RF band pass filter, filter component and duplexer

The present invention refers to RF band pass filters, filter components and duplexers that may be used in frontend systems of mobile communication devices.

In mobile communication devices filter functionality is re quired that provides a high electric performance, broad pass bands and state of the art attenuation. Further, filters of mobile communication devices should be compatible with co-ex- istence requirements, e.g. for utilizing different frequency bands simultaneously. Further, corresponding components real izing such functionality should have a high integration den sity and a low insertion loss to save energy.

Due to the trend towards an increasing number of wireless functions, there is a necessity to design new filters. How ever, what is wanted is the possibility of designing new fil ters with reduced effort. Also a high degree of flexibility in designing new filter functionality is wanted.

Thus, RF filters complying with these requirements are needed .

A corresponding RF filter, a filter component and a duplexer according to the independent claims are provided. Dependent claims provide preferred embodiments.

An RF band pass filter (RF = radio frequency) comprises an input port and an output port. A signal path is arranged be tween the input port and the output port and electrically connects the input port to the output port. Further, the bandpass filter has one or more inductance elements and one or more capacitance elements. Additionally, the RF band pass filter has a first electroacoustic resonator. The RF band pass filter provides at least one pass band. The first elec troacoustic resonator is acoustically inactive and frequen cies within the pass band. The RF band pass filter is a hy brid filter.

Such an RF band pass filter is a hybrid filter that utilizes LC elements and electroacoustic elements. An inductance ele ment has a specific inductivity and can be realized as a coil arrangement or as a patch of conductor segments, for example. A capacitance element has a specific capacity and can be re alized as a capacitor comprising a dielectric material be tween two electrodes.

In contrast, an electroacoustic resonator is a structure that can be acoustically active at least in a specific frequency range. An electroacoustic resonator usually comprises a pie zoelectric material and electrode structures. Electrode structures are connected to the piezoelectric material and if an RF signal is applied to the electrode structure then the electrode structure and the piezoelectric material - due to the piezoelectric effect - convert between RF signals and acoustic waves. Electroacoustic resonators usually provide a resonance frequency and an anti-resonance frequency being ar ranged above the resonance frequency. The transfer function reaches a minimum insertion loss at the resonance frequency and a maximum insertion loss at the anti-resonance frequency. Capacitance elements and inductance elements can be realized as pure capacitance elements and inductance elements, respec tively. A pure capacitance element or a pure inductance ele ment is acoustically inactive at every possible relevant fre quency .

However, due to the complex interaction between acoustic ef fects and electric properties of an electroacoustic resonator there are frequency ranges in which an electroacoustic reso nator provides only an electric response that equals the electric response of a pure capacitance element. In frequency ranges in which an electroacoustic resonator is acoustically active the electroacoustic resonator provides a more complex behaviour. In its acoustically active frequency range an electroacoustic resonator provides an electric response cor responding to that of a parallel connection comprising a first branch and a second branch. In the first branch of the parallel connection a capacitance element is contained. In the second parallel branch a series connection of a capaci tance element and an inductance element is contained.

From WO 2006/032366 A1 hybrid filters are known.

However, in contrast to known hybrid filters the present RF band pass filter uses an electroacoustic resonator that is acoustically inactive in the pass band. In the context of the present application the term "acoustically inactive at fre quencies within the pass band means that the transfer func tion of the filter would be essentially unchanged if the electroacoustic resonator was replaced by a pure capacitance element having a capacity equal to the static capacity of the electroacoustic resonator. In conventional hybrid filters the acoustic properties of electroacoustic resonators are used to shape the pass band. Thus, the approach presented with the above-described bandpass filter is counterintuitive. However, the present bandpass filter bases on the idea that for the sake of keeping design efforts simple, known LC filters could be simply replaced by the presented bandpass filters and no further consideration with respect to the filter characteris tics in the pass bands are necessary. In particular, the electroacoustic resonator' s properties are not used to shape the pass band but to improve the transfer characteristics outside the pass band, e.g. for creating notches outside the pass band.

Correspondingly, an RF band pass filter is provided that com bines pure capacitance elements, pure inductance elements and electroacoustic resonators.

The at least one inductance element and the at least one ca pacitance element establish an LC element. One or several LC elements can be used to provide the wanted filter functional ity at pass band frequencies. An inductance element and a ca pacitance element can be arranged in a parallel configuration to create a parallel LC circuit. However, it is possible that an inductance element and a capacitance element can be ar ranged in a series configuration establishing a series LC circuit .

Circuit elements can be cascaded to improve the out-of-band attenuation .

The bandpass filter can have a ladder-type like configura tion. In a ladder-type like configuration series elements are electrically connected in the signal path between the input port and the output port and shunt elements are electrically connected in shunt paths electrically connecting the signal path to ground.

It is possible that the RF band pass filter comprises two or more electroacoustic resonators and a plurality of inductance and capacitance elements.

It is possible that the bandpass filter' s electroacoustic resonators are selected from SAW resonators (SAW = surface acoustic wave) , TF-SAW resonators (TF = thin-film) , BAW reso nators (BAW = bulk acoustic wave) , and GBAW resonators (GBAW = guided bulk acoustic wave) .

An SAW resonator comprises a piezoelectric material. On the piezoelectric material interdigitated comb-like electrode structures are arranged. The electrode structures comprise electrode fingers. Electrode fingers can be electrically con nected to one of two opposite busbars. Between electrode fin gers electrically connected to opposite busbars an excitation area is obtained. The electrode structure together with the piezoelectric material converts between RF signals and acous tic waves when an RF signal is applied to the electrode structure's busbars. In the case of SAW resonators, acoustic surface waves are generated that propagate at the surface of the piezoelectric material. Reflector structures such as structured metal stripes are arranged at opposite ends of the acoustic track and act as acoustic reflectors confining acoustic waves and the waves' energy within the acoustic track such that a resonating structure is obtained.

In contrast, BAW resonators have a bottom electrode, a top electrode and a piezoelectric material arranged between the bottom electrode and the top electrode in a sandwich-like layer stack. Acoustic waves are generated in the form of lon gitudinal waves. The acoustic energy is confined to the reso nating structure by applying an acoustic mirror or a cavity below the bottom electrode. The acoustic mirror can comprise a plurality of layers of different acoustic impedance.

TF-SAW resonators have a construction similar to SAW resona tors. However, while SAW resonators base on electrode struc tures arranged on a single crystal piezoelectric material, TF-SAW resonators have electrode structures arranged on a pi ezoelectric thin-film. Thus, the TF-SAW resonators' piezoe lectric materials obtained by depositing the piezoelectric material utilizing thin-film deposition techniques such as sputtering, chemical vapor deposition, physical vapor deposi tion, molecular beam epitaxy and the like.

GBAW resonators have a piezoelectric material and an inter- digitated comb-like electrode structure arranged thereon. The electrode structure is covered by one or more further layers. Acoustic waves are generated that propagate at the interface between the piezoelectric material and the further material. As a further material a temperature compensation layer can be used. A temperature compensation layer has temperature-de- pendent characteristics that compensate unwanted temperature- dependent characteristics of the electrode structures and the piezoelectric material. Thus, electroacoustic resonators with a reduced or even eliminated frequency drift of characteris tic frequencies can be obtained.

It is possible that in the pass band the electroacoustic res onator is characterized by its static capacitance only. Thus, in the pass band the electroacoustic resonator resonator acts as a pure capacitance element and the other filter components only "see" the electroacoustic resonator's static capacity.

It is possible that the electroacoustic resonator is acousti cally active in a rejection band outside the pass band.

The acoustics of the electroacoustic resonator can be uti lized to create frequency regions having a high insertion loss that is accompanied by steep rejection band skirts.

LC filters can provide - depending on their degree of cas- cadation - good out-of-band attenuation. However, the typical transition from a low insertion loss frequency range to a high insertion loss frequency range demands for a wide fre quency range. The characteristic properties of electroacous tic resonators utilizing the resonator's resonance and anti resonance, however, can be used to add the functionality of steep insertion loss transitions to the functionality of pure LC filters.

It is possible that a series impedance element is electri cally connected in series in the signal path.

Also, it is possible that a parallel impedance element is electrically connected in a shunt path electrically connect ing the signal path to ground.

This means that the position of pure impedance elements such as pure inductance elements and pure capacitance elements are not limited to the signal path only or to shunt paths only. The signal path and one or more shunt paths can comprise pure inductance elements and pure capacitance elements, respec tively. Also, the position of electroacoustic resonators is not lim ited to either the signal path or to a specific shunt path. The signal path can comprise one or more electroacoustic res onator and each shunt path can comprise one or more electroa coustic resonator. However, it is not necessary that each shunt path has an electroacoustic resonator.

Correspondingly, it is possible that an electroacoustic reso nator is electrically connected in the signal path or in a shunt path.

It is possible that a pure capacitance element is electri cally connected in the signal path or in a shunt path.

In one possible configuration the RF band pass filter com prises two electroacoustic resonators and two inductance ele ments in the signal path. Further, the filter comprises two shunt paths. Each shunt path has an electroacoustic resona tor. Further, the filter comprises one shunt path having an inductance element. Further, the filter comprises a capaci tance element electrically connected in parallel to the sig nal path.

Also, it is possible that the RF band pass filter comprises four electroacoustic resonators and two inductance elements in the signal path. This configuration has four shunt paths. Each shunt path has an electroacoustic resonator. One shunt path has an inductance element and a capacitance element is electrically connected in parallel to the signal path.

Further, it is possible that the RF band pass filter has three electroacoustic resonators, one capacitance element and two inductance elements in the signal path. The filter has three shunt paths. Each shunt path has an electroacoustic resonator. Further, the filter has one shunt path having a capacitance element and one shunt path having an inductance element. Additionally, a capacitance element is electrically connected in parallel to the signal path.

The provision of a hybrid filter as described above allows the provision of a corresponding filter module that can be created in an ultra-compact package while simultaneously ex cellent acoustic properties and that is compatible with the integration of several filter functionalities in a single component .

In particular, pure capacitance elements or pure inductance elements or more general: impedance elements that are acous tically inactive in every frequency region can be associated with a first carrier substrate while electroacoustic resona tors can be associated with a second carrier substrate. The, electric circuit elements of a similar construction have a similar environment that is chosen according to the electric requirements of the corresponding circuit elements. The envi ronment of capacitance elements can be chosen such that the dielectric between the electrodes of the capacitance elements complies with requirements concerning the dielectric con stant. Conductor segments of inductance elements can be inte grated in a multilayer substrate or arranged as an SMD compo nent (SMD = surface mounted device) on a corresponding car rier substrate.

In contrast thereto, electroacoustic resonators have special demands concerning protection and encapsulation. The sensi- tive components of electroacoustic resonators need to be pro tected against environmental influences while must be simul taneously separated from structures draining acoustic energy. Thus, the need for an encapsulated cavity in the vicinity of the sensitive structures of the electroacoustic components may exist. Correspondingly, the second carrier substrate can be chosen such that the acoustic resonators' needs are ful filled.

Correspondingly, it is possible that a filter module com prises a first substrate and a second substrate. The second substrate may be arranged on the first substrate and electri cally connected to the first substrate. At least two capaci tance elements and two inductance elements are arranged in or at the first substrate. At least one electroacoustic resona tor is arranged in or at the second substrate. Impedance ele ments and at least one electroacoustic resonator establish an RF band pass filter as described above.

Further, it is possible that a duplexer or a multiplexer of a higher degree comprises at least one filter as described above .

Thus, ultra-compact, high performance and broadband RF fil ters are realized. The filters can have additional acoustics- based notch filtering functionality to fulfill state-of-the art attenuation and co-existence requirements.

A carrier substrate for pure passive impedance elements can comprise an LTCC material (LTCC = low temperature co-fired ceramics) or a laminate material comprising an organic mate rial . The high quality factor of electroacoustic resonators allows enhancing LC filters in particular at frequency positions where high flank steepness is required. The mechanical and electrical connection of two substrates, each substrate being optimized to either passive or acoustically active components provides compact geometric dimensions compatible with the on going trend towards miniaturization.

Central aspects of the filter and details of preferred embod iments are shown in the accompanying schematic figures.

In the figures:

Fig. 1 shows a possible equivalent circuit diagram of a band pass filter;

Fig. 2 illustrates the dual nature of an electroacoustic res onator;

Fig. 3 illustrates a component comprising two substrates;

Fig. 4 illustrates an equivalent circuit diagram of a band pass filter outside a pass band;

Fig. 5 illustrates the frequency-dependent transfer function if no acoustic effects are taken into account;

Fig. 6 illustrates the corresponding frequency-dependent transfer function with acoustic effects taken into account;

Fig. 7 illustrates the performance of a duplexer utilizing the proposed filter; Fig. 8 illustrates the possibility of having more than one rejection band;

Fig. 9 illustrates an equivalent circuit diagram correspond ing to the transfer function of Fig. 8; and

Figs. 10 and 11 illustrate corresponding frequency character istics and a filter topology.

Figure 1 shows an equivalent circuit diagram of an LC filter establishing a band pass filter BPF. The filter has an input port IN and an output port OUT. A signal path SP electrically connects the input port IN to the output port OUT. In the signal path SP a capacitance element CE, an inductance ele ment, an additional inductance element and an additional ca pacitance element are electrically connected in series be tween the input port IN and the output port OUT. A first shunt path electrically connects the node between the first capacitance element and the first inductance element of the signal path SP to ground. In a second shunt path an induct ance element IE connects the two electrodes of the two in ductance elements in the signal path SP to ground. A third shunt path SHP electrically connects the node between the second inductance element and the second capacitance element to ground. An additional capacitance element electrically connects the input port IN to the output port OUT. This addi tional capacitance element is electrically connected in par allel to the signal path SP.

One or more of the capacitance elements CE of the band pass filter BPF of Figure 1 can be realized by an electroacoustic resonator that is acoustically inactive in the pass band. The dual nature of an electroacoustic resonator EAR is illus trated in Figure 2 : In a frequency range away from the reso nance frequency and the anti-resonance frequency of the elec troacoustic resonator EAR, the electroacoustic resonator EAR is seen as a capacitance element CE only. However, in a fre quency range near the resonance frequency and near the anti resonance frequency, the electric response of the electroa coustic resonator corresponds to that of a parallel connec tion having two branches. In the first branch a single capac itance element CE is arranged. The second branch consists of a series connection of a capacitance element CE and an in ductance element IE. The respective capacitance elements CE of the two branches do not necessarily have the same capaci tance value.

Thus, the equivalent circuit diagram shown in Figure 1 corre sponds to a frequency within the pass band. Outside the pass band and near the resonance frequency and the anti-resonance frequency of the electroacoustic resonator, the capacitance element in the equivalent circuit diagram needs to be re placed by the parallel configuration having the two branches as shown at the bottom part of the right-hand side of Figure 2.

Figure 3 illustrates the possibility of electrically and me chanically connecting a first substrate SI and a second sub strate S2 to obtain a single filter component FC . The filter component FC has the pure inductance elements and the pure capacitance elements realized in or at the first substrate SI. The second substrate S2 provides the housing for the sen sitive elements of the at least one electroacoustic resona tor. Bump connections BC mechanically and electrically con nect the two substrates SI, S2. However, different electrical connections between the sub strates, such as wire connections, are also possible.

Figure 4 illustrates an equivalent circuit diagram where four electroacoustic resonators are acoustically active. Thus, in the respective frequency range outside a band pass a first electroacoustic resonator EAR, a first inductance element, a second inductance element and a second electroacoustic reso nator are electrically connected in series in the signal path SP between the input port IN and the output port OUT. In the first shunt path an electroacoustic resonator electrically connects the signal path SP to ground. Also, in the third shunt path SHP an electroacoustic resonator electrically con nects the signal path SP to ground.

Figure 5 illustrates the transfer function of the filter to pology shown in Figure 1 if only pure capacitance elements without acoustic properties are utilized. Within the pass band a low insertion loss is obtained and at frequencies quite far away from the pass band the requirements concerning the rejection level are complied with.

However, if future specifications demand for a high insertion loss at frequency ranges near the pass band, then conven tional topologies would fail to comply with these future specifications .

In order to have the possibility of complying with future specifications while at the same time keeping efforts to de sign improved filters to be low, the above-described replace ment solves the two problems elegantly. Correspondingly, Figure 6 illustrates the effects of utiliz ing electroacoustic resonators in a per se LC filter to im prove the filter characteristics outside the pass band. Com pared to the transfer function shown in Figure 5, the consid eration of the acoustic properties provides an additional frequency range of high insertion loss flanked by narrow transition ranges while the pass band characteristics remain unchanged .

Figure 7 illustrates that corresponding filters can be com bined to create a duplexer. The duplexer provides a pass and for the low band LB and a pass band for the mid band MB. In both transfer functions electroacoustic resonators are uti lized to provide an additional frequency range of high atten uation flanked by narrow transition ranges.

Figure 8 illustrates the possibility of providing electroa coustic resonators such that outside the pass band more than one rejection frequency range is obtained. In this case an additional rejection frequency range below the pass band and an additional rejection frequency range above the pass band are obtained.

Figure 9 illustrates an equivalent circuit diagram that al lows the insertion loss shown in Figure 8. The filter topol ogy has a first and a second electroacoustic resonator elec trically configured in a series configuration before the se ries configuration of the two inductance elements and an ad ditional series configuration of two electroacoustic resona tors after the series configuration of the two inductance el ements. Further, between the first series configuration of acoustic resonators and the series configuration of the in ductance elements, two shunt paths electrically connect the signal path to ground. Each shunt path has one electroacous tic resonator. Also, between the series configuration of the inductance elements and the second series configuration of electroacoustic resonators, two shunt paths electrically con nect the signal path to ground. Each of these two second shunt paths also has an electroacoustic resonator.

Figure 10 illustrates a frequency-dependent transfer function for the circuit topology shown in Figure 11.

Figure 11 provides a filter topology having two electroacous tic resonators replaced by pure capacitance elements compared to the filter topology shown in Figure 9. In particular, with respect to the filter topology of Figure 9, the first elec troacoustic resonator as seen from the input port IN is re placed by a pure capacitance element. Also, the electroacous tic resonator of the first shunt path as seen from the input port IN is replaced by a pure capacitance element.

Thus, depending on the specified filter requirements the fil ter topology can be tailored such that filter requirements can be complied with while design efforts are kept low.

The RF band pass filter, the filter module and the duplexer are not limited to the details and embodiments shown and de scribed above. The filter can comprise further filter compo nents, such as further filter stages, electroacoustic resona tors and LC elements and the component can comprise further structures and connections. List of Reference Signs

BC: bump connection

BPF : band pass filter

CE: capacitance element

EAR: electroacoustic resonator FC: filter component

IE : inductance element

IL: insertion loss

IN: input port

LB: low band

MB: mid band

OUT: output port

PP: parallel path

SI : first substrate

S2 : second substrate

SHP : shunt path

SP: signal path