Patent Application
20190288663
This application claims the benefit of German Patent Application Serial No. 102016114071.6, filed Jul. 29, 2016, which is herein incorporated by reference in its entirety.
The invention pertains to electroacoustic filters with reduced perturbances caused by plate modes, duplexer with such filters and procedures for creation of such optimized filters.
Electroacoustic filters are well suited for use as HF filters in modern communication equipment and have a piezoelectric material and electrodes connected to the piezoelectric material. Such filters include resonators by which, based on the piezoelectric effect, upon application of a HF signal on the electrodes, this is converted between HF signals and acoustic waves. Thus, acoustic waves are propagated in the piezoelectric material or on its surface.
The spatial distances between the electrode structures and the wave velocity are essentially determining the operation frequency of the transducer. The temperature variations are problematic, as they can modify the parameters determining the operation frequency (e.g. distances, wave velocity). For example, if the resonators in the filter are interconnected to band-pass filters, a temperature change would lead to a shift of the center frequency of the pass-band. As the specifications impose strict requirements on the performance of a filter, temperature changes can easily lead to not meeting the specifications.
A possibility to compensate a frequency drift caused by temperature consists in the use of so-called temperature-compensation layers, e.g. a silicon dioxide layer on the surface of SAW transducers (SAW=Surface Acoustic Wave).
Such temperature compensation layers modify however usually the acoustics of the transducer, so that additional and undesired electro-acoustic perturbations, e.g. plate modes can be propagated in the transducer system and compliance with the specifications becomes also difficult. Plate modes are acoustic modes that propagate along the propagation direction of the desired acoustic modes and usually have frequencies above the frequencies of the desired modes.
By variations of the thickness of the temperature compensation layer, the frequencies of the plate modes can be shifted to less critical ranges. However, the range of the less critical frequencies is reduced with the increase of additional functions of a piece of communication equipment and a deviation from the optimal layer thickness means a decrease of temperature compensation.
The frequencies of the plate modes can also be shifted by local variations, e.g. of the distance between the fingers of the electrode fingers on SAW resonators.
A degradation of the bandwidth accompanying such pitch scaling is also problematic.
The use of band suppressors for inhibiting unwanted frequency components is known e.g. from the U.S. Pat. No. 8,125,300.
Therefore, there is a requirement for temperature-compensated electroacoustic HF filters in which the perturbations by plate modes are reduced by maintaining the remaining electroacoustic chat of the filters. Such filters are necessary when a piece of communication equipment has to operate several bands, e.g. for LTE Carrier Aggregation, or the band gap between the sending and receiving bands is relatively large.
For corresponding electroacoustic filters and procedures for manufacturing of electroacoustic filters have to be taken from independent claims. Dependent claims specify advantageous embodiments.
An electroacoustic filter with reduced stimulation strength of plate modes includes a transducer system with two or more partial electroacoustic transducers. In the transducer system a desired mode is able to propagate. The static capacities of the transducer system correspond to the sum of the static capacities of the split transducers. Each split transducer has a lower electroacoustic coupling of the desired mode than the transducer system. The transducer system has an electroacoustic coupling of a plate mode corresponding to the electroacoustic coupling of the plate mode of a split transducer. The electroacoustic split transducers are connected preferably in parallel.
The following surprising relation was identified: Upon separating an electroacoustic transducer in split transducers, so that the number of stimulating electrode structures (e.g. electrode fingers in case of SAW transducers) of a split transducer is reduced compared to the original transducer, the stimulation strength and the strength of the electroacoustic coupling are also reduced. Concomitantly, the stimulation maximum is propagated in proportion to the degree of separation. Thus, the coupling of the desired modes and the coupling of the plate modes are reduced accordingly. In case the electroacoustic split transducers are connected electrically just so as the sum of the stimulating electrode structures corresponds to the number of the electrode structures of the original converter, and the transducer system has the same static capacity as the original transducer, the electroacoustic coupling of the desired modes is restored. Concomitantly, the electroacoustic coupling of the undesired plate modes remains at a significantly lower level of a single split transducer. At virtually unchanged desired electroacoustic chat, the stimulation strength of the undesired plate modes is thus significantly reduced.
It is possible to have a number of electroacoustic split transducers of n=2, 3, 4, 5, 6, 7, 8 or more. Here, n gives the degree of separation of the transducer system and determines the factor by which the coupling of undesired plate modes is reduced at unchanged desired acoustic chat.
It is possible and preferred to do the separation of the transducers into split transducers in a uniform mode, i.e. the split transducers have the same area, the same aperture and the same finger length.
It is possible to have SAW transducers or GBAW transducers (GBAW=Guided Bulk Acoustic Wave) as split transducers.
It is possible for the transducer system to replace a single transducer with the transducer system having the same static capacity as the replaced transducer. In addition, the transducer system replacing the single transducer has the same electroacoustic coupling of the desired mode as the single transducer. The transducer system replaces however a single transducer with the n-times the electroacoustic coupling of the plate mode.
It is possible that each of the electroacoustic split transducers has an equal number of electrode fingers.
It is possible that each of the electroacoustic split transducers has the same static capacity.
Besides, it is possible for the electroacoustic filter to feature an inductive element. The inductive element is connected in series or in parallel to the transducer system or to one of the transducer systems in case there are several transducer systems.
The interconnection of an inductive element with a transducer or a transducer system replacing the transducer can lead to an additional null point or to an additional pole point in the admittance (conductivity) of a transducer. Especially, the series connection can present on the transducer or transducer system an additional pole point. A parallel connection to the transducer or the transducer sys, respectively and inductance can present an additional null point of admittance.
An additional null point or an additional pole point provides the developer of an HF filter the possibility to specifically suppress frequencies in the range of the null point or the pole position. Therefore, even with the transducers which are replaced by the above-mentioned transducer systems, it is possible to render harmless residual disturbances caused by plate modes by the targeted placement of a null point or of a pole position.
The electroacoustic filter may contain basic elements of a so-called ladder-type filter circuit. A ladder-type filter circuit has serial resonators, interconnected in a signal path, and parallel resonators interconnecting to the ground various circuit nodes in the signal path to the ground. An element of a ladder-type circuit has a series resonator and a parallel resonator. By series circuits of such elements, one can achieve easily band-pass filters or notch filters. Electroacoustic resonators have a resonance frequency at which the admittance is very high, and an anti-resonance frequency with lower admittance. At band-pass filters, the serial and parallel resonators are matched so that basically, the anti-resonance frequency of the parallel resonator matches the resonance frequency of the serial resonator. In case of notch filters, basically, the anti-resonance frequency of the serial filter corresponds to the resonance frequency of the parallel resonator.
Thus, the resonance frequencies generate basically pole points of admittance, while anti-resonance frequencies generate null points of admittance.
Due to the possibility of adding extra pole points—especially to parallel resonators—and null points, respectively, especially to the transmission behavior of serial resonators, one can suppress undesired modes, significantly over or under the pass-band frequencies or the stopband.
It is thus possible to interconnect the transducer system or a remaining transducer to a signal path and to connect the inductive element in parallel to such transducer system or transducer. Alternatively or additionally, it is possible to interconnect the transducer system or a transducer to a parallel path and to connect the inductive element in series to the transducer system or transducer, respectively.
In the first case, one achieves an additional pole point with the inductive element. In the second case, one achieves with the inductive element an additional null point in the admittance.
It is also possible to connect the inductive element in parallel and create an additional null point of the admittance and/or to connect the inductive element in series to the transducer system and to obtain an additional admittance pole point.
It is possible that the transducer system be a multigate-resonator with at least one acoustic reflector between the split transducers. A multigate-resonator is thus a multiport resonator with interconnected reflectors, in order to compensate the higher need for surface in transducer systems split into split transducers.
The reflectors reduce the possible distances of the split transducer, which could cause acoustic perturbations without a reflector.
It is possible that the above electroacoustic filter be part of a duplexer, e.g. part of a sending filter or part of a receiving filter.
Besides, it was seen that not any electroacoustic transducer of a HF filter is equally detrimental regarding the stimulation of plate modes. The main disadvantage of increased surface requirement can thus be limited at electroacoustic transducers with especially high perturbation inputs. Accordingly, a procedure for obtaining an electroacoustic filter includes the following steps:
An alternative procedure includes the following steps:
Usually, one does not attempt to suppress with an inductance the plate mode of the transducer. The reason is that the admittance of a transducer is subjected to a strong change in the range of the plate mode and the desired behavior (pole/null point) is not stable. Therefore, one prefers to fit another transducer with an inductance connected in parallel or in series and to select the inductance so that the pole point (in case of a transducer with parallel circuit) or the null point (in case of a transducer with serial circuit) is placed so that the plate mode of the critical transducer in the filter transferor function appears to be significantly reduced.
The possibilities of reducing perturbations caused by plate modes can be used in combination.
The above filters can have transducers with temperature compensation layers, e.g. of silicone dioxide. The perturbations caused by the additional temperature compensation layer are avoided or, at least sufficiently diminished. Typical application fields of such filters are mobile radio filters, Wireless-LAN applications and GPS receivers.
In SAW transducers or in GBAW transducers, there are periodically connected, intertwined electrode fingers applied to a piezoelectric material and alternatingly interconnected to two bus bars.
The undesired plate modes have frequency components significantly distinguished from the center frequency of the pass-bands or stop-bands of the filters. At frequencies significantly outside this frequency range of the filters, an electroacoustic transducer acts as a capacitive element. The transducer, as capacitive element, interconnected to an inductance, forms a serial oscillating circuit or a parallel oscillating circuit, respectively. Its resonance frequency or anti-resonance frequency, respectively, results in the creation of additional mull points or pole pointe, respectively.
The surprising improvements of the admittance of HF filters were confirmed in good concordance across simulations and physically performed tests setups.
The electroacoustic filter and its operation mode and electrical chat as well as execution examples of the invention are further explained based on the schematic figures.
Shown are:
A conventional acoustic element consists of two reflectors and the transducer itself. The reflectors are limiting the acoustic wave to the transducer area. Two conventional transducers have thus four transducers in total.
In case a transducer is split as per above, n split transducers result and 2n reflectors. Thus, one can replace two adjoining reflectors by a single one, saving thus place on the expensive piezoelectric monocrystal. Between two split transducers only one reflector is thus placed. In order to reduce the number of reflectors and thus save place, all split transducers can be placed in a row. In case of split transducers, there are thus necessary only n+1 reflectors, instead of 2n.
The admittance course in the pass-band range is virtually not changed when replacing a transducer by the transducer system.
The electroacoustic filter, the duplexer and the procedure for creating of filters are not limited by the described chat and execution examples. Filters with additional circuit elements such as additional resonators or additional inductive and capacitive elements are also included.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 114 071.6 | Jul 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/044417 | 7/28/2017 | WO | 00 |
