Electronic RF filters

Abstract

An electronic filter includes a high-pass section (110) and a low-pass section (120). The high-pass section includes at least one filter stage comprising an acoustic resonator (111) connected in series and an inductor (114) connected in parallel. The low-pass section includes at least one filter stage comprising an inductor (121) connected in series and an acoustic resonator (123) connected in parallel. The filter is useful for communication equipment covering the n79 5G frequency band.

Description

Technical Field

[0001] The present disclosure relates to an electronic radio frequency (RF) filter comprising an acoustic resonator. The present disclosure also relates to the use of the electronic RF filter in an electronic device. Background Art

[0002] Electronic filters are used in the RF front-end of mobile devices to select electronic signals in a desired frequency band. As more and more communication services are established in the spectrum available for mobile communications, the bandwidth and selectivity requirements of RF filters are becoming increasingly challenging. For example, the spectrum of fifth-generation (5G) mobile communication services is located in the sub-6 GHz range, which is just below the 5 GHz WiFi band and above the 4G LTE band. RF filters for 5G services should cover a fairly wide 5G bandwidth with relatively steep skirts to reject the LTE band and WiFi band, especially at the upper end of the 5G frequency bandwidth.

[0003] Conventional LC filters require many LC stages to achieve steep filter skirts, so the physical size and insertion loss of the filter become issues. Conventional filters based on wave mechanisms such as surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators are widely used in filters to achieve high skirt steepness. Conventional filters with acoustic resonators typically use a ladder topology, as discussed in Andreas Link and Phil Warder: "The Golden Age for Filter Design," IEEE Microwave Magazine, August 2015, pp. 60-72, Figure 3.

[0004] The bandwidth achieved using a ladder-type approach is relatively narrow. Additional inductors can be used to extend this, but this reduces the rejection level and may introduce additional repelling poles. The filter tends to become complex and sensitive. Furthermore, because the filter passband is implemented via acoustic resonators, power handling capabilities are limited. Therefore, conventional approaches using pure LC filters or ladder-type acoustic filters may be limited for mobile communication services below 6 GHz.

[0005] There is a need for an improved RF filter topology that can handle the high frequencies of currently established communication services such as 5G, has high selectivity relative to other communication services, and can handle high transmission powers.

[0006] An object of the present disclosure is to provide an electronic filter which covers a wide bandwidth, has a steep filter skirt and is capable of handling considerable power. Summary of the Invention

[0007] A filter for solving the above-mentioned problem includes a first terminal and a second terminal; a high-pass section, which is coupled to one of the first terminal and the second terminal, and the high-pass section includes at least one filter stage, and the at least one filter stage includes an acoustic resonator connected in series and an inductor connected in parallel; a low-pass section, which is coupled in series with the high-pass section and coupled to the other terminal of the first terminal and the second terminal, and the low-pass section includes at least one filter stage, and the at least one filter stage includes an inductor connected in series and an acoustic resonator connected in parallel.

[0008] According to one embodiment, an electronic RF filter includes a high-pass section and a low-pass section, each comprising one or more stages having an LC topology. Instead of conventional capacitors, acoustic resonators are used in the series path of the high-pass section and the parallel path of the low-pass section. This concept combines the advantages of conventional LC filter topologies, such as high power durability and wide bandwidth, with the advantages of acoustic resonators, such as highly selective acoustic resonance, thereby achieving a steep filter skirt and a desired rejection level. Furthermore, because the acoustic resonators act as both capacitors and acoustic notches in the bandpass LC filter, the filter topology becomes very compact.

[0009] The topology of the electronic RF filter according to the present disclosure can be considered a bandpass filter with LC characteristics combined with an acoustic bandstop filter utilizing the characteristics of an acoustic resonator. The acoustic resonator provides two distinct functions: that of a capacitor and that of a repelling element. The filter is capable of handling a wide bandwidth, has a steep skirt, and exhibits high power durability.

[0010] For the current 5G communication standard, the electronic filter according to the present disclosure achieves high selectivity just above the passband to suppress the 5 GHz WiFi band. Variations of the design for other mobile communication fields can also achieve high selectivity below the passband or even above and below the passband.

[0011] Each of the high-pass section and the low-pass section includes at least one or more stages, which include a series path and a parallel path. Two or more stages can be cascaded in each of the high-pass section and the low-pass section. In this regard, the high-pass section can include another second filter stage having an acoustic resonator connected in series and an inductor connected in parallel, wherein the second filter stage is connected in series with the first filter stage. The low-pass section can include another second filter stage, which includes an inductor connected in series and an acoustic resonator connected in parallel, wherein the first filter stage and the second filter stage are connected in series. The high-pass filter section can include a third acoustic resonator connected in series, which is coupled to the input of the low-pass filter section. The acoustic resonator in the first stage of the low-pass filter section can be connected to an inductor or to a coil coupled to a ground potential.

[0012] The acoustic resonator may be of any known type, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators. Depending on the application area and target frequency, the skilled person may select a suitable type of SAW resonator or BAW resonator useful for the intended design.

[0013] The electronic filter can be sized to provide a passband between 4.4 GHz and 5.0 GHz, which is dedicated to 5G mobile communication services, and to reject frequencies between 5.15 GHz and 5.925 GHz, which is the 5 GHz WiFi band. Because the two bands are so close, the right skirt of the 5G filter's passband must be very steep, approximately 3%, to provide a sufficiently low rejection level to reject crosstalk from the WiFi band into the desired 5G band.

[0014] The rejection level can be increased by adding additional stages in the high-pass section and / or the low-pass section, however, at the expense of greater complexity, larger size, and increased insertion loss. Technicians can determine the appropriate solution in terms of passband, rejection level, and power endurance to achieve a suitable solution for 5G communication services and even other services in other RF bands.

[0015] The electronic RF filter mentioned above is targeted at mobile communication devices such as cellular phones or smart phones or any other electronic devices including mobile communication capabilities. Specifically, for RF signals in the 4.4 GHz to 5.0 GHz frequency range, the filter implements the RF front end of the transmit path and the receive path.

[0016] It should be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide further understanding and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate one or more embodiments and, together with the description, serve to explain the principles and operation of the various embodiments. In the drawings, identical elements in different figures are represented by the same reference numerals. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the attached figure:

[0018] Figure 1 A diagram showing an electronic RF filter according to the principles of the present disclosure;

[0019] Figure 2 Shown Figure 1 The admittance curve of the filter; and

[0020] Figure 3A and Figure 3B Shown are the admittance curves associated with the acoustic resonators used in the illustrated filters. DETAILED DESCRIPTION

[0021] The present disclosure will now be described more fully below with reference to the accompanying drawings that illustrate embodiments of the present disclosure. However, the present disclosure may be implemented in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure fully conveys the scope of the present disclosure to those skilled in the art. The accompanying drawings are not necessarily drawn to scale, but are configured to clearly illustrate the present disclosure.

[0022] Figure 1 A schematic diagram of a filter according to the principles of the present disclosure is depicted. The filter can be an RF front-end filter in a mobile communication device such as a smartphone. The filter includes a first external terminal or port 131 and a second terminal or port 132 to connect the filter to an antenna and a receiving (Rx) circuit and a transmitting (Tx) circuit of a communication device such as a smartphone. One of the ports (such as port 131) can be connected to an antenna. Another of the ports (such as port 132) can be connected to a signal processing circuit of the communication device, such as a receiving circuit and a transmitting circuit. The filter consists of a high-pass section 110 and a low-pass section 120, which are connected in series with each other, so that the signal received at the antenna terminal 131 is forwarded to the terminal 132 after being filtered by the high-pass section, and the signal received at the terminal 132 is forwarded to the antenna terminal 131 after being correspondingly filtered in the low-pass section 120 and the high-pass section 110.

[0023] High-pass section 110 includes several stages, each comprising a series-connected acoustic resonator 111 and a parallel-connected inductor or coil 114. Specifically, series-connected resonator 111 is connected to external terminal 131 and to further stages 112 and 115. Inductor 114 is connected between resonator 111 and a ground terminal 116. A second stage, comprising series-connected resonators 112 and parallel-connected inductors 115, is connected to the first stage. A third stage, comprising series-connected acoustic resonators 113, is connected to the second stage.

[0024] The low-pass section 120 includes a first stage including an inductor 121 connected in series and an acoustic resonator 123 connected in parallel. Resonator 123 is also connected in series with an inductor 125, which is connected to ground terminal 116. The high-pass section 120 includes a second stage including an inductor 122 connected in series and an acoustic resonator 124 connected in parallel. Inductor 121 is connected to resonator 113 of the high-pass section. Acoustic resonators 111, 112, 113 and 123, 124 can be surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators. The filter can include one type of resonator (only SAW or BAW) or a mixture of both types of resonators.

[0025] In other words, the high-pass section 110 includes a series connection of resonators 111, 112, and 113, wherein the nodes between the resonators 111, 112 and 112, 113 are connected to the ground terminal 116 through respective inductors 114, 115. The low-pass section 120 includes a series connection of inductors 121, 122, wherein the node between the inductors is connected to the ground terminal 116 through the series connection of resonator 123 and inductor 125. The node between inductor 122 and the external terminal 132 is also coupled to the ground terminal 116 through resonator 124.

[0026] Figure 2 Shown Figure 1 Figure 2 shows the admittance curve or transfer curve of the attenuation (dB) achieved by the filter as a function of frequency (GHz). The filter is specifically designed for the 5G communication standard to handle the n79 band 211 and to reject other frequency bands for other communication services, such as several LTE bands 210 and the 5 GHz Wi-Fi band 212 as well as Tx harmonics 213.

[0027] The n79 band consists of a frequency band between 4.4 GHz and 5.0 GHz, which is about 13% relative bandwidth, as Figure 2 The filter is shown at 211. The filter implements a passband 221 to cover the n79 band. Near the right of the n79 band 211 is the 5 GHz WiFi band 212 ranging from 5.15 GHz to 5.925 GHz. The n79 filter must suppress the 5 GHz WiFi band to avoid unwanted crosstalk, so that Figure 1 The filter requires sufficient rejection in a stopband 222 immediately to the right of the passband 221. Therefore, the filter must have a steep skirt 225 of approximately 3% relative to the filter center frequency to the right of the passband 221. Furthermore, the rejection level on the left skirt 220 to the left of the passband 221 must be sufficient to suppress the LTE band 210 up to 2.69 GHz. Furthermore, the stopband 224 suppresses harmonics 213 of the Tx modulation. Figure 1 The filter must satisfy the following opposing characteristics: wide relative passband 221 to target band 211, steep filter skirt 225 to the right of the passband to suppress band 212, and high power handling capability for band 221 Tx operation.

[0028] To achieve a rejection level 222 for the 5 GHz WiFi band and a steep skirt 225 of approximately 3% between the passband 221 and the rejection level 222, the resonant frequencies of the series resonators 111, 112, and 113 of the highpass stage 110 are positioned at the lower portion of the WiFi rejection. Peaks 222a, 222b, and 222c are generated in the admittance curve at these resonant frequencies. The parallel resonators of the lowpass stages 121 and 122 are used to cover the upper portion of the 5 GHz WiFi rejection, as indicated by peaks 222d and 222e.

[0029] The use of acoustic resonators in the disclosed topology achieves the following benefits. The entire filter passband 221 is generated by the LC characteristics of the acoustic resonator. The LC characteristics of the acoustic resonator have high power durability, allowing them to handle high powers during Tx operation, especially high transmit powers. The acoustic characteristics of the acoustic resonator are used to achieve the desired rejection level, without the filter having to sustain high power. The filter topology is relatively compact because only a few LC and acoustic elements are used. The acoustic resonator acts as a capacitor for the passband portion with the LC characteristics 221, and as an acoustic notch 222 for the rejection level. Figure 1 The filter combines the filter functions of a bandpass LC filter and an acoustic band-reject filter. The acoustic resonator provides two different functions: LC filter characteristics and acoustic band-reject filter characteristics. This allows the design of a filter suitable for the n79 5G band, which includes a passband 221 and has high selectivity 222 just above the passband.

[0030] Further modifications of the filter concept according to the present disclosure for more specific or additional application areas may allow the filter to be modified to achieve high selectivity below the passband or even above and below the passband.

[0031] Figure 1 The filter combines a wide relative bandwidth of approximately 13% with a high skirt steepness of approximately 3%. Conventional filter designs using LC components can only achieve this steepness through a large number of stages, resulting in very high-order filters. This conventional approach can be large in size and generate high insertion loss. Conventional acoustic filters in conventional ladder topologies can achieve very high steepness but only achieve relatively narrow bandwidths, for example, less than 6%, which is insufficient for the n79 band. Although the bandwidth of the ladder topology can be extended using several additional inductors, this reduces the rejection level and introduces a repelling pole at certain frequencies, making this conventional approach complex and sensitive.

[0032] To optimize the currently available Figure 1To develop a filter topology, the filter designer can use a starting point that is an LC filter topology rather than an acoustic ladder topology. First, a conventional bandpass LC filter with a small number of stages is considered to meet as many specifications as possible, including rejection far from the passband. Then, starting from the starting point of the method, the capacitors of the different low-pass and high-pass stages of the LC topology are replaced with acoustic resonators with the same static capacitance as the original C elements, which results in Figure 1 This maintains the same out-of-band filter response as before and introduces a highly selective acoustic resonance for 5GHz-WiFi rejection. Figure 1 The filter is particularly suitable for mobile communication devices such as smartphones to transmit and / or receive RF signals in the 4.4GHz to 5.0GHz frequency range because it combines high selectivity with high power handling capability.

[0033] Figure 3B Depicts a synthetic admittance diagram showing the admittance curve / transmission curve of the filter and the Figure 2 The passband and rejection level of the acoustic resonators 111, 112, 113, 123, 124 are described as individual admittance curves. Figure 3A The ones marked in Figure 3B Components and component combinations whose RF frequency characteristics are described. Specifically, Figure 3A The series connection of the acoustic resonator 113 and the inductor 121, which is marked as 313 in FIG. Figure 3B Generate the admittance curve 313 in Figure 3A The series connection of the acoustic filter 123 and the inductor 125, which are marked as 323 in FIG. Figure 3B An admittance curve 323 is generated. Figure 3A The acoustic resonators 111, 112 and 124 are Figure 3B The combination of these admittance curves achieves the following Figure 3B The overall transfer characteristics of the filter are shown in Figure 2. Figure 3B It can be seen that elements 111, 112, and 313 result in the lowest suppression peak of the repelling strip 222. Elements 323 and 124 result in the highest suppression peak. Figure 3B It can also be seen in FIG that the C characteristic of the acoustic filter results in a passband 221 , which is a portion of each admittance curve corresponding to the left side of the resonant frequency, which are those portions with frequencies lower than the resonant frequency.

[0034] It will be apparent to those skilled in the art that various modifications and variations may be made without departing from the spirit or scope of the present disclosure as defined in the appended claims. Since modifications, combinations, sub-combinations, and variations of the disclosed embodiments that incorporate the spirit and substance of the present invention may be conceived by those skilled in the art, the present disclosure should be construed to include all within the scope of the appended claims.

Claims

1. An electronic RF filter comprising: a first terminal (131) and a second terminal (132); A high-pass section (110) coupled to the first terminal, the high-pass section comprising: a first acoustic resonator, a second acoustic resonator, and a third acoustic resonator connected in series; a first inductor coupled to a ground terminal and to a first node between the first acoustic resonator and the second acoustic resonator; and a second inductor coupled to the ground terminal and to a second node between the second acoustic resonator and the third acoustic resonator; and a low-pass section (120) coupled in series with the high-pass section and coupled to the second terminal (132), the low-pass section comprising: a third inductor and a fourth inductor connected in series; a fourth acoustic resonator coupled to a third node, the third node being located between the third inductor and the fourth inductor; a fifth inductor coupled to the fourth acoustic resonator and to the ground terminal; and a fifth acoustic resonator directly coupled to the ground terminal and to a fourth node between the fourth inductor and the second terminal, The resonant frequencies of the first acoustic resonator, the second acoustic resonator, and the third acoustic resonator are placed in a lower part of a rejection band, and the fourth acoustic resonator and the fifth acoustic resonator are used to cover an upper part of the rejection band, and wherein the frequency of the rejection band is higher than the frequency of a passband provided by the electronic RF filter.

2. The electronic RF filter of claim 1, wherein at least one of the acoustic resonators (111, 112, 113, 123, 124) comprises a surface acoustic wave resonator.

3. The electronic RF filter of claim 1, wherein at least one of the acoustic resonators (111, 112, 113, 123, 124) comprises a bulk acoustic wave resonator. 4 . The electronic RF filter of claim 1 , wherein the electronic RF filter is configured to provide the passband between 4.4 GHz and 5.0 GHz, and is configured to provide the rejection band between 5.15 GHz and 5.925 GHz.

5. The electronic RF filter of claim 1 , wherein: The electronic RF filter is implemented as a component in a mobile communication device; and The mobile communication device is configured to transmit or receive at least one of RF signals in a frequency range of 4.4 GHz to 5.0 GHz.

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