ACOUSTIC FILTER WITH IMPROVED REFLECTIVITY

Abstract

A filter circuit comprises a main operating unit (MU) arranged in the series signal line providing most of the filter function of the filter circuit. A micro acoustic last series resonator (RL.sub.s) as a last element of the main operating unit in the series signal line is prone to excite a spurious mode that is damped with a final series capacitance (CE.sub.s) circuited between the last element and the antenna terminal (AT).

Claims

1. A filter circuit that provides a pass band, comprising: a series signal line (SLS) that connects a first terminal (T1) and an antenna terminal (AT); a main operating unit (MU) arranged in the series signal line providing most of the filter function of the filter circuit; a micro acoustic last series resonator (RSL) that is a last element in the series signal line and part of the main operating unit; and a final series capacitance (CES) circuited between the last element and the antenna terminal.

2. The filter circuit of claim 1, wherein the filter circuit is formed on a substrate, and wherein the final series capacitance is embodied as an external discrete element or as an integrated capacitance on the substrate or is a resonator acting with its static capacitance only and having a resonance frequency outside the pass band.

3. The filter circuit of claim 1, wherein a further node (N) is arranged in the series signal line between the last element and the final series capacitance, and wherein a further parallel branch is circuited between the further node and a ground terminal, the further parallel branch comprising an impedance element (IE) chosen from a further parallel resonator, a parallel coil and a parallel capacitance.

4. The filter circuit of claim 1, wherein an additional parallel capacitance (CP.sub.A) is arranged in the further parallel branch, and wherein an additional series coil (IS.sub.A) is circuited in parallel to the final series capacitance (CE.sub.S).

5. The filter circuit of claim 1, wherein a phase shifting circuit is circuited between the main operating unit (MU) and the antenna terminal, the phase shifting circuit comprising the last series resonator, an additional parallel coil (IP.sub.A) arranged in the last parallel branch and the final series capacitance in the series signal line.

6. The filter circuit claim 1, wherein the main operating unit ends with a last node and a last parallel branch is connected to the last node, and a parallel micro-acoustic resonator is arranged in the parallel branch, and wherein said last node is directly connected to the final series capacitance and the final series capacitance is directly connected to the antenna terminal.

7. The filter circuit of claim 1, wherein the main operating unit comprises a ladder type arrangement of series and parallel resonators.

8. The filter circuit of claim 1, wherein the resonators of the ladder type arrangement comprise SAW or BAW resonators.

9. The filter circuit of claim 1, wherein the main operating unit comprises a DMS filter series-connected to a micro-acoustic series resonator.

10. The filter circuit of claim 1, wherein the last series resonator is a temperature compensated thin film SAW resonator or a BAW resonator.

11. The filter circuit of claim 1, wherein a first operating unit comprising the filter circuit and a second micro-acoustically operating unit chosen from a passband filter circuit or a notch-filter are connected to the same antenna terminal in a permanent or switchable form, wherein the first or the second operating unit comprises a micro acoustic resonator next to the antenna terminal, wherein this resonator next to the antenna terminal is prone to produce a spurious mode in the operating frequency band of the respective other unit, and wherein the final series capacitance suppresses this spurious mode.

Description

[0025] In the following the invention is explained in more detail with reference to specific embodiments and the related schematic figures.

[0026] FIG. 1 shows an admittance curve of a filter circuit with a main mode and a plate mode as a spurious mode

[0027] FIG. 2 shows at A the antenna reflectivity around the frequency of the spurious mode and at B a pass band of another operating unit that is lying within the frequency range of the spurious mode

[0028] FIG. 3 shows a first embodiment of the invention with a ladder type arrangement of resonators as main operating unit

[0029] FIG. 4 shows a generalized structure of the first embodiment where the main operating unit may be an arbitrary filter circuit ending with a last series resonator at the antenna side thereof

[0030] FIG. 5 shows another general structure of a further first embodiment where the main operating unit may be an arbitrary filter circuit ending with a last parallel branch and a parallel resonator at the antenna side thereof

[0031] FIG. 6 shows another embodiment where the main operating unit is chosen similar to the embodiment of FIG. 3 may be an arbitrary filter circuit ending with a last parallel branch and a parallel impedance element at the antenna side thereof

[0032] FIG. 7 shows an embodiment where the main operating unit is a ladder type arrangement of resonators ending with a last parallel branch and a parallel resonator arranged therein

[0033] FIG. 8A to 8D show embodiments where the main operating unit is a ladder type arrangement of resonators ending with a last series resonator and different additional elements comprising a last series capacitance are added

[0034] FIG. 9 shows an embodiment where a first and a second operating unit are coupled to the same antenna terminal and one of first and second operating unit is ending with a last series resonator.

[0035] FIG. 1: In example, a B1/B3 quadplexer is connected at the same antenna with a B40 filter. A spurious mode coming from B3 TX filter negatively affects the performance of the B40 filter. A severe spike in the antenna reflectivity is visible as shown in FIG. 2A. This leads to a drop in the pass band of the B40 filter as shown in FIG. 2B. The solid line represents a B3 TX filter affected by a spurious mode, whereas the dashed line is the same filter without the effect of a spurious mode.

[0036] According to the invention, the real challenge is to find a way to completely suppress the spurious modes rather than trying to move them out of the main bands. In the example represented in FIG. 1, a resonator designed to have his main mode in Band 3 (uplink) has its plate mode in Band 1 (downlink). If such a resonator would be used in a multiplexed application (band 1+band 3), the overall performance would suffer considerably due to a mediocre cross-isolation between the two operating bands.

[0037] One could try to move the plate mode in frequency, but this will result in violations of other constraints (e.g. in case of carrier aggregation with Band 40).

[0038] From a circuit level point of view, the nature of the unwanted mode is rather irrelevant. Here, the attention is more focused on how an unwanted mode from a filter “A” negatively affects a second filter “B” which is connected to the same antenna terminal. The answer to such question can be found in the reflectivity at the antenna, as represented in FIG. 2A. A bad reflectivity in a specific frequency range (e.g. the spike of the unwanted mode) means that there is energy leaking in another filter. This effect can be thought as a very pronounced frequency-localized loss caused by one filter to the other.

[0039] As it will be clearer after the following sections, the invention aims to cover not only unwanted modes from SAW filters, but more generally, seeks to cover a broader range of cases (e.g. spurious modes in BAW filters or other filter technologies like DMS filter when circuited with a micro-acoustic resonator).

[0040] The idea comes from the observation that the spikes in the reflectivity are caused mainly from the resonators close to the antenna. The effects of other resonators are quickly vanishing while moving along the ladder topology from the antenna to the first terminal (i.e. the other port). This can be mathematically proven by separating each stage of the filter (e.g. shunt/series resonator+series/shunt resonator) into a cascade of multiple 2-ports networks.

[0041] Since the spikes are caused only/mainly by the last stage at the antenna side of the filter, the idea is to add an additional stage or replace one or two elements of the existing last stage) with one of the following solutions: [0042] Adding a passive capacitor (this can be a series or a shunt resonator) and leaving the original resonator (if its unwanted mode does not create any trouble) [0043] Adding a stage with two passive capacitors (a series and a shunt capacitor) and leaving the original resonator (if its unwanted mode does not create any trouble) [0044] Adding a passive capacitor and a resonator that does not create any trouble) [0045] Replacing the resonator causing the unwanted mode by a passive capacitor. This can be a series or a shunt resonator which has to be replaced by a respective series or a shunt capacitor.

[0046] FIG. 3 shows a filter circuit wherein the main operating unit MU is the original filter unit with a last series resonator RLS at the side of the antenna terminal AT. The main operating unit MU comprises a ladder type arrangement of series resonators RS arranged in a series signal line SL between an antenna terminal AT and a first terminal T1. Parallel branches with parallel resonators RP arranged therein are connecting nodes N in the series signal line SL to ground terminals. A final series capacitance CES is added and inserted into the filter circuit between the last series resonator RLS and the antenna terminal AT.

[0047] FIG. 4 shows a more general embodiment using the same idea. The main operating unit MU providing most of the pass band filter function may be embodied in an arbitrary technology that can provide the aimed pass band. Preferably an acoustic filter is used such like SAW, BAW or DMS. However, an LC filter made from discrete or integrated passive elements is possible too. This main operating unit MU if circuited in series with a last series resonator RLS may cause spikes that may disturb other bands. Hence, here too a final series capacitance CES is circuited between antenna terminal AT and last series resonator RLS to suppress the spurious mode.

[0048] FIG. 5 shows another embodiment wherein an arbitrary main operating unit MU ends up with a last node NL and a parallel branch with a last parallel resonator RLP is connected to this last node. This last parallel resonator RLP is prone to excite spurious modes that are suppressed by the final series capacitance CES.

[0049] According to the embodiment shown in FIG. 6 the same main operating unit MU ending with a last series resonator RLS is used. A parallel impedance element IE is added as a new shunt element in the filter circuit and a final series capacitance CES is added between last series resonator and the antenna terminal. The added final series capacitance may be an external capacitor or a capacitor in the filter circuit for example a resonator with a resonance frequency that is far away from the used bands.

[0050] In this way the following effects are obtained: [0051] Matching of the filter is practically unaltered. Same topology can be used and only one more stage is necessary. This causes presumably only small losses due to the greater number of elements, but at the same time there is more freedom in the trade-off between selectivity/in-band attenuation. Adding stages usually helps with selectivity, but on the other hand increases the losses (therefore the insertion loss). If the level of selectivity achieved before the introduction of this additional stage was already sufficient, one can re-optimize the filter to recover the small additional insertion loss. Small additional losses can in principle be compensated through the re-design of the filter. [0052] The last series resonator RLS (with its plate mode) is not the last element of the filter circuit anymore. At the antenna terminal AT, incoming waves will now see first the final series capacitance CES, and thus they will not excite any plate-mode.

[0053] It is worth to mention that the plate-mode will be still excited, but its intensity will be much less pronounced since there is an additional stage between the last series resonator RLS and the antenna.

[0054] Further, the proposed approach does negative impact on other bands that may be used in connection with the filter circuit and may be connected to the same antenna terminal AT.

[0055] Once again, for sake of clarity, the series element can be either external or be directly integrated on-chip. In FIG. 7 the main operating unit MU ends at the antenna side with a parallel branch and a parallel resonator arranged therein. The final series capacitance is represented as a capacitor, since its main purpose is to act as capacitor. However, there is the possibility of also using an additional series resonator instead of a capacitor. This approach is efficient in damping a spurious mode coming from the last series resonator but reduces the intensity of the parallel as well.

[0056] In case the main operating unit ends with a last series resonator at the antenna different damping stages can be added, as schematically represented in FIG. 8:

[0057] A) An additional parallel resonator RPA can be introduced (on-chip) between main operating unit MU and final series capacitance CES. In this way the last series resonator causing the spurious mode is not the closest to the antenna anymore, its spurious mode dis damped and the filter keeps the same ladder topology with only one stage more.

[0058] B) In a variant of case A) the additional parallel resonator RPA is substituted with an additional parallel capacitance CPA. This is useful when both last resonators, last series and last parallel resonator, are causing problems with unwanted modes in counter-bands. Other more complicated networks are allowed too. In these topologies, two (or more) elements are added in series to the filter to cause a phase-shifting of the filter at the antenna terminal.

[0059] C) In this case an additional parallel coil IPA is introduced between main operating unit MU and final series capacitance CES. If the static capacitance of the last series resonator RSL is considered, a high pass T phase-shifter (series capacitance, shunt coil, series capacitance) is achieved.

[0060] D) In this last case a parallel resonant circuit comprising a final series capacitance CES parallel to an additional series coil ISA creates a resonance in the frequency range of the unwanted spurious mode. The final series capacitance close to the filter helps in recovering the matching.

[0061] FIG. 9 shows an embodiment where a first and a second operating unit OU1,OU2 are coupled to the same antenna terminal and one of first and second operating unit is ending with a last series resonator RLS. The first operating unit OU may be a filter circuit as described above according to FIG. 4 for example. The second operating unit OU2 may be an arbitrary filter or notch circuit operating with a frequency band that complies with the frequency of a spurious mode the last series resonator is prone to excite. By the proposed final series capacitance CES this mode is damped and the disturbance in the operating frequency band of the second operating unit is minimized.

[0062] All presented filter circuits have shown the capability of damping the spurious modes by replacing an element (or in case necessary, even both elements) of the last stage of a ladder-topology filter or another filter technology/topology with passive elements which do not present any spurious mode.

[0063] A preferred application of the presented filter circuit in a system is shown in FIG. 9. [0064] A system consists of minimum two different acoustical operating units (e.g. BAW-/SAW-filter, -notch) which are connected temporary or permanent in a system (e.g. module). [0065] One operating unit creates spurious modes which affect in a critical frequency range the performances of second unit (e.g. filter, duplexer of a quadplexer). [0066] Very critical are spurious modes if they are excited in an acoustical operating unit close to the connection of the two operating units (e.g. antenna terminal within a quadplexer). [0067] The progressive handling of spurious modes may demand a rework of the last sub-unit of the main operating unit (in case of a SAW filter the last IDT) or/and the electro-magnetic environment between the last sub-unit and the connection point of the two operating units. [0068] The rework could include a partial (parallel capacity, resonator with different pitch) or fully replacement of the last sub-unit by an alternative electric element (e.g. coil, capacitor, damping circuit . . . ) which does not create any spurious modes in general or at least not in the critical frequency range.

[0069] The invention has been explained with reference to a limited number of embodiments but is not limited to these specific embodiments. The full scope of the invention is defined by the claims.

LIST OF USED TERMS AND REFERENCE SYMBOLS

[0070] AT antenna terminal [0071] CES final series capacitance [0072] CPA additional parallel capacitance [0073] EL last element [0074] IPA additional parallel coil [0075] ISA additional series coil [0076] MU main operating unit [0077] N node [0078] NL last node [0079] OU1,OU2 operating units [0080] RLP last parallel resonator [0081] RLS last series resonator [0082] RP parallel resonator [0083] RPA additional parallel resonator [0084] RS series resonator [0085] SLS series signal line [0086] T,T′ terminals [0087] T1 first terminal