Acoustic filter device In SAW designs e.g. filters, multiplexers, etc. embodied in a ladder type configuration of resonators out-of-band (OOB) attenuation very much depends on coupling between different resonators within one filter design, the grounding of

individual resonators as well as the grounding of the whole filter when soldered on a printed circuit board (PCB) .

Even carefully designed filters with high OOB attenuation can have significant performance degradation as soon as they are soldered on a PCB. Such degradation is due to non-ideal grounding connection compared to the simulation with ideal grounding or compared to the design implemented for a special evaluation board with grounding optimized for low inductance. Such evaluation boards are used for performance evaluation purposes during filter design. Such said special evaluation boards are normally designed with many grounding vias in order to minimize the grounding inductance. For layout implementation on typical customer board, however, it is often not possible to use many grounding vias due to the space limitation on the board which leads to an increased grounding inductance and hence to a degraded OOB performance.

To solve this problem different approaches have been done. Filters have been designed with some performance margin in order to compensate possible performance degradation due to the soldering on a customer's PCB. The drawback of this approach is that additional performance margin in view of OOB attenuation normally leads to additional insertion loss as well as to increased size of the filter design. Another possibility is to design the entire circuitry

strongly optimized and therefore limited to one particular customer PCB stack-up and component footprint. Thereby the flexibility to re-use the same design for different PCB stack-ups and component footprints is significantly reduced.

Furthermore, the stack-up information is often available only in a later design phase. Hence some fast fine-tuning methods are required for the performance optimization.

It is an objective of the present application to provide an acoustic filter device that allows a fine tuning of the filter performance to adapt it with low effort to a specific customer PCB.

These and other objectives are met by an acoustic filter device according to the independent claim. Further

advantageous embodiments are given by the dependent

subclaims .

An acoustic filter device comprises a filter chip with filter elements that are arranged in a series signal line as well as in parallel branches. Each of the parallel branches is connected to a ground termination on the filter chip.

The filter chip is mounted on a carrier substrate comprising grounding vias for connecting the ground terminations to an external ground. Other elements like signal lines as well as necessary and optional passive impedance elements are

integrated in the carrier substrate. At least one primary grounding inductor is integrated within the carrier substrate and coupled between one of the ground terminations and the external ground. When mounting such a carrier substrate with a filter chip to a specific PCB additional grounding inductance by the solder contact and the PCB results in reduced filter performance as the matching of the filter deteriorates. However, it can be shown that simply reducing the primary grounding inductor does not solve the problem and does not recover the

originally designed performance. Hence, it is proposed to couple additional tunable elements between a ground termination and the external ground that allow optimal adaption to the customer' s PCB environment with low effort. By doing so the filter chip and its acoustics can remain unchanged and thus acoustically optimized. Adapting the ground connection simply within the carrier substrate by redesigning and tuning the additional tunable elements is much more easier and cheaper than redesigning and optimizing again the acoustics on the filter chip. When describing the filter device relative locations are given by the terms "bottom", "upper" and the like that are for orientation purpose only. These terms refer to an

arrangement where the filter chip is mounted on top of the carrier substrate. The connection to a PCB and hence to external ground are on the bottom surface side of the carrier substrate .

According to a first embodiment the additional tunable elements comprise a series circuit of a first tunable

capacitance and an additional series inductance arranged like a series resonance circuit to ground. The object of this series circuit is to improve OOB performance without any degradation to matching and pass band performance. The additional tunable elements in the series circuit can be embodied by elements that may already be present in the carrier board. The first tunable capacitance comprises an upper ground plate as a first capacitor electrode and a second metal area as a second capacitor electrode connected to the upper end of the primary grounding inductor and hence to a ground termination of the filter chip. The upper ground plate is connected to the external ground by one or more grounding vias. These grounding vias form at the same time the necessary additional series inductance. Hence, the upper ground plate and second metal area are arranged above the primary grounding inductor. The first tunable capacitance can be tuned by varying the surface area of the second metal area and/or by adapting the distance between the capacitor

electrodes. The additional series inductance can be tuned and modelled by the number and length of the grounding vias connecting the upper ground plate to the external ground. Alternatively, the second metal area may be as small as the cross-section of a grounding via. In another example the function of the second metal area with respect to the second electrode of the tunable capacitor may be performed by a winding of the primary grounding inductor capacitively coupling to the upper ground plate. The capacitance can then be tuned by varying the distance of upper ground plate to the upper winding.

According to a second embodiment the additional tunable elements comprise a second tunable capacitance circuited in parallel to the primary grounding inductor between a ground termination and the external ground thereby providing a parallel resonance circuit with primary grounding inductor to ground. Similarily like in the first embodiment the object of this parallel circuit is to improve OOB performance without any degradation to matching and pass band performance. The second tunable capacitance can be embodied by elements that may already be present in the carrier board. A first

electrode of the second tunable capacitance is formed by a bottom ground plate arranged at a bottom section of the carrier substrate below the windings of the primary grounding inductor. The capacitance may be formed between the bottom ground plate and the windings above and may tuned by setting an adapted distance between the bottom winding and the bottom ground plate. Alternatively, the bottom ground plate may have an opening the size of which can be varied to adapt the overlap to the winding and thus to tune the capacitance. The bottom ground plate may be indentical with the ground of the complete filter device.

Alternatively, the second tunable capacitance may be formed by a third metal area arranged near the bottom side connected by a via to the upper ground plate end of the primary

grounding inductor and capacitively coupled to the bottom ground plate.

According to an embodiment, the carrier board has at least one ground pad for connecting the carrier board to the external ground. This may be the above mentioned ground of the complete filter device or the bottom ground plate

respectively. This does not exclude that more than one solder contacts have to be made to external ground at a customer' s PCB.

As already indicated the carrier substrate is a multi-layer board comprising an alternating sequence of dielectric layers and wiring planes. A wiring is formed by structured metallizations in the wiring planes and interconnecting vias. The wiring comprises impedance elements integrated within the carrier substrate. The additional tuning elements are formed from such integrated impedance elements. But other circuits may be present in the multi-layer board too providing further functions and matching of the filter device.

As already indicated the primary grounding inductor is a coil comprising one or more windings within a wiring plane each. According to a further embodiment shielding strips are provided and arranged near the perimeter of a respective winding in the wiring planes. The shielding strip may follow the perimeter of the respective winding in a short distance over a substantial part of the perimeter. Each shielding strip has two ends, one of them is connected to a grounding via, the other end is an open end. The shielding strip is a metal structure in the wiring plane and may be a narrow strip. It is preferred that each winding is shielded. Each winding may be neigbored by two assigned shielding strips arranged on mutually opposite sides of the respective

winding .

Though the proposed amendment concentrates on elements within the carrier substrate the structure of the according filter or filter chip has to be observed. The filter device

comprises a filter realized on the filter chip that comprises a ladder type arrangement of series and parallel reactance elements as filter elements. The reactance elements are chosen from resonators, capacitors and inductances. Some of the resonators may be substituted by capacitors. The parallel reactance elements are arranged in the parallel branches and connected to a ground termination each. Resonators are preferably formed by acoustic resonators and may be the SAW or BAW type. Any known or still unknown filter may be used in connection with proposed filter device. Advantageous effects are observed for band pass filters. In the following embodiments of the invention are explained with reference to accompanied drawings. The drawings are schematical only and are not drawn to scale when showing real structures . Figure 1 shows a block diagram of a first ladder-type filter of the art mounted on a carrier substrate with grounding inductances

Figure 2 shows a block diagram of the same ladder-type filter mounted on carrier substrate with a reduced number of

grounding inductances

Figure 3 shows a block diagram of a second ladder-type filter on a carrier substrate that is mounted on a PCB

Figure 4 shows a block diagram of the filter device of figure 3 with additional tunable elements integrated into the carrier substrate Figure 5 shows a wide-band transfer curve of the filter device of figure 4 compared with those of devices according to figures 2 and 3

Figure 6 shows a narrow-band section of the same curves shown in figure 5 Figure 7 shows in a perspective view a section of the

metallization of a multilayer carrier substrate realizing the additional tuning elements and shielding strips. Figure 8 shows in a perspective view a similar multilayer carrier substrate with tuning elements and shielding strips.

Figure 1 shows a block diagram of a first ladder-type filter known from the art. On a filter chip FC filter elements are arranged in an ladder-type configuration that is the filter elements are arranged in a series signal line SL between a first and a second terminal T1,T2 as well as in parallel branches PB1, PB2, .. PB4. Here, four parallel branches PB1 to PB4 are connected between the series signal line SL and a ground termination GT each. In the example, the filter elements comprise five series resonators RSI to RS5 and four parallel resonators RP1 to RP4. The resonators may be

embodied as SAW resonators or BAW resonators as well.

Possible other ladder-type configurations not shown can comprise a series capacitor replacing one of the series resonators .

The filter chip FC is mounted on a carrier substrate SU that comprises grounding vias for connecting the ground

terminations GT to an external ground. Each grounding via is assigned to a grounding inductance LG1 to LG4. At least one grounding inductance is realized as a coil whose windings are integrated into the carrier substrate SU. On the bottom of the carrier substrate SU towards the external ground all parallel lines and branches with grounding vias and/or grounding inductances may be connected to a common ground plane such that at least one ground contact would be

necessary . Usually, the integrated grounding inductances LG1 to LG4 are used to optimize the OOB performance. On the other hand, in case of a small package used for the filter device, these integrated inductances are electromagnetically coupled. This coupling usually degrades the OOB performance, especially in extractor/multiplexer designs where this coupling is very critical and may degrade the performance significantly in other multiplexer channels. Also, the coupling makes the design more sensitive to production tolerances. However, the integrated inductances/ coils LG1 - LG4 can be tuned in order to compensate the parasitic ground inductance of the PCB for some frequency range. When grounding inductances LG are embodied as integrated coils additional notches in the filter transfer curve are achieved due to a respective series shunt resonance formed by the coils LG in combination with the static capacitances of the parallel resonators RP .

A filter as shown in figure 1 may be optimized in its performance using simulation tools and/or an evaluation board into which the carrier substrate is implemented in. In many cases, where the coupling between the LG1 to LG4 is very critical, schematics as shown in the block diagram of figure 2 for example are more preferable. In these cases only one grounding inductance embodied as a coil LG2 is used instead of assigning a coil to each of the parallel branches PB . However, coil LG2 cannot be used to compensate the parasitic effect of the PCB. Further, useful notches that can be set by creating a respective series resonances of static capacitance of RP and LG may no longer be present. For layout implementation on typical customer board, however, it is often not possible to use many grounding vias due to the space limitation on the board which leads to an increased grounding inductance and hence to a degraded OOB performance.

Figure 3 shows a block diagram of a second ladder-type filter that is similar to the first ladder-type filter. Here, second series resonator RS2 of the filter in figure 1 or 2 has been substituted by a series capacitor CSS. Further, it is shown that implementing the filter device on a printed circuit board PB by soldering it on the PCB creates some additional grounding inductance due to interconnections (solder contacts SC) between the filter package (carrier substrate SU) and the ground of the printed circuit board PB . This additional grounding inductance is denoted in figure 3 by LGconmon . Though the total grounding inductance may be enhanced by the

interconnect only slightly by an amount of -O.OlnH or greater it is not possible to compensate that effect by simply reducing the inductance value of grounding inductance LG2 having a typical inductance value of about 0.1 to 3nH. In most cases the effect leads to a substantial degradation of the filter performance as can be seen at curve 3 of figures 4 and 5 to be discussed later.

Figure 4 shows an advantageous possibility to compensate that degrading effect explained with reference to figure 3 by adding tunable elements TE1,TE2 in the grounding lines of the carrier substrate SU.

The first additional tunable element TE1 comprises a series circuit of a first tunable capacitance CG and an additional series inductance LP arranged like a series resonance circuit to ground. This series circuit is coupled between a grounding termination GT (at the interface between filter chip FC and carrier substrate SU) and the package ground that is usually a bottom ground plate GPB.

The second additional tunable element TE2 provides further advantage when used in addition to the first additional tuning element TE1 and comprises a second tunable capacitor CP coupled between the grounding termination GT and the package ground that is usually a bottom ground plate GPB.

Figure 5 shows a wide-band transfer curve 4 of the improved filter device of figure 4 compared with the curves 2 and 3 of filter devices according to figures 2 and 3. The curves are numbered from 2 to 4 according to the number of the figure the device is assigned to. Curve 2 accords to a filter device whose acoustic performance is optimized on an evaluation board with nearly ideal ground. Insertion loss and OOB attenuation is as required by the customer's demands.

Curve 3 shows a wide-band transfer performance of the same filter device when mounted onto an arbitrary customer PCB denoted as PB . It can be seen that the OOB attenuation above and below the filter pass band has worsened.

Curve 4 shows a wide-band transfer performance of the

improved design with first and second additional tuning elements TE1,TE2. It can be seen that the performance is improved compared to curve 3 of uncompensated device of figure 3. Below the pass band OOB is even improved compared to the optimized design of figure 2/curve 2. Above passband a useful notch appears at about 3.3GHz . This shows that by the invention the performance is improved in same aspects even when compared to the optimized filter design of figure

2 /curve 2.

Figure 6 shows the same transfer curve in an enlarged

narrow-band depiction around the pass band. Here it can be seen that for all figures/curves the insertion loss and return loss performance are idententical .

Figure 7 shows in a perspective view an embodiment of how the additional tuning elements TE1,TE2 can be transformed into a metallization of a real multi-layer carrier substrate SU. The figure depicts a section of the metallization only. The metallization comprises wiring planes and interconnecting vias. Conducting vias can go through a single layer of dielectric (dielectric not shown in the figure) or can be stacked to go through a multitude of dielectric layers or through the complete carrier substrate from grounding

termination to bottom ground plate GPB. The grounding vias LG1, LG3 and LG4 usually provide parasitic inductance.

Circuiting in parallel several grounding vias LG can be used to reduce inductance thereof.

The primary ground inductor LG2 comprises several windings WG arranged in stacked wiring planes and interconnected by

"short" vias thereby coupling a grounding termination GT (not shown) and an external ground that is to be connected to the bottom ground plate GPB. Above the upper winding WG of primary ground inductor LG2 an upper ground plate GPu is arranged that is connected to the bottom ground plate GPB by a stacked through-going grounding via VG. A second metal area MP2 capactively couples to the upper ground plate GPu thereby forming additional tunable capacitance CG. An additional capacitance forms by coupling of the upper ground plate GPu to the windings WG. Both stacked vias that connect upper ground plate GPu second metal area MP2 to bottom ground plate GPB represents a grounding inductance Lp of the first

additional tuning element TE1.

The second tuning element TE2 comprises second tunable capacitor formed by capacitive coupling of bottom ground plate GPB to the bottom wiring arranged above. To keep this coupling small and/or to tune the second tunable capacitance an opening OP just below the bottom most winding is provided in bottom ground plate GPB. Further, shielding strips MS are arranged along the perimeter of each winding WG.

The shielding strips MS also serve to improve the isolation between input and output of the filter

Figure 8 depicts these shielding strips more distinctly. One end of each shielding strip MS is connected to a grounding via and the other end is open. Moreover, this figure 8 shows how second metal area MP2 can be minimized to comply with the cross-sectional area of a grounding via. Here the major part of the tunable capacitance CG forms directly between upper ground plate GPu and the windings WG of primary grounding inductor LG2.

CG first tunable capacitance

CG, CP first and second tunable capacitance

CSS series capacitance in signal line SL

FC filter chip

GP _B bottom ground plate

GPD upper ground plate

GT ground termination (on filter chip)

LG1 , 3, 4 grounding via having parasitic inductance only

LG2 primary grounding inductor

LGcorrmion equivalent grounding inductance of SC and PB

MP2 second metal area

MS shielding strip

OP opening in bottom ground plate

PB parallel branch

PB printed circuit board (PCB)

RP parallel resonator

RS series resonator

SC solder contact of SU to PB

SL series signal line

SU carrier substrate

Tl, T2 terminals of series signal line SL

TE1, E2 additional tunable elements

VG grounding via

WG winding