DE102019121866A1 - SAW resonator cascade - Google Patents

Abstract

Cascades of SAW resonators with different apertures can reduce the effects of unwanted transverse modes and other interference modes of the SAW resonator. As part of a filter circuit, cascades of a SAW resonator have the potential to improve insertion loss, edge steepness, group delay ripple, compression and performance stability of SAW filters and their sensitivity to manufacturing tolerances.

Description

The invention relates to filter circuits which include cascaded SAW resonators modified to improve spurious modes and other parameters of filter performance.

SAW resonators are used to form filter circuits, e.g. B. by the same are interconnected in a junction type or cross member type arrangement. Furthermore, SAW resonators can be part of a filter circuit implemented in a technology other than branch type or even different from SAW in order to act as a single element that improves the selectivity or forms additional poles in the transfer characteristic of the filter circuit.

In addition to the desired main SAW mode (typically a Rayleigh or SH-like mode), there are some spurious modes in SAW resonators. These interfering resonances can in z. B. transverse modes, differently polarized modes, volume modes and other unwanted modes. In certain cases, due to design limitations, manufacturing tolerances or other competing optimization goals, such as large bandwidth, good out-of-band suppression, large edge steepness or low insertion loss, it is not possible to completely suppress all of these unwanted modes. In a SAW filter comprising such resonators, these unwanted resonances worsen the target performance, affect e.g. B. the passband evenness, the edge steepness, the compression and performance stability limits or the out-of-band behavior. The problem increases with increasing production tolerances, which are unavoidable at higher frequencies and therefore smaller structures.

It is therefore an object of the present invention to provide a filter circuit which provides SAW resonators with improved spurious mode rejection.

This and other objects are achieved by a filter circuit according to the independent claim. Further advantageous features and embodiments are the subject of the dependent claims.

A filter circuit comprises a first SAW resonator with a first aperture and a second SAW resonator with a second aperture. Both SAW resonators are connected in series or in parallel. The first and second apertures are set to different values. It is preferred that the apertures differ in value by at least 5%.

With such a modification, it is possible to reduce the effects of unwanted transverse modes produced by a single SAW resonator.

In this context and hereinafter two resonators connected in series or in parallel are referred to as cascaded resonators and form a circuit element which is referred to as a resonator cascade. Such a cascade can replace one or more SAW resonators in any filter circuit. If the first and second resonators are cascaded in series, some design modifications, e.g. B. increasing the resonator area, may be necessary to adjust the static capacitance, which is reduced compared to a single resonator in a series connection of resonators.

Integrated within a filter circuit, such cascades can reduce the effects of spurious modes on filter performance. In particular, such cascades can improve an insertion loss, an edge steepness, a group delay ripple, a compression and a performance stability of SAW filters as well as their sensitivity to manufacturing tolerances.

In SAW filters, resonator cascades are widely used to improve power stability, compression, and linearity, as well as to reduce static capacitance and to equalize impedance levels. Cascades can include resonators connected in series and in parallel. The individual resonators within a cascade are usually the same in order to enable their admittances to be perfectly superimposed.

The proposed cascade of two or more resonators can be particularly advantageous if a full suppression of transverse or interference modes in a filter circuit cannot be achieved by simple design measures due to manufacturing tolerances, design limitations or other competing design goals.

The basic idea is that the resonance frequency of some interference modes (in particular of transverse modes and other modes with a non-uniform transverse profile) depends on the aperture of the resonator. Accordingly, a cascade of two or more resonators with different apertures makes it possible to distribute unwanted features in the admittances of the resonators over different frequencies and in this way to reduce the effect of the unwanted resonances on the admittance of the entire cascade. By changing the aperture of a SAW Resonator, the frequency of each higher mode is shifted by a specific amount. A shift by a larger amount is achieved for interference modes with a respective increased frequency.

Both the first and the second SAW resonator as well as any other SAW resonator with a higher number in the cascade comprises only one IDT , which is arranged in an acoustic track between two reflectors.

A filter circuit may comprise a branch type or cross member type structure of reactance elements. One or more of these reactance elements can be replaced by a SAW resonator cascade comprising two or more SAW resonators with two, three or more different apertures. In the resonator cascade, a first, second and optionally any desired higher-numbered SAW resonator can be connected directly to one another in series in order to represent a single cascaded resonator. Some of the resonators connected in series can themselves represent a parallel connection of two or more individual resonators. A cascade results in two or more "stages" being present, each stage comprising a single resonator or a circuit of two or more individual resonators connected in parallel. In addition, a first and a second resonator or optionally more resonators that have different apertures and are connected in parallel can form a cascade with only a single stage and can provide the already mentioned advantage of distributing the resonance frequencies of interference modes.

In the resonator cascade, a difference of at least 5% between the highest and lowest aperture of the SAW resonators is preferred. However, larger differences of 10%, 20% or more are possible and can provide a further benefit.

If there are more than two SAW resonators in the cascade, the apertures of two resonators directly following one another in the cascade can deviate by a smaller amount if the difference between the highest and lowest aperture is high enough to produce the desired one To achieve suppression of interference modes. In any case, it is preferred that the values of apertures are evenly distributed over the entire range of aperture values. This includes the optional requirement that each aperture value is only present once in a cascade.

According to one example, the aperture values are distributed symmetrically around a central aperture.

The filter circuit can be designed as a bandpass. Alternatively, the filter circuit can be a low pass or a high pass. A notch filter is another possible embodiment, since in all of these filter circuits it is possible to suppress interference modes with a cascade of at least two SAW resonators, each with a different aperture. A preferred use of the filter circuit is as a Tx filter.

As mentioned above, interference modes and high production tolerances mainly occur at higher frequencies due to their smaller IDT structures and finger / finger spacing. Therefore, a preferred application of the proposed cascade is a filter circuit that operates at high frequencies, e.g. B. works in a band with a frequency of about 2.3 GHz and more.

The proposed cascaded SAW resonators can be a reactance element of a known filter circuit, e.g. B. a branch type filter replace. However, it is possible to add such a resonator cascade as a parallel or series element to any other filter circuit. If a known filter circuit already comprises cascaded SAW resonators, the new cascade can be implemented without any design adaptation being necessary.

If a known filter circuit comprises individual SAW resonators which tend to produce spurious modes, the filter topography only needs to be adapted in the case of resonators cascaded in series, because the replacement of individual SAW resonators with a series resonator cascade compared to a single non-cascaded resonator consumes a multiple of the chip area. However, if the performance of the filter circuit can be sufficiently improved by introducing the new cascade, the increased area requirement is overcompensated and need no longer be a real disadvantage.

If a known filter circuit already comprises cascaded or non-cascaded resonators, it is preferred to use those resonators or resonator cascade which generate interference modes at critical frequencies, e.g. B. at or near a strip edge, generate / generated to replace by the new cascade. For example, in a ladder type filter, the lowest frequency series resonator tends to produce a spurious mode near the upper band edge.

Apart from the different apertures, the new resonator cascade can be designed like a well-known cascade. Hence it will It is preferred that the direct series connection of two adjacent SAW resonators or their IDTs is realized by combining the two adjacent busbars of the resonators in order to form a common busbar that is shared by the two resonators.

• 1 zeigt einen ersten und einen zweiten SAW-Resonator mit unterschiedlichen Aperturen, die in Reihe geschaltet sind, gemäß einer allgemeinen Ausführungsform,
• 2 zeigt eine Abzweigtypstruktur aus Resonatoren einschließlich eines ersten und eines zweiten SAW-Resonators gemäß einer Ausführungsform,
• 3 zeigt eine Kreuzgliedtypstruktur aus Resonatoren einschließlich eines ersten und eines zweiten SAW-Resonators gemäß einer Ausführungsform,
• 4 zeigt eine Struktur eines Filterschaltkreises einschließlich eines ersten und eines zweiten SAW-Resonators gemäß einer Ausführungsform,
• 5 zeigt einen hybriden Filterschaltkreis einschließlich eines ersten und eines zweiten SAW-Resonators und eines LC-Filters gemäß einer Ausführungsform,
• 6 zeigt eine Reihenschaltung aus einem ersten, einem zweiten und einem dritten SAW-Resonator mit unterschiedlichen Aperturen,
• 7 zeigt eine berechnete Admittanz einer Reihenschaltung aus einem ersten, einem zweiten und einem dritten SAW-Resonator mit unterschiedlichen Aperturen,
• 8 zeigt einen ersten und einen zweiten SAW-Resonator mit unterschiedlichen Aperturen, die parallelgeschaltet sind, gemäß einer zweiten allgemeinen Ausführungsform.

The invention is explained in more detail below with reference to specific embodiments and the associated figures. The figures may only be drawn schematically and not to scale. Therefore, no relative or absolute dimensions can be taken from the figures.

• 1 shows a first and a second SAW resonator with different apertures, which are connected in series, according to a general embodiment,
• 2 12 shows a branch type structure of resonators including first and second SAW resonators according to an embodiment;
• 3 Fig. 10 shows a cross member type structure of resonators including first and second SAW resonators according to an embodiment;
• 4th Fig. 10 shows a structure of a filter circuit including first and second SAW resonators according to an embodiment;
• 5 FIG. 3 shows a hybrid filter circuit including first and second SAW resonators and an LC filter according to an embodiment; FIG.
• 6th shows a series connection of a first, a second and a third SAW resonator with different apertures,
• 7th shows a calculated admittance of a series connection of a first, a second and a third SAW resonator with different apertures,
• 8th shows a first and a second SAW resonator with different apertures, which are connected in parallel, according to a second general embodiment.

1 shows a cascade of a first and a second SAW resonator R1 , R2 with different apertures AP1 , AP2 connected in series according to a first general embodiment of the invention. In the left part of the figure a block diagram of the two resonators is shown. The second resonator is along a transverse direction, which is the direction normal to the direction of propagation R2 with regard to the first resonator R1 shown enlarged.

In the right part out 1 a schematic finger structure of the resonators cascaded in series is shown. Each of the two resonators includes one IDT (Interdigital transducer) between two reflectors REF is arranged in an acoustic path.

A second general embodiment that provides the same advantages and includes a cascade of first and second resonators connected in parallel will be discussed later with reference to FIG 8th explained.

2 Fig. 16 shows a filter circuit comprising a branch type structure of reactance elements, which for simplicity are called resonators RS , RP are shown. Some of the reactance elements can be implemented as coils or capacitors. At least one of the reactance elements includes a cascade of a first and a second SAW resonator with different apertures AP according to the embodiment 1 . These two SAW resonators form a cascaded resonator. Such a cascaded resonator - if it comprises resonators connected in series - has to be adapted in terms of size in accordance with the design rules of a branch type filter circuit in order to establish correct static capacitances of the resonators including the cascaded resonator. If the cascaded resonators R1 , R2 according to 8th are connected in parallel, no adaptation of the design of the filter is necessary.

The branch type structure is formed as usual and comprises a number of at least one base portion, each of which is a series resonator RS in a series arm, which is formed between a first and a second terminal T1, T2, and a parallel branch with a parallel resonator arranged therein RP connected to ground includes. The number of base sections BS _LT can be increased in the filter circuit if a higher selection of the filter circuit is necessary.

The number of resonators that are cascaded can be set as desired. But each of the cascaded resonators is formed according to the first embodiment. It is therefore sufficient to replace each cascaded resonator of a known branch-type structure of resonators according to the prior art with a cascade of resonators according to the invention. The number of resonators in the cascade is at least two, but a larger number can be set. Each of the resonators in the cascade advantageously has a different aperture.

3 Fig. 10 shows a filter circuit comprising a cross-member type structure of reactance elements, usually represented by series and Parallel resonators RS , RP are formed. Again, there is at least one of the resonators RS , RP formed as a cascade of SAW resonators according to the invention. Similar to the branch type structure, a cross link type structure includes base sections BS _LC each including series and parallel resonators. The parallel resonators, which are arranged in parallel branches, connect two series lines which are connected in parallel. The parallel branches cross each other to form a kind of twisted ladder.

4th shows a filter circuit comprising a first filter element F1 and a resonator connected in series RS includes. A parallel resonator RP is connected in a parallel arm that is connected to ground.

The first filter F1 is a filter circuit which is implemented in any technique. The first filter can be, for example, a DMS filter, an LC filter which is formed from passive L and C elements (inductances and capacitances), or any other desired filter circuit.

The parallel resonator RP and / or the series resonator RS comprise a cascade of resonators according to the invention. If the first filter comprises further resonators, these can also be formed from such cascades.

5 Figure 11 shows, as an example, a schematic hybrid filter with a series impedance element IES formed as a capacitor, for example, and a parallel impedance element IEP formed as an inductor, for example. A base portion BS of a branch type structure is connected in series therewith and comprises a series resonator RS and a parallel resonator RP . The parallel resonator RP and / or the series resonator RS comprise a cascade of resonators according to the invention. Of course, the filter circuit can be off 5 comprise a larger number of LC units or base portions of a branch type structure.

6th shows a cascade of three SAW resonators connected in series R1 , R2 and R3 according to an embodiment in which each aperture AP of a resonator RS , RP is different from the other two. In the figure, the aperture decreases from the first aperture AP1 , which is assigned to the first resonator in the cascade, to the second aperture and the third aperture AP3, so that AP1 <AP2 <AP3. However, this is an exemplary arrangement and the aperture does not have to increase or decrease continuously in the cascade. Further, these three resonators include bus bars that are commonly owned and therefore shared by the two adjacent IDTs from two adjacent resonators.

Likewise, a cascade according to the second general embodiment can comprise three resonators which are connected in parallel and each have a different aperture (not shown in the figure).

Higher cascades which include more than three resonators with different apertures and which are likewise possible embodiments are not shown.

7th shows a calculated admittance for a cascade of three resonators with the same design and the same pitch, but with different apertures, compared to admittances of respective individual resonators with three different apertures. The upper diagram shows the real part of the admittance and the lower diagram shows the respective amount.

The three thin lines 1 , 2 and 3 are assigned admittances of resonators with different apertures. line 2 corresponds to a resonator with a target aperture. line 1 corresponds to a resonator with an aperture that is 10% larger than a target aperture according to the line 2 is. line 3 corresponds to a resonator with an aperture that is 10% smaller than a target aperture according to the line 2 is. The bold line C. corresponds to the aperture of the cascade from the three individual resonators. All resonators have the same static capacity as the resonator with the target apparatus according to the line 2 normalized. The lines are for better visibility 1 and 3 from the lines 2 and C. shown offset. The same goes for the diagram below.

As can best be seen in the diagram above, the admittance of the three individual resonators shows according to the lines 1 , 2 and 3 a ripple with relative maxima that originate from different interference modes, which are unavoidable due to production tolerances, for example. The frequencies of these interference modes appear above the resonances of the resonators.

The line C. corresponds to the aperture of the cascade from the three individual resonators. It can be from the even course of the line C. it can be seen that the disturbance modes and therefore the ripple of the cascade is considerably reduced according to the invention. This is proof of the positive effect of the invention.

8th shows a first and a second SAW resonator R1 , R2 with different apertures AP1 , AP2 which are connected in parallel according to a second general embodiment of the invention. In the upper part of the figure, a block diagram of the two resonators is shown, the second resonator being shown enlarged with respect to the first resonator along a transverse direction, which is the direction normal to the direction of propagation. In the lower part from 8th a schematic finger structure is shown. Each of the two resonators includes one IDT (Interdigital transducer) between two reflectors REF is arranged in an acoustic path. The two resonators R1 , R2 use a common center reflector that is common to the two resonators. However, each resonator can have two reflectors that are not shared by two adjacent resonators. In addition, two resonators cascaded in parallel do not have to be arranged directly adjacent to one another. Cascading only uses advantages that result from the electrical connection of two resonators with different apertures.

Embodiments are not shown which comprise resonators cascaded in series and in parallel, which form a single cascade which can replace a single resonator in any filter circuit.

List of reference symbols

acoustic trace

Branch type structure

cascaded resonator

common busbar

1,2,3

Admittance of individual resonators

AP1, AP2

first and second apertures

BSLC

Base portion of a cross link type structure

BSLT

Base section of a branch type structure

C.

Admittance of a cascade of resonators with different apertures

F1

First filter

FC

Filter circuit

IDT

IDT / interdigital converter

IE

Reactance element

IEP

Parallel reactance element

IES

Series reactance element

R1, R2

first and second resonator

REF

reflector

RP

Parallel resonator

RS

Series resonator

Claims (10)

Filter circuitry comprising: - A first SAW resonator (R1) which has a first aperture (Ap1), - A second SAW resonator (R2) which has a second aperture (AP2) and is connected in series or in parallel to the first SAW resonator, wherein the first and second apertures are different in terms of value. A filter circuit according to the preceding claim, wherein the first and second apertures differ in value by at least 5%. Filter circuit according to the preceding claim, wherein each SAW resonator (R) comprises only one IDT which is arranged in an acoustic track between two reflectors (REF). Filter circuit according to one of the preceding claims, - wherein the filter circuit comprises a branch type structure of reactance elements (R, IE), - wherein the first and second resonator are directly connected to each other so that they together represent a cascaded resonator, - wherein the cascaded resonator is included in the branch type structure as a reactance element. A filter circuit according to any preceding claim, wherein the cascaded resonator is a series reactance element of the branch type structure. Filter circuit according to one of the preceding claims, wherein three or more SAW resonators form a cascaded resonator, the SAW resonators of the cascaded resonator having at least three different apertures. Filter circuit according to one of the preceding claims, which is designed as a band pass or a low pass for use as a Tx filter. Filter circuit according to the preceding claim, which is designed to operate in a band above 2.3 GHz. Filter circuit according to one of the preceding claims, wherein the first and second resonators form a cascaded resonator which is connected as a parallel or a series element within the filter circuit. Filter circuit according to one of the preceding claims, wherein the series connection of the IDTs of the first and second resonators is realized by a common busbar, which is of the first and second resonator or of two any other adjacent resonators is shared.

Images