Filter and Method of Designing an RF Filter

Abstract

A filter and a method for forming a filter are disclosed. In an embodiment a filter includes a first port, a second port and a signal path between the first port and the second port. The filter further includes a plurality of series resonators electrically connected in series in the signal path, a plurality of shunt paths, each electrically connecting the signal path to ground and one parallel resonator electrically connected in each shunt path, wherein at least one series resonator is an electroacoustic resonator, and wherein at least one parallel resonator comprises one acoustically inactive capacitor or an electrical connection of an acoustically active resonator and a de-tuning coil.

Claims

1. An RF filter comprising: a first port; a second port; a signal path between the first port and the second port; a plurality of series resonators electrically connected in series in the signal path; a plurality of shunt paths, each electrically connecting the signal path to ground; and one parallel resonator electrically connected in each shunt path, wherein at least one series resonator is an electroacoustic resonator, and wherein at least one parallel resonator comprises one acoustically inactive capacitor or an electrical connection of an acoustically active resonator and a de-tuning coil.

2. The RF filter of claim 1, wherein the RF filter comprises the acoustically active resonator and the de-tuning coil electrically connected in series.

3. The RF filter of claim 1, wherein all series resonators are electroacoustic resonators.

4. The RF filter of claim 1, wherein an acoustically active parallel resonator has a capacitance C.sub.active in a shunt path or in the signal path instead of an acoustically inactive capacitor of the capacitance C.sub.inactive in the same shunt path or in the signal path, and wherein the capacitance C.sub.active of the acoustically active resonator is between 0.5 C.sub.inactive and 2.0 C.sub.inactive.

5. The RF filter of claim 1, wherein each electroacoustic resonator is a BAW resonator or a SAW resonator.

6. The RF filter of claim 1, wherein all acoustically inactive resonators comprise a LC resonance circuit.

7. The RF filter of claim 1, wherein the RF filter provides a first pass band.

8. The RF filter of claim 7, wherein the pass band has a center frequency 3 GHz.

9. The RF filter of claim 7, wherein the RF filter provides a second pass band with a center frequency 3 GHz.

10. The RF filter of claim 1, wherein each parallel resonator comprises the electrical connection of the acoustically active resonator and the de-tuning coil, wherein a resonance frequency of the acoustically active resonator is tuned to a frequency higher than a resonance frequency of a resonator of another shunt path, and wherein the de-tuning coil tunes the resonance frequency of the electrical connection of the acoustically active resonator and the de-tuning coil to a frequency lower than the resonance frequency of the acoustically active resonator.

11. The RF filter claim 1, wherein bulk waves increase an insertion loss in a frequency range outside a pass band.

12. A method for forming an RF filter, the method comprising: providing a first port, a second and a signal path between the first port and the second port; providing a shunt path electrically connecting the signal path to ground; electrically connecting a parallel electroacoustic resonator and a de-tuning coil in the shunt path; tuning the electroacoustic resonator to a higher resonance frequency; and tuning a resonance frequency of the electrical connection of the parallel electroacoustic resonator and the de-tuning coil to a frequency lower than the resonance frequency of the electroacoustic resonator.

13. The method of claim 12, where bulk waves of the electroacoustic resonator are used to increase out-of-band suppression of the RF filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Central aspects of the present RF filter and details of preferred embodiments are shown in the schematic accompanying figures.

[0045] FIG. 1 shows a possible equivalent circuit diagram of an RF filter;

[0046] FIG. 2 shows an equivalent circuit diagram including further filter elements;

[0047] FIG. 3 shows a filter topology with more than one series electroacoustic resonator;

[0048] FIG. 4 shows an equivalent circuit diagram of an electroacoustic resonator;

[0049] FIG. 5 illustrates the working principle of tuning the electroacoustic resonator and the tuning back via the de-tuning coil;

[0050] FIG. 6 illustrates the possibility of further electrically connecting a low pass filter;

[0051] FIG. 7 shows a possible implementation of a low pass filter;

[0052] FIG. 8 illustrates frequency-dependent characteristics of an RF filter including a low pass filter;

[0053] FIG. 9 illustrates the performance of an RF filter without a low pass filter; and

[0054] FIG. 10 illustrates the performance of an RF filter including the performance of a low pass filter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0055] FIG. 1 shows a possible implementation of the RF filter F. The filter F has a first port P1 and a second port P2. A signal path SP is arranged between the first port P1 and the second port P2 and electrically connects the first port P1 to the second port P2. In the signal path series resonators RS are electrically connected in series. One series resonator RS is an electroacoustically active resonator EAR. The other series resonator RS is an LC resonator LCR comprising a capacitance element and an inductance element connected in series.

[0056] The filter topology of the filter F shown in FIG. 1 has three shunt paths electrically connecting the signal path SP to ground. In the first shunt path as seen from the point of view of the first port P1, a parallel resonator RP and a de-tuning coil DTC are electrically connected in series. Thus, the shunt path PS electrically connects the first port P1 to ground.

[0057] It is to be noted that each of the first port P1 and the second port P2 can be an input port provided to receive an RF signal form an external environment. The respective other port is then the output port provided to transmit the filtered RF signal to an external circuit environment.

[0058] In the two other shunt paths PS a respective further resonator R being realized as an LC resonator LCR is arranged.

[0059] It is to be noted that the de-tuning coil DTC connected to the electroacoustic parallel resonator RP differs from conventional inductive elements that may be present in shunt paths of ladder-type like structures. Conventional inductive elements can be realized as unavoidable external connections to ground, e.g., bump connections and their inductance value is chosen such that filter characteristics without consideration of spurious modes and parasitic effects are optimized. In contrast, the inductance value of the de-tuning coil DTC is chosen such that a frequency shift of the same absolute value but of the opposite direction compared to the frequency shift applied to the resonator considered alone, is obtained.

[0060] FIG. 2 illustrate the possibility of arranging further electrical components such as capacitance elements and inductance elements in the filter. In addition to basic elements of a ladder-type configuration comprising a series element and a shunt path, two further inductance elements in the signal path, a further inductance element in a further shunt path and two further capacitance elements parallel to the additional inductance elements are possible. The two inductance elements in the signal path SP are electrically connected in series. Each of the two capacitance elements is electrically connected in parallel to a respective inductance element in the signal path. The additional shunt path comprises a series connection of a capacitance element and an inductance element. One electrode of the capacitance element in the additional shunt path is electrically connected to one electrode of each inductance element in the signal path and to one electrode of each of the capacitance elements parallel to the inductance elements in the signal path.

[0061] FIG. 3 illustrates the possibility of having each resonator in the signal path SP being realized as an electroacoustic resonator.

[0062] At least one shunt path comprises an electroacoustic resonator in combination with a de-tuning coil. The remaining shunt paths, or some of the remaining shunt paths, can be endowed with electroacoustic resonators or with LC resonators.

[0063] FIG. 4 illustrates the equivalent circuit diagram of an electroacoustic resonator EAR. An electroacoustic resonator EAR can be regarded as a parallel connection of a capacitance element in parallel to a series connection including an inductance element and a capacitance element.

[0064] FIG. 5 illustrates a central aspect of the RF filter: curve 1 shows the frequency-dependent matrix element Y.sub.11 of a conventional electroacoustic resonator. The conventional resonator has a resonance frequency and an anti-resonance frequency at a frequency slightly higher than the resonance frequency. In the frequency range denoted as BW parasitic effects, e.g., caused by bulk waves, can take place.

[0065] In a first step the electroacoustic resonator is detuned by shifting characteristic frequencies to higher frequency positions. Correspondingly, curve 2 shows the shifted resonance frequency, anti-resonance frequency and parasitic frequencies (BW). The electroacoustic resonator is preferably detuned such that the frequencies of parasitic effects are shifted to a frequency position where the unwanted excitations do not harm a proper functioning of the filter, or preferably to a frequency position where the unwanted effects can contribute to enhance the filter characteristics.

[0066] Finally, in a second step, the characteristic resonance frequency is shifted back utilizing the de-tuning coil while mainly maintaining the frequency position of the parasitic effects at their preferred position.

[0067] As a consequence, parasitic effects do not further harm the frequency characteristics but contribute to enhance the frequency characteristics and by separating the resonance frequency and the resonance frequency a possibly wider bandwidth can be obtained.

[0068] FIG. 6 illustrates the possibility of arranging a low pass filter LPF between the first port P1 and the second port P2. The other circuit elements establish a band pass filter BPF with one or two pass bands.

[0069] FIG. 7 illustrates an equivalent circuit diagram of a possible low pass filter LPF. The low pass filter LPF can have two inductance elements electrically connected in series. For each inductance element of the inductance elements in the signal path one capacitance element is provided in parallel to the corresponding inductance element. Further, three shunt paths electrically connecting the signal path to ground are provided. In each shunt path one capacitance element is electrically connected. All three shunt paths are shunted to the ground connection utilizing a single further shunt inductance element.

[0070] FIG. 8 illustrates the return loss (RL) and the return loss RL of a filter having the topology as shown in FIG. 7.

[0071] FIG. 9 illustrates a frequency response of an RF filter corresponding to the topology of FIG. 6 without a consideration of the effects of the low pass filter LPF. The resonance frequencies of the combinations of the electroacoustic resonators and the corresponding de-tuning coils from the first shunt path and the third shunt path are chosen to coincide at the resonance 1. The resonance frequency of the combination of the electroacoustic resonator and the de-tuning coil of the centered shunt path is chosen to coincide with the resonance 3.

[0072] Resonances in shunt paths correspond to poles in the insertion loss of the corresponding filter. Thus, resonance 1 causes pole 2 and resonance 3 causes pole 4. At frequency ranges above 5 GHz the dashed ellipses indicate the position of formally unwanted and now preferred parasitic effects now helping to improve the filter characteristics.

[0073] Correspondingly, FIG. 10 illustrates the filter characteristics of the filter topology shown in FIG. 6 while the effect of the low pass filter is also considered. The necessary filter requirements concerning low insertion loss in the pass band and high attenuation outside the pass band are fulfilled.

[0074] Neither the RF filter nor the method for designing an RF filter is limited by the presented subject-matter and its technical features. RF filters comprising further filter elements and methods for designing an RF filter comprising further designing steps are also comprised.