It is possible by means of a notch filter to reflect a frequency range in a frequency spectrum such that it cannot pass through the notch filter or can only pass through it with high damping. Notch filters are also called band-stop filters. Notch filters can be used to selectively filter out certain frequencies, which cannot completely be damped by normal pass filters, i.e. high- pass, low-pass, or bandpass filters. The stopband of a notch filter is preferably set to frequencies or frequency ranges, where interference signals and in particular strong interference signals of adjacent bands can occur.

From US 7,583,936 Bl is known an extractor arrangement, with which a signal can in particular be extracted from a signal path. This extractor arrangement comprises a notch filter, which is arranged in the signal path and which constitutes a band-stop filter for the frequency to be extracted. Between the signal input and the notch filter, an extractor path branches off, in which a bandpass filter for the frequency to be extracted is arranged. It is possible with such an extractor arrangement, for example, to filter out GPS or Glonass or even WLAN signals from the entire spectrum of wirelessly transmitted signals. Alternatively, signals fed into the input of the bandpass filter can also be transmitted.

A notch filter can be realized in a ladder-type arrangement, which differs from the ladder-type bandpass arrangement in that the frequency positions of serial and parallel impedance

elements are interchanged in comparison to the bandpass filter.

It has however been found that so-called spurious modes occur in the signal path downstream of the notch filter, which spurious modes result in breaks in the passband and therefore bring about losses in the signal path. Spurious modes are caused by undesired secondary modes of acoustic filter elements, occur preferably at a frequency that is shifted in comparison to the main mode, and can interfere in other bands used. Most of the time, they can be suppressed only with difficulty and rarely completely.

Figure 1 shows three passband curves 1-3 of different notch- filters, which are all set to the same stopband but are designed in different technologies. It is shown that in each of the technologies used, interference modes occur, which cause

significant breaks in particular in the upper stopband in the transmission curve. For notch filters designed in SAW technology on lithium tantalate, such interference modes shown in curve 3 are caused by undesiredly excited Rayleigh waves and bulk waves.

In SAW filters on lithium niobate substrates, bulk waves and plate modes can also occur and cause losses. Such filters can also be realized in HQTCF technology (HQTCF = high quality temperature compensated filter) , wherein the temperature coefficient TCF of the frequency is compensated. These filters are then produced on lithium niobate substrates and covered with a compensation layer for the temperature coefficient in the form of a thick S1O ₂ layer. Curves 1 and 2 show interference modes of a temperature- compensated SAW notch filter. In a notch filter formed from BAW resonators, the second and third harmonic are excited as well as reflector modes, which also interfere in the upper stopband.

The task of the present invention is therefore to specify a notch filter, in which the interference modes are suppressed better and which therefore shows an improved pass behavior above the stopband.

This task is achieved according to the invention by a notch filter according to claim 1. Advantageous embodiments of the invention as well as an extractor arrangement comprising this notch filter are contained in additional claims.

A notch filter constructed of impedance elements is proposed, which notch filter comprises at least electro-acoustic resonators and also inductances as impedance elements. These impedance elements are arranged in a ladder-type arrangement in a serial branch or in shunt branches branched off therefrom. In order to suppress the interference modes, resonators are now, according to the invention, connected to capacitors in the serial branch and/or in one or more shunt branches. The connection can take place in series and/or in parallel to one of the resonators.

Several resonators can also be connected to capacitors.

The impedance elements can be selected from

an acoustic resonator

an acoustic resonator in series with a coil a parallel LC oscillator circuit

a serial LC oscillator circuit

a capacitor alone.

The ladder-type arrangement can exclusively comprise acoustic resonators, which are arranged as impedance elements in the serial branch and in the shunt branches. It is also possible that not all possible positions of the ladder-type arrangement are occupied by acoustic resonators, but that individual

positions are occupied by other impedance elements.

The capacitors can be connected to resonators that are only arranged in shunt branches, only in the serial branch or in both the shunt branches and the serial branch. The capacitors can respectively be connected in parallel or in series to individual resonators, to a portion of the resonators, or to all resonators. Ultimately, a notch filter is obtained, in which the

interference modes are significantly suppressed and which shows a significantly improved pass behavior above the stopband.

If one of the resonators of the notch filter is connected to a capacitor, it must be ensured that the total capacity of the circuit, i.e. the sum of the static capacity of the resonator plus the capacity added by the capacitor, remains the same. As

reference serve the resonators of a filter, which is optimized for an optimally designed stopband but not connected to capacitors.

If a capacitor is connected in parallel to a resonator, the resulting capacity of the circuit consisting of resonator and capacitor is increased such that a reduction of the static capacity of the resonator is required in order to obtain the total capacity. If a capacitor is connected in series to a resonator, the total static capacity is reduced if the capacity of the capacitor is about in the order of magnitude of the static capacity of the resonator. In this case, the resonator connected in series is designed with a correspondingly higher static capacity.

A notch filter according to the invention can be constructed only of resonators, which can be realized in different

technologies. It is, for example, possible to construct the resonators in SAW technology, BAW technology, as LC resonators, or as dielectric resonators. Interference modes can occur practically in all resonator technologies but can effectively be suppressed using the invention.

The notch filter can be designed in a ladder-type arrangement, wherein the impedance elements arranged in either the serial branch or in the shunt branches are designed as inductances. The capacitors can be designed as individual components that are mounted separately onto a circuit board. It is however

advantageous to integrate the capacitors into the structure of the notch filter.

If a resonator is, for example, designed as a SAW resonator, a capacitor can be formed as additional interdigital structure directly on the substrate of the SAW resonator and connected to the resonator in parallel or in series. On a SAW filter substrate, plate capacitors can also be formed and layer deposition processes already used for the SAW structures can be used to this end. For example, the metallization of the interdigital transducer can be used for the bottom electrode of the plate capacitor. A

dielectric, such as S1O ₂ or BCB, which already serves to insulate conductor path crossings, can also be used as dielectric of the plate capacitor, while the top electrode of the capacitor can be formed from the pad reinforcement.

Outside their resonance frequency, resonators in BAW technology also act as capacitors and can therefore already be used by themselves with a changed, non-interfering resonance frequency as capacitors. A shifting of the resonance frequency is possible with a BAW resonator by adding additional layers between the top and bottom electrode, by applying an additional layer on the top or bottom electrode, or by omitting a sublayer of the resonator, or by reducing a layer thickness, in particular a layer

thickness of the dielectric.

It is advantageous to use at least one of the electrodes of a BAW resonator as first capacitor electrode. As second capacitor electrode, a high-impedance layer of the acoustic mirror can then also be used, wherein a low-impedance layer arranged between them can serve as dielectric. It is however also possible to produce the capacitor laterally adjacent above the top or bottom electrode of the BAW resonator by applying a dielectric layer and a second capacitor electrode.

Another possibility of integrating the capacitors required for the notch filter according to the invention into the filter structure consists in using a multilayer substrate, such as an LTCC or a laminate. In a multilayer structure, any number of integrated capacitors can be produced in a structured manner and realized by a layer sequence of conductive layer, dielectric, and additional conductive layer.

Such a multilayer substrate can serve as carrier for the notch filter. The resonators can be constructed directly on the

multilayer substrate or mounted onto it. The multilayer

substrate can also comprise the resonators integrated into the multilayer structure.

When connecting the resonators to capacitors, the fact that the pole-zero distance of the resonator, which determines its

bandwidth or the bandwidth of the filter produced from the

resonator or the resonators, is reduced must be accepted as a disadvantage. A reduced pole-zero distance is often times only acceptable if a small bandwidth is required for the notch filter or if the substrate has an innately large pole-zero distance.

A piezoelectric substrate with a large pole-zero distance is, for example, lithium niobate. Onto it can be constructed SAW notch filters, which can, as necessary, comprise additional layers, such as in particular an S1O ₂ layer for temperature compensation of the TCF (temperature coefficient of the frequency) , above the metal structure of the electro-acoustic transducer.

It is however advantageous to use a notch-filter, which comprises an innately high bandwidth. A notch filter constructed of microacoustic resonators is therefore preferably constructed on highly interconnecting substrates in order to bring about a sufficiently high pole-zero distance. A preferable exemplary embodiment of a highly interconnecting substrate is lithium niobate, such as LN41 or LN128. The model mentioned last is also preferably used for HQTCF resonators so that a preferred notch filter comprises resonators of the HQTCF type.

In all embodiments, in which a resonator of the notch filter is connected to a capacitor, the linearity of the filter is also simultaneously increased, in particular if the capacitors are significantly more linear than the resonators. Both the increase in linearity and the suppression of interference modes are

pronounced the more, the more the resonators are connected to capacitors. The effect can furthermore be increased by selecting the capacity of the capacitors used in a parallel connection to the resonator as high as possible, in a connection in series however as low as possible.

A notch filter according to the invention can advantageously be used in the signal path of a wireless system, for example for mobile communication, cordless phones, WiFi applications, WLAN, satellite reception, radio data transmission, or other systems.

As already mentioned above, a possible use of a notch filter is in an extractor, in which the notch filter is arranged in the signal path, while immediately upstream of it between the notch filter and the antenna connector, an extractor path, in which a bandpass filter is arranged, branches off from the signal path. The notch filter has a stopband and is impermeable to signals in this frequency range. The bandpass filter comprises a passband, which is completely within the stopband in an extractor

arrangement. It is also possible to arrange two and more

extractors in series to the signal path, wherein the notch filters used to this end can be connected to capacitors as described. The different extractors can then serve to extract different frequency bands from the signal path.

The bandpass filter can also be constructed of resonators, which act as capacitors outside the passband. The resonators of the bandpass filter can also produce interference modes in the signal path, which interference modes can occur in a frequency range that is also used by the same device.

In an embodiment of the invention, the bandpass filter, in addition to the notch filter, is also modified accordingly by connecting its resonators to capacitors. It is shown that a connection in series or in parallel of resonators of the

bandpass filter to capacitors achieves an additional improvement of the signal quality by interference modes caused by the bandpass filter being reduced.

To capacitors are preferably connected those resonators of the bandpass filter, which are closest to the common node of the extractor path with the signal path. For example, the first three resonators closest to the common node are connected to capacitors irrespectively of whether the resonators are arranged in the serial branch or in shunt branches.

The signal path itself connects an antenna connector to a

send/receive unit.

Since the stopband of the notch filter and the passband of the bandpass filter are almost congruent, the resonators used for the two filters can be realized in the same technology and preferably also on a common identical substrate. It is however also possible to design the notch filter and the bandpass filter as separate units on separate substrates. It is also possible to design the notch filter and the bandpass filter in different technologies. A preferred use of the extractor arrangement results for GNSS or WLAN frequencies, wherein the stopband of the notch filter is impermeable for these frequencies, while the bandpass filter lets the GNSS or WLAN frequencies pass. In this way, it is possible for the GNSS or WLAN signals to be extracted from the signal path via the extractor path.

The invention will be explained in greater detail below with reference to exemplary embodiments and the associated figures. Filter arrangements are partially designed as schematic block diagrams, in which only the most important or necessary

components are illustrated. Actual filter arrangements can comprise additional components. In addition, the structures illustrated are not to scale so that neither absolute nor relative dimensions can be taken from the figures. Rather, for the sake of better comprehensibility, individual components can be illustrated in an enlarged or reduced scale.

Figure 1 shows the transmission behavior of various notch filters, which are realized in different technologies or on different substrates,

Figure 2 shows the transmission behavior of a notch filter according to the invention in comparison to a known notch filter,

Figures 3 to 7 show various embodiments of the notch filter according to the invention,

Figure 8 shows an extractor arrangement with the notch filter,

Figure 9 shows an extractor arrangement with additional measures in the bandpass filter, Figure 10 shows the real part of the admittance of a resonator connected to a capacitor in comparison to a resonator without additional capacitor,

Figure 11 shows the imaginary part of the admittance for the same element,

Figure 12 shows the pass behavior of another notch filter with and without connected capacitors,

Figure 13 shows a section of the curves of figure 12,

Figure 14 shows a filter circuit with two extractor arrangements, including notch filters according to the invention,

Figure 15 shows an alternative filter circuit with two extractor arrangements, including notch filters according to the invention.

Figure 1 shows the pass behavior of known notch filters using the matrix element S21. Curves 1 and 2 are determined on two different notch filters, which are designed as SAW components i HQTCF technology. It is shown that such a notch filter has a pronounced break in the transmission behavior above the

stopband. Curve 3 represents a notch filter, which is designed in SAW technology on lithium tantalate. This notch filter also shows a break in the transmission behavior in a somewhat lower frequency but also above the stopband.

Figure 3 shows a simple exemplary embodiment of a notch filter according to the invention. This filter is designed in a ladder type arrangement and comprises three serial impedance elements ZS designed as resonators in a serial branch. Illustrated branching off from the serial branch are two shunt branches, in which a parallel impedance element ZP is respectively arranged. The parallel impedance element ZP can also be a resonator but can also be designed as an inductance. In the latter case, the notch filter constitutes a high-pass filter.

According to the invention, a capacitor CESP is now respectively connected in the serial branch in parallel to the serial impedance elements ZS or in parallel to the resonators. In order to further optimize the notch filter according to the invention, the static capacities of the resonators bridged in parallel by a capacitor are now adapted in order to obtain the normal functionality of the notch filter, in particular the impedance and the adaptation in the passband. The adaptation is carried out such that the static capacity between the two nodes Kl and K2, which are arranged in the serial path and at which the parallel path with the capacitor respectively merges into the serial path, remains unchanged by the additional capacitor. Since the capacity value of the capacitor is added in this circuit to the static capacity of the resonator, the static capacity of the resonator must be reduced appropriately depending on the size of the capacitor.

Such a notch filter as in figure 3 effectively reduces the interfering spurious modes. Figure 2 shows the passband curve (S21) of a notch filter realized in SAW technology on a lithium niobate substrate with an intersecting angle of 41° with and without a capacitor connected in parallel. It can be clearly seen that the curve 4b of a notch filter according to the invention is significantly improved in the pass behavior above the stopband in comparison to the curve 4a, which corresponds to a notch filter without capacitors connected in parallel. In this case, the improvement appears over a larger range and is not limited to certain frequencies associated with an interference mode. Another advantage consists in the transition from the lower passband to the stopband also being steeper so that the barrier effect of the notch filter is more clearly limited to the stopband and subsequently transitions into a pass behavior with low damping without any transition. As an additional effect of such capacitors connected in the serial branch in parallel or in series to resonators, the resonators in the notch filter according to the invention also show an improved linearity. This improvement comes from the voltage or current being divided between the non-linear

resonators and the decisively more highly linear capacitors. A description of this effect can be found, for example, in the German patent application DE 102014112676 Al .

Figure 10 shows the real part of the admittance of a resonator bridged by a parallel capacitor in comparison to the admittance without bypass, while figure 11 shows the magnitude of the admittance of the same resonator with and without capacitor connected. The resonator is realized as SAW resonator on an LN41 substrate. Curve 5a in figure 10 shows the real part of the admittance of a corresponding resonator without capacitor

connected in parallel, while curve 5b shows the behavior of a resonator according to the invention connected in parallel to a capacitor. It is shown that the admittance curves are pronounced substantially more strongly or transition more quickly into resonance and antiresonance if a capacitor is connected in parallel to the resonator. Curve 5b furthermore shows a reduced admittance value above the resonance, which indicates that interfering modes, in this case bulk waves, are suppressed.

The magnitude of the admittance according to figure 11 shows that the pole-zero distance of curve 6b, which corresponds to the resonator with a capacitor connected in parallel, is reduced in comparison to the non-connected resonator in curve 6a. The difference is Δ. The reduced pole-zero distance is also the reason for only filters with a smaller bandwidth being

realizable with such a resonator. This can however be accepted if, as in this case, a highly interconnecting substrate (lithium niobate 41) is used for the structure of the filter. Applications that require the filters and in particular notch filters with smaller bandwidth can also be improved in this way.

Figure 4 shows another exemplary embodiment of a notch filter according to the invention. The ladder-type arrangement

illustrated with five impedance elements Z comprises two shunt branches, in which a resonator ZP is respectively arranged, which is bridged in parallel by a capacitor CEPP. The impedance elements ZS arranged in the serial branch can also be designed as

resonators but can also be inductances. In the case mentioned last, the notch filter shows a low-pass nature.

In this case, it also applies that the total capacity of the static capacity and bypass capacitor in the filter according to the invention remains constant in comparison to a filter without bypass capacitor. This means that the capacity determined between the nodes K3 and K4 in the shunt branch remains constant. This can also be achieved in the same way as in figure 3 by the respective static capacity of the resonator being reduced

depending on the additively acting capacity of the capacitor.

Figure 5 shows another exemplary embodiment of a notch filter according to the invention, which again comprises a ladder-type arrangement with three serial impedance elements ZS and two shunt branches with parallel impedance elements ZP. In this case, serial capacitors CESS are connected in the serial branch in series to the resonators ZS arranged there. The impedance elements ZP in the shunt branches can also be resonators or even inductances. In this embodiment, each additional capacitor CE also shows an effect with respect to the suppression of interfering modes. In this case, the mentioned secondary effect of a reduced pole-zero distance and of the serial resonators also occurs and associated therewith a reduced bandwidth of the notch filter. Another exemplary embodiment is illustrated in figure 6. There, the parallel impedance elements ZP are designed as resonators and connected in the shunt branch in series to a capacitor CEPS The impedance elements in the serial branch ZS can again be resonators or inductances.

In this embodiment, the advantages according to the invention o improved linearity and of the improved suppression of

interference modes are also achieved.

Figure 7 shows another exempla y embodiment, in which resonators arranged as impedance elements ZS, ZP in the serial branch and in one or more shunt branches are also bridged in parallel by capacitors CESP or CEPP.

Not illustrated are variations of the notch filter, in which serial capacitors and/or parallel capacitors are connected to resonators in a ladder-type arrangement at the same time. In all cases, the effects according to the invention are achieved.

Figure 8 shows an extractor arrangement using a notch filter N according to the invention. The notch filter N is connected in series between an antenna connector A and a send/receive unit SE . Branching off from a node K between the antenna connector A and the notch filter N is an extractor path EX, in which a bandpass filter BP is arranged. The passband of the bandpass filter BP is within the stopband of the notch filter N. By means of such an extractor arrangement, it is possible to completely reflect signals entering through the antenna connector A at the notch filter N and to extract them from the serial path between the antenna connector and the send/receive unit via the

extractor path by means of the bandpass filter. By means of the notch filter N according to the invention, the functionality of the extractor arrangement is improved, since spurious modes can also act in an interfering manner there if their frequency position is at a different operating frequency.

Figure 9 shows a further developed extractor arrangement, in which, as in figure 8, the notch filter N is designed according to the invention, for example according to one of the exemplary embodiments of figures 3 to 7. Additionally, the first stages of the bandpass filter are also designed in a ladder-type

arrangement, which comprises, both in the serial branch and in the at least one shunt branch, resonators bridged in parallel by a capacitor CESP or CEPP. The dashed illustration of the bypass path indicates that these capacitors are optional. It is

advantageous to not bridge all of the resonators accordingly using capacitors, but to only bridge those closest to the antenna connector A. In figure 9, these are the three resonators closest to the antenna connector A.

It is clear that notch filters N can comprise a higher number of impedance elements Z and in particular of resonators, which can increase the order of the respective notch filter and thus also the bandwidth. Since the losses of the notch filter also

increase with its order, each of the notch filters constitutes a trade-off between improved selectivity, higher bandwidth, and suppression of interference modes.

As already explained with reference to the various embodiments of the notch filter N, the bandpass filter BP can also be connected in various ways to capacitors CE, which can be arranged in parallel to resonators in the serial branch, in parallel to resonators in the shunt branch, in series to serial resonators, and in series to resonators in the shunt branch. Furthermore possible are combinations, in which a capacitor can be connected both in parallel to a resonator and in series to a resonator. Figure 14 shows a filter circuit with two extractor arrangements, which can be realized with the notch filters according to the invention. A serial signal path connects an antenna connector A to a send/receive unit SE . The two extractor arrangements are

arranged in series and respectively comprise a notch filter Nl, N2 arranged in the serial path and an extractor path EX1, EX2, in which a bandpass filter BP1, BP2 is arranged. The extractor path EX1, EX2 is respectively connected in the serial path to a node Kl, K2, which is arranged between the respective notch filter N and the antenna connector A. In the respective extractor path EX, between the bandpass filter BP and the node K, a matching network M is drawn, which can optionally be present in the filter circuit. Additional optional matching networks M can be arranged in the serial path between each node K and the antenna connector A, between each node and the notch filter N, and at the end of each notch filter N facing away from the antenna connector A. Such a matching network M can comprise serial and/or parallel adaptation elements, which are selected from capacitors and inductances.

Figure 15 shows an alternative filter circuit with two extractor arrangements, including notch filters according to the invention. In contrast to the embodiment according to figure 14, the two extractor paths EX1, EX2 are in this case connected in the serial path to two nodes Kl, K2, which are both arranged between the first notch filter Nl and the antenna connector A and directly follow one another. In this case, matching networks M can also optionally be arranged respectively between two components each, selected from bandpass filter, node, and notch filter.

In addition to the filter circuits of figures 14 and 15, in which only two extractor arrangements are illustrated, a filter circuit can also contain additional extractor arrangements connected in series in the serial path. In this way, it is possible to extract several bands from the serial path with relatively low loss via one extractor path each. The notch filters according to the invention now makes this possible with improved selection so that the illustrated filter circuits expanded by additional extractors as necessary constitute improved multiplexers.

Figure 12 shows with reference to another exemplary embodiment, in which the notch filter is realized on an HQTCF substrate in SAW technology, the positive effect of the invention.

Curve la shows the matrix element S21 of the notch filter, the resonators of which are not connected to capacitors. Curve lb shows the same matrix element on a notch filter otherwise constructed in the same manner with capacitors connected

according to the invention in parallel to the resonators.

Figure 13 shows the curve in a section above the stopband in order to better illustrate the positive effect. It is shown that in this case as well, in the connection to capacitors according to the invention, an improved pass behavior of the notch filter according to the invention is obtained in accordance with curve lb. The breaks in the pass behavior, which can be attributed to interference modes, are reduced. This shows that the invention can be realized in different technologies and in different connections and shows in each case a positive effect with

respect to the suppression of interference modes.

The invention was explained only with reference to a few

exemplary embodiments and is therefore not limited to these. Considered as notch filters according to the invention are also arrangements that constitute combinations of the exemplary embodiments shown. Also according to the invention are notch filters and extractor arrangements, which show a larger or smaller number of impedance elements in the respective ladder- type structure. For the effect according to the invention

(reduction of the interference modes), it is also of no concern, in which technology the capacitors are realized. Preferred are however highly linear plate capacitors, which can be realized in a laminate, an LTCC on or below the filter chip. As already mentioned, highly linear plate capacitors can be produced in an integrated manner on SAW substrates. Highly linear plate

capacitors can also be produced in an integrated manner on BAW filter substrates. The pad reinforcement can then, for example, serve as the top electrode and one of the resonator electrode layers can serve as dielectric S1O ₂ or SiN and as bottom

electrode. With appropriate structuring of these layers, the plate capacitors can be produced in parallel with the resonators on the respective substrates.

Capacitors that are particularly simple to produce are realized as interdigital structure on the surface of a SAW resonator, which interdigital structure can be produced at the same time and in the same work step together with the resonator structures or their metallizations.

List of reference symbols

1, 2, 3, 4, 5 Impedance and admittance curves

A Antenna connector

BP Bandpass filter

CE Capacitor

EX Extractor path

K Node

M Matching network

N Notch filter

SE Send/receive unit

Z _p , Z _s Parallel and serial impedance element

Δ Bandwidth reduction