DE19818826A1 - Surface acoustic wave filter for mobile communications - Google Patents

Abstract

The surface acoustic wave filter has a lithium tantalate substrate provided with at least two electrically coupled acoustic paths, each with at least two transducers and two reflectors. At least one of the transducers for each acoustic path is used as a coupling transducer for coupling to the adjacent acoustic path. The coupling transducers of a pair of adjacent acoustic paths are electrically coupled via a connection line (V), with an inductance (I) connected in series or parallel with the coupling line.

Description

With the growing number of participants in cellular mobile radio It is necessary to make new frequency bands available to meet the increasing need for frequency bands to satisfy. For example, this shows in Europe GSM system a bandwidth of 25 MHz in 900 MHz Area on. For the E-GSM system planned as a successor an extended bandwidth of 35 MHz in the same frequency range richly provided. The distances between the assigned bands less. For example, it decreases se the distance between the transmit (Tx) band and the receive (Rx) band in the step from GSM to E-GSM from 20 MHz to 10 MHz. Similar However, broadband systems are also found in Japan and around the world provided in the range around 2GHz.

To design end devices according to this new E-GSM standard usually the architecture of the GSM device largely continue to be used. However, they all have to be replaced HF filters are using the new larger bandwidth to do justice to the narrower band gap in E-GSM.

For the previously used RF filters in surface waves technik (OFW-Technik) this means the bandwidth signifi edge and at the same time the slope to maintain or enlarge. Remote selection is allowed do not deteriorate in the process.

This cannot be done with conventional known SAW filters achieve without at the same time other serious disadvantages To have to buy.

Reactance filter on a temperature stable substrate Lithium tantalate enables the required bandwidth and also offer the desired steep flanks. Problems  but arise with the necessary remote selection and with Transition from single-ended operation at the entrance to balanced Operation at the outlet of the filter (balun). It is also with them Filtering not possible, which is urgent for modern devices required impedance jump from 50 ohms at the input to 200 Ohm to set at the output without additional external network len.

Known filters with two cascading tracks (double mode SAW = DMS) on a high coupling lithium niobate substrate 64 ° red Y / X cut achieve the desired bandwidth and the desired remote selection and enable both balun as well as impedance transformation. However, with this Do not filter the necessary slope and the ge desired low insertion loss achieved because of the substrate has too high a temperature response at which the Fre quenzablage of the filter depending on the temperature greatly changed.

Two-track strain gauge filters on LiTaO ₃ 42 ° red y / x offer all properties apart from the necessary bandwidth. Because of the low ren electro-acoustic coupling of this substrate, such a strain gauge filter in the pass band has a pronounced mini mum, which is not acceptable for the E-GSM system, because, on the one hand, an excessively high maximum insertion loss occurs in the minimum range , suffer from the sensitivity and noise figure of the receiver, and because the ripple in the entire pass band is too high, which complicates the performance control of the system.

The object of the present invention is therefore to provide an SAW Specify filter with which for E-GSM or similar wide bandy systems required bandwidth is achieved without the disadvantages of the known filters just mentioned in purchase to have to take.  

According to the invention, this object is achieved by an SAW filter Claim 1 solved.

Further refinements of the invention are given in the subordinate sayings.

An SAW filter according to the invention has sufficient band width of at least 35 MHz an extremely low input damping and shows neither in the passband embossed minimum, still an impermissibly high ripple, like previously known filters with such a bandwidth grasslands. This improved passband is achieved, oh ne that compared to the known prior art deterioration in the other properties of the fil mentioned ters such as edge steepness, remote selection and Temperature response occurs.

An SAW filter according to the invention is also included as a DMS filter Coupling up at least two cascaded acoustic tracks built up the lithium tantalate. Because DMS kilter of the resonator type each acoustic track is on both sides of a total of two Reflectors limited within which there is a resonant Can build up vibration. There are at least two in each track Converter provided. In the first lane this is at least one Input converter and at least one coupling converter. About one The connecting conductor is the coupling converter of a first track the coupling converter of a second track electrically connected. In a SAW filter with two acoustic tracks, the second track next to the coupling converter at least one output transducer arranged on which the output signal is tapped becomes. Via the connecting conductor is serial or parallel to the associated coupling transducers ge switches.

In a preferred embodiment, the lithium tan a crystal cut xx ° red Y / X for which preferably 30 ≦ xx ≦ 46 or 210 ≦ xx ≦ 226 applies  Substrate with such a crystal cut has depending on the Aus management of the metal electrodes and a possible passive particularly low runtime losses and a special one high coupling and a good temperature response.

An SAW filter according to the invention has at least two acoustics traces. More than two tracks are possible however, no further advantages are achieved.

There is at least one input / output converter and one in each track Coupling converter provided. With more than two converters per battery The static track are input / output converters and coupling converters ty pically arranged alternately. If the number is odd converters can have more coupling converters or more input or Output converter may be provided.

With the at least one connecting conductor there is an inductor electrically connected or connected in parallel. Alternatively, the inductance is serial between two couplers transducers in different acoustic tracks or serial to the connecting conductor between these coupling transducers tet. The inductance can be on the surface of the Be arranged substrate. However, it is also possible that Arrange inductance externally, for example discretely a carrier substrate or in a housing in which the SAW Filter is installed. The corresponding connection with the Transducer structures on the substrate can then, for example be made over bond wires. In the simplest way the inductance is a printed strip conductor or a printed spool that goes with the rest Metallization can be produced. With an external arranged inductance can correspond to this one the analogue processes. For example the inductance must be printed on the inside of the housing. Poss However, it is also a concrete component as an inductor te to use.  

In a specific embodiment of the invention, In a SAW component used in the fragli Chen frequency range shows an inductive behavior. At for example, a one-gate resonator can be between two tracks arranged the substrate and electrically in series with two heads be switched. This version has the front partly that with an otherwise free filter design the induct activity can take effect precisely at the frequency at which an adjustment of the passage area with a filter after the state of the art is required.

An inductance effective according to the invention has a size order of approx. 10 nH. In general, for an inventor a larger inductance connected in parallel according to the invention Value required as for a series-connected inductor vity.

The SAW filter according to the invention is constructed so that the Output operated either symmetrically or asymmetrically can be. While in asymmetrical mode of operation one of the two outputs is at ground, can be symmetrical Operating a positive or the correspondingly symmetrical negative signal at either one of the outputs be gripped. For symmetrical operation at the output Transducers are preferred arrangements that have two output have transducers whose outputs are symmetrical to each other and therefore polarized differently in the operation of the component are. This has the advantage that the required electrical connections outside the converter area can be, so that no additional traces between traces must be brought out.

A good impedance transformation, for example an Impe jump from 50 ohms at the input converter to 200 ohms at Output converter is reached when the output converter sym is divided metrically. This can be parallel to the acoustic Tracks or parallel to the direction of propagation of the acoustic  Surface wave occur and is called a so-called H split realized. The output converter is replaced by an additional parallel busbar in the middle of the acoustic Track divided so that each sub-converter of the output converter half the acoustic track width and thus double the impe danz has. The middle track serves as a ver binding for the two outer busbars. You can extend over the entire length of the output transducer, or even over a part of its entire length.

Another way to make an impedance jump in the SAW filter To realize is to split a busbar of the output converter into two electrically different ones Halves, the so-called V-Split. The electrode fingers on the Busbars are arranged so that on the two Halves of the divided busbar symmetrical to each other, This means that signals with different polarity are tapped can.

A preferred metallization for building the transducer and the Reflectors are made of aluminum Al, aluminum copper AlCu (Alloy) or aluminum magnesium AlMg or has one Sandwich construction with several different layers that each consist of one of the materials mentioned. A preferred total layer thickness of the metallization is in the loading ranging from 1 to 15 percent of the operating wavelength of the SAW Filters. This operating wavelength is the frequency true, with which the SAW filter is operated and is additional Lich still dependent on the rate of propagation of the SAW in the substrate, including the substrate material and the cut.

In the converters and reflectors, a metallization ver ratio η of significantly more than 0.5. Preferably the metallization ratio η fulfills the condition 0.65 ≦ η ≦ 0.8. Such a high metallization ratio increases the manufacturing stability of the product and reduces signi  fictional losses in the propagating acoustic wave. In some system applications with a moderately large bandwidth is the use of a high metallization according to the invention ratio η alone is sufficient to meet the necessary to achieve smooth and low loss passband.

In the following the invention is based on exemplary embodiments play and explain the associated thirteen figures tert.

Figs. 1 to 10 show exemplary embodiments of the invention SAW filters.

Fig. 11 shows the transmission response of a known filter, and

Fig. 12 shows the transmission response of a SAW filter according to the invention.

Fig. 13 shows an embodiment of a special electrical connection rule.

Fig. 1 shows a two-track DMS filter according to the prior art in a schematic representation. The track A on the input side comprises three transducers K1, E1 and K2, which are arranged between two reflectors R1 and R2, which limit the acoustic track on both sides. Track B has an identical arrangement of three transducers K1B, A2B and K2B. The two coupling converters K1A and K1B or K2A and K2B are each connected by connecting conductors V1 and V2.

Fig. 11 shows the frequency response of such a known Fil ters. A frame is drawn under the pass band, which represents the system requirements for E-GSM. It is clear to know that the pass curve in the right short-wave region has a dent in which it does not meet the system requirements.

Fig. 2 shows an SAW filter according to the invention, which shows a converter and reflector design like the known filter from Fig. 1, but in which, however, according to the invention an inductor I is connected in parallel to the coupling converters. The electrical connection is made, for example, as shown, via the connecting conductor V, with which the inductance is connected. However, it is also possible to connect the inductance to the coupling converters in parallel on the ground side.

Fig. 12 shows the transmission curve of this filter according to the invention. It can be clearly seen that the waviness of the curve in the pass band is significantly reduced and that the pass curve fulfills the system requirements for E-GSM requirements drawn in the form of a rectangle over the entire pass band. The steepness to the low-frequency side, which is followed by the next band, is also well fulfilled.

FIG. 3 shows an arrangement similar to FIG. 2, in which, in contrast to this, the two connecting conductors V1 and V2 are connected to one another by an additional conductor piece L. This increases the symmetry in the arrangement and thus also the symmetry in the signal processing, in particular in balun operation.

Fig. 4 shows an embodiment of the invention with two Spu ren A, B, in which two input and output converters are hen vorgese. A coupling converter K between the two input and output converters and a connecting conductor V, which connects the two coupling converters in the different tracks A, B, complete the arrangement. The two input converters E1A and E2A are connected in parallel and connected to the input IN. The output converters A1B and A2B are also connected in parallel to the output OUT. In parallel to the connecting conductor V, an inductance I is switched.

Fig. 5 shows a filter with two lanes each having two converters, namely a coupling transducer K1 and an input or output transducer E, A. The coupling transducers of the two tracks K1A, K1B are connected via an inductor I to each other in series. This inductance I is arranged in the schematic FIG. 5 between the two coupling transducers, but in the real design it is preferably outside the active area defined by the two tracks on the substrate or even outside, for example in the housing. The serial connection according to the invention is a Inductivity I between two coupling converters is also not limited to the connection shown between the two internal busbars facing the coupling converter. It is also possible to connect the busbars lying on the ground in the figure via a connecting conductor (not shown) and to install an inductor serially therein. The two remaining internal and mutually facing busbars of the two coupling walls ler K1A and K1B can be connected to one another via a further connecting conductor.

Fig. 6 shows a filter with the same converter / reflector arrangement as Fig. 5, but as a difference here the inductance I is connected in parallel to the coupling converter K.

A serial connection of the coupling converter K, as shown in FIG. 5, can also be transferred analogously to the already described exemplary embodiments according to FIGS. 2 to 4 and to the exemplary embodiments yet to be described according to FIGS . 9 and 10.

Fig. 7 describes an arrangement with two input or output converters per track and a coupling converter K arranged in between. In contrast to Fig. 4, the two coupling converters K1A and K1B are converters by symmetrically dividing the internal busbars into two electrically symmetrical part split. This is achieved in that the electrode finger arrangement of the two coupling transducers is axially symmetrical. The two parts of the internal busbar of both split coupling converters are separately connected to connecting conductors V due to the different electrical polarity. In parallel to the connecting conductor marked here with V1, an inductance I is switched, while the other connecting conductor V2 can optionally be connected to ground. The two input converters E1A and E2A are connected in parallel, as are the two output converters A1B and A2B.

Fig. 8 shows a two-track filter with two wall learners per track, which corresponds to the principle of the arrangement shown in FIG. 5. While in Fig. 5, however, a general Induk tivity in series between the two coupling transducers K1A and K1B connected, the inductor I in Fig. 8 is formed as a schematically indicated one-port resonator, the inductor in the desired range of the operating frequency behavior. By appropriate design of this Eintorresona gate, the transmission curve can be influenced and modeled specifically in the area in which an improvement in the insertion loss or a reduction in the ripple is required.

Fig. 9 shows an arrangement with each Koppelwand learn and an input or output converter. In contrast to FIG. 2, however, the output converter is here electrically split symmetrically by symmetrical division of a busbar of the output converter A1B. Accordingly, the electrode fingers of the output transducer are arranged axisymmetrically. The two halves of the split busbar are each connected to an output and represent a balanced out. The advantage of this arrangement is that it has an impedance jump from the input converter to the output converter, for example from 50 to 200 ohms. The second busbar of the output converter, which is not connected to the output, can be grounded, as shown in the figure, but does not have to be connected to an external potential.

Also Fig. 10 shows a filter, the transition from the entrance to the Off denotes an impedance jump. Regarding the number and circuitry of the transducers, this filter also corresponds to that shown in FIG. 2 with the difference that the output transducer A1B is split into two coupled partial transducers, each with half the track width, by an additional internal busbar. An inductance I is connected in parallel with the two coupling walls per track.

In all of the embodiments shown in the figures only with an output (single ended) are shown, in which case the second output is at fixed potential, i.e. at ground, balanced operation is also possible. For this purpose you can the busbars of the output or the outputs lying on ground converter with a second one symmetrical to the other output Output can be connected. This can be done with the help of additional Conductor tracks are made which are defined by the traces Lead out the active area of the filter. It is possible however, also the mass or the second balanced output loop through, that is, the corresponding connection an extended and extended electrode finger to make. This is not just for output converters for egg NEN second balanced output, but also for all Ground connections of the input and output converters possible.

Fig. 13 shows a variation of the filter shown in Fig. 3, in which in the output transducer A1B an external electrode finger in the transducer is extended so that the electrical connection of the associated busbar over the end of this current finger outside of the defined by the acoustic spo ren active area of the filter can take place.

Only the parts of the filter or filters that are essential to the invention are shown throughout the figures. Of course, these filters can also be connected to other reactance elements that can be connected to the input and / or the output. One-gate resonators can be provided as reactants, which can be connected in series or in parallel. Ladder type arrangements are also possible. The exact configuration of the individual coupling, input and output converters, in particular the size and arrangement of the electrode fingers, can also be as desired, as is known from conventional filters. With these known design rules, it is also possible in a simple manner to set the desired width of the pass band. With the invention, however, it is then possible for the first time to smooth this widened pass band and to ensure the required low insertion loss over the entire pass band, as is convincingly demonstrated by the measurement curve of FIG. 12, for example.

The range of variation allowed in the invention also applies all other parts of the filter not previously mentioned or its packaging, without this being explained in detail here would be.

Claims (15)

1. Resonator type surface wave filter (SAW filter) for high frequency applications

• - With a substrate made of lithium tantalate
• - With at least two electrically coupled acoustic tracks arranged on it
• - With at least two transducers per track and two reflectors delimiting the acoustic track on both sides
• - Wherein at least one of the transducers represents a coupling transducer to an adjacent track per acoustic track
• - At least two coupling transducers from two adjacent tracks are electrically connected to one another via a connecting conductor and
• - Wherein ei ne inductance is connected in series or parallel to this connecting conductor.

2. SAW filter according to claim 1, where the substrate consists of lithium tantalate xx ° red Y / X with 30 ≦ xx ≦ 46 and 210 ≦ xx ≦ 226. 3. SAW filter according to claim 1 or 2, where each track has three transducers and two reflectors. 4. SAW filter according to claim 3, with two acoustic tracks, each over the two outer Outer converter as coupling converter with the help of connecting lines tern are coupled, the two connecting conductors between between the two tracks with an additional ladder section are electrically connected. 5. SAW filter according to claim 1 or 2, with at least two acoustic tracks and five walls per track learn and two reflectors.   6. SAW filter according to one of claims 1-5, with two balanced outputs (balanced out), which on one or two output converters are implemented. 7. SAW filter according to claim 6, with an output converter that has a first and a second Has busbar, the second busbar axes is divided symmetrically, with both halves of the second Busbar are connected to outputs and being connected to the a symmetrical output signal (balanced out) can be tapped. 8. SAW filter according to claim 6, with an output converter, which is supplemented by a medium che busbar at least partially in two parallel ordered coupled via the additional busbar pelte partial converter with half the acoustic track width is divided, with the electrode fingers in the two Partial transducers are arranged so that on the two outer Busbars of the output transformer a symmetrical off output signal (balanced out) can be tapped. 9. SAW filter according to one of claims 1-8, in which a housing is provided for receiving the SAW filter is a coil, a strip conductor, as the inductance or the like is provided, which is integrated in the housing are. 10. SAW filter according to one of claims 1-8, in which an inductive SAW component as an inductor, ins special one connected in series between the coupling converter ter gate resonator is provided. 11. SAW filter according to one of claims 1-10, in which additional reactance elements precede the SAW filter are connected downstream.   12. SAW filter according to one of claims 1-11,

• in which the metallization for at least the transducers and the reflectors consists of Al, AlCu alloy or AlMg alloy or has a sandwich structure made up of several different layers of the materials mentioned,
• - in which the layer thickness of the metallization is in the range from 1% to 15% of the operating wavelength of the SAW filter.

13. SAW filter according to one of claims 1-12, with a metallization ratio η in the transducers and Re flectors of more than 0.5 and in particular of 0.65 ≦ η ≦ 0.8. 14. Resonator type surface acoustic wave (SAW) filter for high frequency applications

• - With a substrate made of lithium tantalate
• - With at least two electrically coupled acoustic tracks arranged on it
• - With at least two transducers per track and two reflectors delimiting the acoustic track on both sides
• - Wherein at least one of the transducers represents a coupling transducer to an adjacent track per acoustic track
• - At least two coupling transducers from two adjacent tracks are electrically connected to one another via a connecting conductor
• - With a metallization ratio η in the transducers and reflectors of more than 0.5 and in particular of 0.65 ≦ η ≦ 0.8.

15. Use of the filter according to one of the preceding An say as RF filter in mobile phones, especially after the E-GSM standard.