The present invention relates to a resonator filter structure according to the preamble of claim 1.

More specifically, the invention relates to a resonator filter structure arranged for providing a passband which can be defined by frequencies as a center frequency fc, a lower cut off frequency fL, a upper cut off frequency fu comprising between an input port and an output port at least one resonator filter section with first resonator elements having a resonance frequency fiR and an anti-resonance frequency fiA and with second resonator elements having a resonance frequency f2R and an anti-resonance frequency f2A.

The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. This leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This concerns radio frequency (RF) filter structures, which despite the increasing miniaturization should be able to withstand considerable power levels, have steep passband edges, and low losses. Due to use of high frequencies in the range of GHz special circuit elements for building RF filter structures are required and high frequency related concerns have to be dealt with.

For example, receive band filters for modern telecommunication standards, like PCS, need steep transition from stopband to passband since Tx and Rx are closely separated. For instance, extended GSM (EGSM) is the standard for European second generation 1 GHz mobile communication. The Rx and Tx bands are centered at 942. 5 and 897. 5 MHz, respectively. Both of these have a bandwidth of 35 MHz, resulting in fractional bandwidth of 3. 71% and 3. 9% for the Rx and Tx ; respectively. Moreover, some newer applications, for example, GPS or TV up conversion filter require even smaller bandwidths.

Accordingly, it is known to use mechanical resonator characteristics in filter circuits for electrical signals. These resonators can be divided into two classes that are derived from the utilized kind of mechanical vibration. In a first case, surface acoustic vibration modes of a solid surface are utilized, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. In other words, a surface acoustic wave is travelling on the surface of the solid material, wherein the mechanical or

acoustic waves, respectively, are coupled in and out via applicable formed electrical connections that cause a frequency selective behavior. Due to the used surface acoustic waves such elements are called Surface Acoustic Wave (SAW) filters or SAW resonators. A SAW resonator typically comprises a piezoelectric solid and two interdigitated structures as electrodes. Various circuits as filters or oscillators containing resonator elements are produced with SAW resonators, which have the advantage of very small size, but unfortunately a weakness in withstanding high power levels.

In the second case, a mechanical vibration of a bulk material is used which is sandwiched between at least two electrodes for electrical connection. Typically the bulk material is a single piezoelectric layer (piezo) disposed between the two electrodes. When alternating electrical potential is applied across the metal-piezo-metal sandwich, the entire bulk material expands and contracts, creating the mechanical vibration. This vibration is in the bulk of the material, as opposed to being confined to the surface, as is the case for SAWs.

Therefore, such elements are called Bulk Acoustic Wave (BAW) resonators. BAW resonators are often employed in bandpass filters having various topologies. Further known BAW resonator elements are thin film bulk acoustic resonators, so called FBARs, which are created using a thin film semiconductor process to build the metal-piezo-metal sandwich in air in contrast to the afore-mentioned BAWs, which are usually solidly mounted to a substrate.

The electrical behavior of a SAW or BAW resonator is quite accurately characterized by the equivalent circuit, which is shown in the accompanying Fig. 5. In Fig. 5 there is a branch comprising a series combination of an equivalent inductance Ls, an equivalent capacitance Cs, and an equivalent resistance Rs. Ls and Cs are the motional inductance and capacitance respectively and Rs represents the acoustic losses of the resonator. These series elements are connected in parallel to a capacitance Cp that follows from the dielectric properties of the piezoelectric material. Therefore, each SAW or BAW resonator comprises two characteristic resonance frequencies, which is a series resonance frequency and a parallel resonance frequency. The first is mostly called resonance frequency f and the second is also known as anti-resonance frequency fA.

Circuits comprising BAW or SAW elements in general are better understood in view of above-introduced element equivalent circuit. The series resonance of the individual resonator element is caused by the equivalent inductance Ls and the equivalent capacitance Cs. At frequencies that are lower than the series resonance frequency, the impedance of the resonator element is capacitive. At frequencies higher than the series

resonance frequency of the resonator element, but which are lower than the parallel resonance frequency of the resonator element, caused by the parallel capacitance Cp, the impedance of the resonator element is inductive. Also, at higher frequencies than the parallel resonance frequency impedance of the resonator element is again capacitive.

As to the impedance characteristic of the resonator element with respect to signal frequency, at the (series) resonance frequency fR of the resonator element, the impedance of the resonator element is low, i. e. in an ideal case, where there are no losses in the element, the resonator element functions like a short circuit. At the parallel or anti- resonance frequency fA, respectively, the impedance of the resonator element is high, i. e. in an ideal case without losses the impedance is infinite and the device resembles an open circuit at the anti-resonance frequency. Therefore, the resonance-and anti-resonance frequencies (fR and fA) are important design parameters in filter design. The resonance and anti-resonance frequencies are determined by process parameters like the thickness of the piezoelectric layer of each resonator element and/or the amount of massloading.

A first known filter type with BAW resonator elements is constructed in a topology known as ladder type topology. For the purposes of this description, ladder type filters that are built primarily of BAW resonator elements are referred to as"BAW ladder filters". BAW ladder filters are typically made so that one or more BAW resonators are series-connected within the filter and one or more BAW resonators are shunt-connected within the filter. Further, a BAW ladder filter is typically designed so that series-connected resonators also called"series resonators", yield series resonance at a frequency that is approximately equal to, or near, the desired, i. e. design or center, frequency of the respective filter. Further, shunt-connected resonators, also referred to as"shunt resonators"or"parallel resonators", yield parallel resonance at a frequency that is approximately equal to, or near, the desired center frequency of the respective filter.

A second known circuit topology for filters is the BAW lattice circuit, which circuit topology is also called balanced bridge design. Such a BAW lattice circuit has a stopband when all branches have approximately equal impedance and a passband when one branch type, i. e. the series arm or the lattice arm, respectively, behaves inductive and the other capacitive. Fig. 6 shows the impedance characteristics of two different BAW resonator elements, BAW-1 and BAW-2, usually used in filter design. BAW-1 and BAW-2 are made such, as anti-resonance frequency fA, of BAW-1 is substantially equal to resonance frequency fR2 of BAW-2. Thus, it can be seen that with such two types of BAW resonators according to the afore-mentioned circuit topologies BAW resonator filters can be constructed, which have

a passband approximately corresponding to the difference Af between the lowest resonance frequency, here frai, and the highest anti-resonance frequency, here fA2. BAW series and lattice resonator elements may be exchanged provided series or horizontal resonators are of one type and lattice or diagonal resonators are of the other type. The bandwidth, i. e. the passband, of the thus created filter corresponds approximately to the difference between the highest anti-resonance frequency and the lowest resonance frequency of the used resonator elements. BAW lattice circuits have the advantage that there is a deep stopband rejection far away from the passband.

According to preventing power losses, which causes attenuation in the passband, impedance matching between circuits within the signal path is crucial. In case generator and load impedance are equal, impedance matching can be achieved by scaling BAW resonator areas. However, when generator, for instance the antenna of a mobile communication unit, and load impedance, for instance, the impedance of a following low noise amplifier (LNA), are different, impedance matching requires also impedance transformation, e. g. from 50Q in to a 150 to 200Q differential-out. Moreover, balanced output is preferred as well; because usually low noise amplifiers (LNA) incorporated after receive filters often require a balanced input signal. Hence, unbalanced-in to balanced-out filters are generally preferred as receive filter for communication systems. However, this combination of specifications will not be met with a pure lattice or ladder filter design, since these filter constructions generally have a poor amplitude and phase balance. For some applications this needs to be improved.

Therefore, it is an object of the present invention to provide a resonator filter structure, which provides improved phase and amplitude balance. It is a further objective to have a resonator filter circuit, which has a steep transition from stopband to passband.

Another objective is to provide impedance transformation between the impedance levels at input and output port of the resonator filter structure. Moreover, it is a further objective to have the input and output impedances of the resonator filter structure substantially matched with the respective loads.

Accordingly, a resonator filter structure of the present invention is characterized by means for balance improvement connected in cascade to a resonator filter section. As resonator elements can be used any circuit element which provides an impedance characteristic over signal frequency as is illustrated as an example in Fig. 6, where for two resonator elements BAW-1 and BAW-2 the impedance characteristic is shown, or as is expected from a circuit as shown in Fig. 5.

Preferably are used as resonator elements acoustic wave resonator elements, more preferably are used bulk acoustic wave (BAW) resonator elements; surface acoustic wave (SAW) resonator elements may also be used. When BAW resonator elements are used in the invention, such a BAW resonator comprises a stack on a substrate with at least one or more acoustic reflective layer, a bottom electrode, a bulk, a top electrode, and an optional massload on top of the top electrode. Thereby, the bulk of the BAW resonator elements comprise a piezoelectric layer having a predetermined thickness and being made of an piezoelectric material such as aluminum nitride (AIN) or zinc oxide (ZnO) and having an optional additional dielectric layer, for instance, silicon oxide (Si02). The combination of silicon oxide (Si02) and aluminum nitride (A1N) in the BAW resonators reduces the coupling coefficient of the BAW resonator elements, as required in some applications with respect, for instance, to bandwidth or temperature stability. According to the fabrication process of such BAW resonator elements, advantageously, the thickness of the component layers of the bulk, and/or the massload, and/or the electrode layers for each BAW resonator element can be used to arrange the BAW resonator elements to have a predetermined resonance frequency and a predetermined anti-resonance frequency.

For so-called thickness modes the frequency of acoustic vibration is approximately inversely proportional to the thickness of the piezoelectric layer. The piezoelectric thickness is therefore of the order of 1 micron, so typically a thin-film semiconductor process is used. In one embodiment, the solidly-mounted bulk acoustic wave resonator (sometimes called SBAR) one or more acoustic layers are employed between the piezoelectric layer and the substrate. An alternative embodiment of thin-film BAW resonator elements (sometimes called FBAR) employs a membrane approach with the metal-piezo- metal sandwich suspended in air. BAW resonators are often employed in bandpass filters having various topologies. One advantage of BAW resonators is the intrinsically better power handling compared to the interdigitated structures used in surface acoustic wave (SAW) resonators, especially at frequencies of modern wireless systems where the pitch of the interdigital structures must be sub-micron.

The resonator filter structure is arranged for providing a passband which can be defined by frequencies as a center frequency fc, a lower cut off frequency fL, a upper cut off frequency fu comprising between an input port and an output port at least one resonator filter section with first resonator elements having a resonance frequency fiR and an anti- resonance frequency fiA and with second resonator elements having a resonance frequency f2R and an anti-resonance frequency f2A.

According to the invention, the means for balance improvement comprise a LC-lattice section wherein inductive elements Lx and capacitive elements Cx, are arranged in lattice circuit configuration. The inductive elements Lx are arranged as series elements of the LC-lattice section and the capacitive elements Cx are arranged as lattice elements of the LC- lattice section, or vice versa. Advantageously, at least at one side of the LC-lattice section for balance improvement the resonator filter structure provides balanced signal guidance. It should be noted that the LC-lattice section can be included in the resonator filters structure at any position.

As to the inductances Lx and the capacitances Cx of the LC-lattice section, the inventors have found that it is advantageous when Lx and Cx are derived from the equations wherein fc is the center frequency of the resonator filter structure, RIN is a input impedance level of the LC-lattice section, and ROUT is a output impedance level of the LC-lattice section.

The LC-lattice section may comprise discrete circuit elements; preferably the LC-lattice section is made with passive integration technologies.

In a first embodiment a resonator filter section is a resonator ladder filter section having one or more of the first resonator elements arranged as series arms alternating with at least one of the second resonator elements arranged as shunt arms. When a resonator ladder type filter section is connected to a balanced side of the means for balance improvement it is symmetrical constructed for providing balanced signal guidance, too.

In a second embodiment a resonator filter section is a resonator lattice type filter section having the first resonator elements arranged as series arms and having the second resonator elements arranged as lattice arms. It should be noted that series resonator elements and lattice resonator elements of a resonator lattice filter section may be exchanged, provided that series resonator elements are of one type and lattice resonator elements are of the other type.

A combination of a resonator ladder filter section and/or a resonator lattice filter section with a specific LC-lattice section according the invention, which is designed for optimum balance at the filter center frequency fc, significantly improves the amplitude

balance and phase balance in a frequency band which is large enough for communication standards like PCS.

In further advantageous development of the resonator filter structure, there is a resonator lattice filter section having resonance frequencies and anti-resonance frequencies of the first and second resonator elements substantially equal. Further, there are capacitances connected in parallel across the lattice arm resonator elements or the series arm resonator elements. This way, very narrow filter bandwidth is realized by using only one type of resonator in the circuit having one resonance frequency and one anti-resonance frequency. As a further advantage, the processing is simplified by eliminating the step of creating an offset in resonance frequencies. For instance, all resonators can be made with same thickness of the piezoelectric layer and no massloading is required. The series resonators of different filter sections may differ in area. The capacitances C, which are connected in parallel to one type of the resonator elements, advantageously move the anti-resonance frequencies of those resonators. Thus, the capacitance value C is used to tune the bandwidth, wherein the smaller the capacitance, the smaller the bandwidth. It has further found by the inventors that to create a good stopband rejection, the total capacitance of all branches needs to be equal in the stopband.

Further, it should be noted that series and lattice branches within one filter section of the resonator filter structure may be exchanged. Furthermore, input or output port or even none of both may be connected to a fixed reference potential. When this technique is combined with other processing techniques that reduce the coupling coefficient between the BAW resonator elements, like combining silicone oxide (Si02) and aluminum nitride (A1N) in the BAW resonators, results are more improved.

In another advantageous development, the resonator filter structure comprises a resonator lattice filter section and at least one additional resonator ladder filter section.

Such combination of these two resonator filter sections adds the best of both topologies. The transition from stopband to passband is much steeper than for a sole lattice design. Further, stopband rejection is much deeper than with a sole ladder design. Furthermore, common mode rejection in stopband is equal to stopband rejection of the applied ladder sections.

Further, the combination provides unbalanced-in to balanced-out, which is preferred for receive filters and fits to the LC-lattice section for balance improvement of the present invention.

Moreover, with the combination of lattice filter sections with ladder filter sections, several implementations of the invention with respect to the application needs are

possible: unbalanced ladder section (s) are connected to the unbalanced port of the resonator filter structure and the lattice section (s) are connected to the balanced port of the resonator filter structure. Furthermore, it is has found that an unrestricted amount of balanced ladder sections can be connected between any two lattice sections or between the lattice sections and the balanced port of the resonator filter structure. It should be noted that the amount of ladder and lattice sections is in general unrestricted. In all designs, typically series resonators have the same resonance and anti-resonance frequencies, but may have different areas on the substrate of the respective device. Also all lattice or shunt resonator elements have the same resonance and anti-resonance frequencies, but may have different areas. The resonance and anti-resonance frequencies (fR and fA) of the resonator elements are related to the center frequency fc of the filter circuit. In other words: series resonator elements have a resonance frequency substantially equal to the center frequency and lattice or shunt resonator elements have a anti-resonance frequency substantially equal to the center frequency of the filter circuit.

As to impedance matching at the ports of the resonator filter structure, there are arranged at the input port and/or the output port inductive elements and/or capacitive elements which are connected in series to and/or parallel across one of the or both of the input port and output port. It goes without saying that when input and output impedance levels are equal, impedance matching can be achieved by scaling the resonator areas.

However, when input and output impedance levels are different, impedance matching requires impedance transforming by the resonator filter structure. The inventors have found that some impedance transformation can be achieved by the LC-lattice section as shown by the above-introduced equations for deriving applicable values for Lx and Cx.

Due to the implementation of a LC-lattice section as heart of the resonator filter structure unbalanced-in to balanced-out signal guidance is provided. Thus, the output port is used as a balanced signal port while at the input port an unbalanced signal as well as a balanced signal can be applied, as it is needed in a specific application. Balanced output is most preferred, because as already mentioned the filter often is connected to the balanced input of a low noise amplifier (LNA). The input port with the unbalanced signal guidance can be connected to a fixed reference potential, if needed, e. g. a ground potential of the circuit.

The above and other objectives, features and advantages of the present invention will become more clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings. It is noted that through the drawings same or equivalent parts remain the same reference number. All

drawings are intended to illustrate some aspects and embodiments of the present invention.

Moreover, it should be noted that in case of different embodiments only the differences are be described in detail. Circuits are depicted in a simplified way for reason of clarity. It goes without saying that not all alternatives and options are shown and therefore, the present invention is not limited to the content of the accompanying drawings.

In the following, the present invention will be described in greater detail by way of example with reference to the accompanying drawings, in which: Fig. 1 shows a circuit diagram of a resonator filter structure wherein a BAW lattice filter section combined with a balanced BAW ladder filter section is connected in cascade to a LC-lattice section and inductances for matching purposes; Fig. 2 depicts a second example for the resonator filter structure of Fig. 1 which is construction optimized ; Fig. 3 is a third example for a resonator filter structure wherein an unbalanced BAW ladder filter section is connected to the unbalanced-in side of the LC-lattice section and a balanced BAW ladder filter section is connected to the balanced-out side of the LC-lattice section and inductances are used for matching purposes; Fig. 4 illustrates a further optimization of a BAW lattice filter section providing very narrow filter bandwidth; Fig. 5 is an equivalent element circuit of an resonator element; and Fig. 6 shows the impedance characteristics of two BAW resonator elements drawn over signal frequency, wherein resonance frequencies, anti-resonance frequencies and filter center frequency are marked.

Fig. 1 shows a resonator filter structure 10 according to the present invention, which comprises a first port 1, which has a connection to a fixed reference potential being ground 5 of the circuit, and a second port 2. There is connected a first load 3 to the first port 1 and a second load 4 towards the second port 2. It is noted that both loads in principal could be equal, however, with respect to the issue of impedance transformation it is assumed that the first load 3 is smaller than the second load 4. Therefore, the first load may represent an internal resistance of a generator that is driving a radio frequency (RF) signal as input for the resonator filter structure 10; in an application the generator, for instance, may be a receiving

antenna of a communication unit. Further, the second load 4 represents the input resistance of a following stage like, for instance, a low noise amplifier (LNA).

Only for the sake of good presentation in the following it will be assumed that the first load is a 50Q resistance and the second load stands for a 200Q differential resistance.

In such case, it is clear for the man skilled in the art that for optimal power transition there is need for an impedance transformation. Moreover, the input impedances of the resonator filter structure have to be matched according to the respective loads 3 and 4, at least within the frequency band that corresponds to the resonator filter structure passband. The passband is defined by a lower cut-off frequency, a center frequency, and an upper cut-off frequency, wherein a cut-off frequency could be derived by a certain signal power level to which the signal has decreased from passband towards the stopband.

The basic section of the resonator filter structure is a BAW lattice filter section 20, which comprises four BAW resonator elements 22a, 22b, 24a, 24b. The structure of this BAW lattice filter circuit is constructed with the four BAW elements 22a, 22b, 24a, 24b in the known principle of bridge circuits. Thus, respective two of the four resonator elements, i. e. 22a and 24a are connected in series building a first series path, and 24b and 22b are connected in series building a second series path. The connection nodes between two resonator elements of the first and second series path represent respective one output node of the resonator lattice circuit. Further, first and second series path of the bridge are connected in parallel to the input nodes of the resonator lattice circuit. Due to the illustration of such lattice circuit, resonator elements 22a, 22b are also called horizontal elements or series elements of the lattice circuit, and resonator elements 24a, 24b are also called diagonal elements or lattice elements of the lattice circuit. Moreover, according to this naming convention each branch of the lattice circuit is called an arm of the lattice circuit, wherein horizontal element builds an horizontal or series arm, respectively, and diagonal element builds a diagonal or lattice arm, respectively.

In the example of Fig. 1 for a resonator filter structure according the present invention, both series arm BAW resonators 22a and 22b, which are of a first resonator element according the invention, are equal, which means they have same resonance frequency fRs and same anti-resonance frequency fA,, and both BAWs 22a and 22b have an equal area on the substrate of the device. In the same manner, both lattice arm BAW resonators 24a and 24b, which are of a second resonator element according the invention, are equal, which means they have same resonance frequency fR2 and same anti-resonance frequency fA2 and both BAWs 24a and 24b have also an equal area size on the substrate. The

BAW lattice filter section has a desired center or design frequency fc in accordance to which the respective resonance and anti-resonance frequencies of the BAW elements 22a, 22b, 24a, and 24b are adapted. Thus, resonance frequencies fRl of the series arm BAW elements 22a and 22b are substantially equal to the center frequency fc of the resonator filter structure, while the anti-resonance frequencies fA2 of the lattice arm BAW elements 22a and 22b are substantially equal to the center frequency fc of the resonator filter structure.

On the right hand side of the BAW lattice filter section 20 is connected a BAW ladder filter section 30 comprising three BAW ladder elements 32a, 32b, 33. Due to the fact, that the output port 2 of the resonator filter structure is connected to a balanced input port of, for instance, a LNA, the BAW ladder filter section 30 provides balanced signal guidance. Therefore, the BAW ladder filter section 30 is symmetrically constructed with respect to the signal which is travelling in balanced condition and thus, both series BAW elements 32a and 32b have identically design parameters as equal area on the substrate of the device and the same resonant frequency fRz which is substantially equal to the center frequency fc of the RF filter structure 10. The vertical or shunt BAW element of the BAW ladder filter section 30 has its anti-resonance frequency fA2 substantially equal to the center frequency fc of the resonator filter structure 10.

On the left hand side of the BAW lattice filter section 20 is connected an impedance matching section 50a with two equal inductances 54a and 54b which are for impedance matching and therefore, will be described further below. Next to impedance matching section 50a there is a LC-lattice section 40, wherein the same naming convention as for a BAW lattice filter section applies. The LC-lattice section 40 comprises two capacitances 42a and 42b as series arm elements and two inductances 44a and 44b as lattice arm elements.

As to the dimension of the capacitances 42a, 42c and the inductances 44a, 44b there are two criterions met: first, the LC-lattice filter section 40 is according to its balanced bridge design in a balanced condition at the desired center frequency fc of the resonator filter structure 10.

In other words the impedances of the inductances 44a and 44b and capacitances 42a and 42b are equal at the center frequency of the resonator filter structure 10. Second, for purpose of some impedance transformation, the value Cx of the used capacitances 42a and 42b is derived from the formula: the value Lx of the used inductances 44a and 44b is derived from the formula :

wherein RIN and RoUT are the impedance levels on the respective input and output side of the LC-lattice section. Thus, the LC-lattice section 40 besides the BAW lattice filter section provides for impedance transformation between the input port 1 and the output port 2.

As to the impedance matching, at the input port 1 of the RF filter structure 10, there are impedance matching sections 50a and 50b, which are arranged for providing impedance matching simultaneously at the input port 1 and the output port 2 of the resonator filter structure. In this example, the parallel inductance 56 is connected to the port with the high impedance level being the output port 2 of the resonator filter structure and the symmetrical constructed series inductances 54a and 54b are connected in series to the right side of the LC-lattice section 40 which transforms these series inductances 54a and 54b to the port with the low impedance level being the input port 1 of the RF filter device 10 where it has a effect as a parallel capacitance (as it will be shown in Fig. 2).

Fig. 2 shows basically the same resonator filter structure as in Fig. 1, therefore, only the differences will be described. The resonator filter structure in Fig. 2 is a variation that has been found to be more optimal for process of manufacture. The two equal inductances 54a and 54b of impedance matching section 50a on the right side of the LC- lattice section 40 in Fig. 1 have been transformed to a capacitance 54c being a matching section 50c on the left side of the LC-lattice section 40 in Fig. 2. This brings size reduction with respect to the chosen manufacturing technology. For instance, the capacitance 54c at the impedance matching section 50c and the capacitances 42a and 42b of the LC-lattice section 40 may be combined on a single passive integration chip.

It shall be noted that it is clear for the man skilled in the art that the invention as described by the example of Fig. 1 and Fig. 2 is not restricted to the depicted configuration. First, it is possible to have any needed amount of BAW lattice filter sections and BAW ladder filter section connected in cascade, for instance, when a higher attenuation within the stopband is needed. Moreover, lattice and ladder filter sections can be arranged at any position within the RF filter structure, also the LC-lattice filter section may be placed at any position. However, when an unbalanced-in to a balanced-out RF filter structure is needed it has been taken care that each section guiding a balanced signal is constructed symmetrical in accordance to the balanced signal condition.

Fig. 3 shows another embodiment of a resonator filter with improved phase and amplitude balance within the passband according to the present invention. In comparison

to Fig. 1 and also Fig. 2 there is as a main difference no BAW lattice filter section 20.

However, the central part of the resonator filter structure 10 is again formed by the LC-lattice section 40 which provides for change-over from unbalanced signal guidance on the left side to balanced signal guidance on the right side of the LC-lattice section 40 of the resonator filter structure 10. The LC-lattice section 40 comprises the same construction as described with respect to Fig. 1 and Fig. 2. On the right hand side of the LC-lattice section there is also a symmetrically arranged BAW ladder filter section 30 as known from Fig. 1 and Fig. 2. LC- lattice section 40 and BAW ladder filter section 30 will not be described again in greater detail. On the left hand side of the LC-lattice section 40 there is a further BAW ladder filter section 30a, which is constructed in t-topology. Since signal guidance on this side of the LC- lattice section 40 is unbalanced there is no need to have this BAW ladder filter section symmetrical. It should be noted that also a BAW ladder filter section in -topology, which comprises two shunt BAW elements and between the two shunt BAW elements one series BAW element, or C-topology, which comprises one series BAW element and one shunt BAW element, may be used. A further difference in comparison to the embodiments of Fig. 1 and 2 is that a impedance matching section 50d at the input port 1 comprises in this case a series inductance 54d.

From here it will be referred to Fig. 4, which highlights another possible improvement of the resonator filter structure, especially when a very narrow passband is needed. In Fig. 4 only a BAW lattice filter section 20a is shown which is used as resonator filter section of the resonator filter structure 10 of the present invention. It goes without saying that it is clear for the man skilled in the art how the BAW lattice filter structure 20a of Fig. 4 can be incorporated in the resonator filter structure according to the present invention as illustrated by example of Fig. 1 and 2.

In Fig. 4 all BAW lattice elements 26a, 26b, 26c, 26d have equal resonance frequency and anti-resonance frequency, which also simplifies the fabrication process by eliminating the step of creating an offset in resonance frequencies between first and second resonator elements. That results all resonators can be made with the same piezo thickness and no massloading is required. Further, the BAW resonator elements 26a, 26b and the BAW resonator elements 26c, 26d differ in area. The parallel capacitances 28a, 28b which are connected in parallel towards the BAW elements 26a, 26b are essential to move the anti- resonance frequencies of those resonators. In other words, providing the tuning capacitances 28a and 28b for adjusting the filter passband can ease the manufacture of the BAW lattice filter section. It has found by the inventors that for good stopband rejection capacitances of

each lattice arm need to be equal. Thus, for example, assumed oppose BAW resonators 26c and 26d have an area A and a capacitance per area CAREA on the substrate of the respective device and BAW resonators 26a and 26b have an fraction x of the area A, wherein x is a value between zero an 1, then making the parallel capacitances according the equation C = (1-x)-A C 4 provides for a good stopband rejection. It should be noted that horizontal, i. e. series, arms and diagonal, i. e. lattice, arms may be exchanged.

The above presented invention has introduced a resonator filter structure applicable for communication devices, for instance, handheld GPS or personal communication units. Accordingly, a resonator filter structure comprising at least one filter circuit with resonator elements that are preferably BAW resonator elements. This at least one resonator filter circuit is combined with at least one LC-lattice section in which inductance and capacitance elements are arranged for balance improvement of the resonator filter structure. The complete resonator filter according to a preferred embodiment of the invention provides improved output amplitude and phase balance in a frequency band which is large enough for communication standards like PCS. Moreover, according to the signal guiding, implementation in unbalanced-in to balanced-out applications is possible.

It should be noted that the present invention is not restricted to the embodiments of the present invention, in particular the invention is not restricted to receive filters which have been used in this specification for reason of example. Moreover, the principle of the present invention can be applied to any application that needs in a high frequency environment a filter that provides narrow bandwidth and high stopband rejection together with good phase and amplitude balance within the passband.