminal T2 of the second port P2.

Correspondingly, the filter topology shown in Figure 7 has an equivalent circuit diagram that is symmetric with respect to the two signal lines between the first terminals and the sec ond terminals, respectively. Thus, it is possible to utilize the filter topology shown in Figure 7 to conduct balanced RF signals.

Figure 8 illustrates the further possibility to arrange a fifth inductance element between the first terminal T1 of the first port PI and the second terminal T2 of the first port PI. Additionally, or as an alternative it is possible to electrically connect a sixth inductance element L6 between the first terminal T1 of the second port P2 and the second terminal T2 of the second port P2.

The filter topology shown in Figure 7 can be used for con ducting balanced RF signals.

Figure 9 shows the possibility of providing each electroa coustic resonator with its own parallel inductance element to enhance bandwidth.

The third resonator R3 can have an inductance element L7 electrically connected in parallel. The resonator between the second terminals of the two ports can have an inductance ele ment L8 electrically connected in parallel.

The lattice filter shown in Figure 9 can also be used to fil ter balanced RF signals.

Figure 10 illustrates a comparison between a six stage ladder-type filter (curve 1) and a lattice filter according to the circuit shown in Figure 9 (curve 2) .

To obtain a sufficient out-of-band attenuation six filter stages of the conventional ladder-type topology are needed. Thus, the ladder-type filter has six series resonators and six parallel resonators in six corresponding parallel paths between the signal path and ground.

In contrast, the lattice filter needs only four inductance elements together with four electroacoustic resonators to fulfil the requirements concerning out-of-band attenuation.

In particular the insertion loss within the passband is smaller than the insertion loss within the passband of the ladder-type filter.

Also, the strongly reduced number of electroacoustic resona tors and of inductance elements allows to realize the corre sponding filter module with much smaller spatial dimensions.

Figure 11 shows the possibility of having a carrier C on which structures of an electroacoustic resonator EAR are ar ranged. The same carrier C can be utilized to carry struc tures of inductance elements L.

Further, Figure 12 shows the possibility of utilizing a sec ond carrier C2 on which conductor elements of inductance ele ments L are arranged. Additionally, carrier C2 has another carrier Cl attached to its surface which carries the struc tures of the electroacoustic resonators EAR.

Generally, the construction shown in Figure 11 allows for more compact packages. However, the construction shown in Figure 12 allows the possibility of choosing material proper ties independently for the electroacoustic resonator EAR and the inductance element L. The lattice filter can comprise further circuit elements, e.g. further resonators or inductance or capacitance ele ments. The filter module also can comprise further electri cal, electromagnetic, electronic or mechanical components, in particular a housing for protecting sensitive MEMS structures (MEMS = micro-electro-mechanical system) of the electroacous tic resonators from an environment.

List of Reference Signs

C, Cl, C2 : carrier

EAR: electroacoustic resonator

ELF: electroacoustic lattice filter

L: inductance element

LI, L2 , L3 , L4 first, second, third, fourth inductance el ement

L5 , L6, L7 , L8 : fifth, sixth, seventh, eighth inductance element

PI, P2 : first, second port

Rl, R2 , R3 , R4 : first, second, third, fourth electroacous tic resonator

Tl, T2 : first, second terminal