Bulk acoustic wave filter circuit structure with high steep drop and high out-of-band rejection

Abstract

The invention discloses a high-abrupt-drop high-out-of-band rejection bulk acoustic wave filter circuit structure, which comprises a high-abrupt-drop circuit and a high-out-of-band rejection circuit, and is characterized in that the output end of the high-abrupt-drop circuit is connected with the high-out-of-band rejection circuit in series; the high-steep-drop circuit is a passive device externally expanded on a Ladder bulk acoustic wave filter circuit, and transmission zero point stepped distribution of a near-transmission band is realized; and the high out-of-band rejection circuit is used for externally expanding an inductor on the Ladder bulk acoustic wave filter circuit so as to construct a new transmission zero point of a specific frequency band at the far end of a transmission band to realize high out-of-band rejection. The circuit not only can realize high out-of-band rejection, but also can avoid the occurrence of a large peak at a near transmission band.

Description

Technical Field

[0001] The invention belongs to the technical field of filter circuits, and in particular relates to a high-steep-drop high-out-of-band suppression bulk acoustic wave filter circuit structure. Background Art

[0002] Bulk acoustic wave (BAW) filters are the main filter devices used in 5G and subsequent communication frequency bands. With the development of existing communication technologies, users of smart mobile terminals have increasingly higher requirements for communication quality under extreme conditions, and conventional communication methods based on communication base stations cannot meet this demand. Therefore, various difficulties in satellite communication functions of mobile smart terminals need to be solved urgently.

[0003] Traditional dedicated satellite phones do not pay too much attention to the size and portability of the equipment. The size of the RF filters they use is mostly 10mm*10mm or larger, and in most cases multi-stage filters are required to meet the filtering requirements. However, current smart mobile terminals are highly integrated and cannot reserve such a large space for the RF front-end of satellite communication functions. Therefore, smaller and higher-performance BAW filters are required.

[0004] The invention patent application with publication number CN113411069A discloses a bulk acoustic wave filter device and a method for improving out-of-band suppression, wherein the bulk acoustic wave filter device includes a substrate, a protective cap wafer and a filter wafer, a thin film bulk acoustic wave filter is arranged on the filter wafer, a metal sealing ring is formed on the periphery of the thin film bulk acoustic wave filter, and the metal sealing ring is connected to the ground plane of the substrate in a flip-chip manner through the metallized via of the protective cap wafer. Furthermore, the parallel resonant arm in the filter is connected to an inductor on the corresponding metal layer and then connected to the ground metal, and two adjacent inductors are separated by the ground metal. The bulk acoustic wave filter device disclosed in this patent application cannot effectively achieve the improvement of out-of-band suppression at the far end of the passband.

[0005] The invention patent application with publication number CN111200418A discloses a bulk acoustic wave filter and a signal processing device, which relates to the field of filters. The bulk acoustic wave filter includes a series branch and multiple parallel branches; the series branch is composed of a plurality of series bulk acoustic wave resonators connected in sequence; a parallel branch is connected to the connection node between two adjacent series bulk acoustic wave resonators; each parallel branch includes a first parallel bulk acoustic wave resonator, a second parallel bulk acoustic wave resonator and a first inductor, the first parallel bulk acoustic wave resonator, the second parallel bulk acoustic wave resonator and the first inductor are connected in series in sequence, and the second parallel bulk acoustic wave resonator and the first inductor are simultaneously connected in parallel with at least one of the second inductors; there is a mutual inductance between at least two adjacent second inductors connected to different parallel branches; the first inductor and the second inductor are both grounded. The circuit structure disclosed in this patent application is complex. The mutual inductance effect between inductors affects the calculation of the series inductance value, and the newly constructed series resonant frequency cannot be estimated more accurately. In addition, such a circuit structure will cause the parallel body acoustic wave resonator to construct a new parallel resonant frequency at the left end of the original series resonant frequency, resulting in a large peak in the transition band on the left side of the filter, affecting the filter performance. Summary of the invention

[0006] The present invention provides a high steep drop and high out-of-band suppression bulk acoustic wave filter circuit structure, which can achieve high out-of-band suppression and avoid the occurrence of large peaks at near passbands.

[0007] A specific embodiment of the present invention provides a high-steep-drop high out-of-band suppression bulk acoustic wave filter circuit structure, comprising a high-steep-drop circuit and a high out-of-band suppression circuit, wherein the output end of the high-steep-drop circuit is connected in series with the high out-of-band suppression circuit; The high steep drop circuit is a passive device that is externally expanded on the ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros near the passband; The high out-of-band suppression circuit is to expand the inductance on the ladder bulk acoustic wave filter circuit to construct a new transmission zero point in a specific frequency band at the far end of the passband to achieve high out-of-band suppression.

[0008] Further, the ladder bulk acoustic wave filter circuit includes a series arm and a plurality of parallel arms; Wherein, the series arm comprises a plurality of series bulk acoustic wave resonators connected in series in sequence; The parallel arm is connected between the series arm and the ground line, at least one series BAW resonator is spaced between two adjacent parallel arms, and the parallel arm includes a parallel BAW resonator.

[0009] Furthermore, the electrode film thickness of the series resonator on the series arm is adjusted so that the impedance curve of the series resonator on the series arm is placed at the low frequency end; By adjusting the electrode film thickness of the parallel bulk acoustic wave resonator on the parallel arm, the impedance curve of the parallel bulk acoustic wave resonator on the parallel arm is placed at the high frequency end.

[0010] Furthermore, the series bulk acoustic wave resonator and the parallel bulk acoustic wave resonator both use AlN as the piezoelectric layer.

[0011] The present invention places the impedance curve of the series resonator on the series arm at the low-frequency end and the impedance curve of the parallel BAW resonator at the high-frequency end, and combines the BAW resonator with a high scandium-doped AlN as a piezoelectric layer that can achieve a higher electromechanical coupling coefficient, thereby achieving a larger series-parallel resonant frequency difference fps of the resonators, which can provide a larger limiting bandwidth for the constructed prototype BAW filter.

[0012] Furthermore, the high steep drop circuit is an externally expanded passive device on the ladder bulk acoustic wave filter circuit, wherein the method of externally expanding the passive device includes connecting a passive device in series or in parallel to the series bulk acoustic wave resonator on the series arm, and / or connecting a passive device in series or in parallel to the parallel bulk acoustic wave resonator on the parallel arm, and the passive device is a capacitor or an inductor.

[0013] Furthermore, the high steep drop circuit includes connecting a capacitor in parallel to at least one series BAW resonator, and the transmission zero point of the right band of the Ladder BAW filter circuit is shifted to the right in a step-by-step manner by setting the thickness of the series BAW resonator and the capacitance of the parallel capacitor, and connecting a capacitor in series to at least one parallel BAW resonator, and the transmission zero point of the left band of the Ladder BAW filter circuit is shifted to the left in a step-by-step manner by setting the thickness of the parallel BAW resonator and the capacitance of the series capacitor.

[0014] Compared with the prior art, the present invention realizes a step-by-step rightward shift of the transmission zero point of the right band by connecting a capacitor in parallel to the series BAW resonator, and a step-by-step leftward shift of the transmission zero point of the left band by connecting a capacitor in series to the parallel BAW resonator, thereby realizing an extremely narrow transition band.

[0015] Furthermore, the high steep drop circuit also includes a capacitor and / or an inductor connected in series with the series BAW resonator, and a capacitor and / or an inductor connected in parallel with the parallel BAW resonator, so that the series resonant frequency of the series BAW resonator and the parallel resonant frequency of the parallel BAW resonator at the same frequency point in the passband are realigned.

[0016] When the film thickness is modified, it is easy to cause the series resonant frequency fs of the series-connected series BAW resonator and the parallel resonant frequency fp of the parallel BAW resonator, which were originally at basically the same frequency point, to be misaligned, causing the center of the filter passband to collapse. In order to solve the above problem, the present invention connects capacitors and / or inductors in series on the series BAW resonator, and connects capacitors and / or inductors in parallel on the parallel BAW resonator to realign the series-parallel resonant frequencies of the series-parallel BAW resonators, thereby ensuring a smooth passband of the filter and reducing ripple.

[0017] Furthermore, the high out-of-band suppression circuit is to connect an inductor in parallel to at least one series BAW resonator in the series arm of the ladder BAW filter circuit, thereby constructing a new transmission zero point, and / or to connect an inductor in series to at least one parallel BAW resonator in the parallel arm, thereby constructing a new transmission zero point, thereby achieving high out-of-band suppression.

[0018] Compared with the prior art which mainly constructs zero points in the transition band, the present invention constructs a new filter transmission zero point in order to achieve ultra-high out-of-band suppression at the far end. The present invention can construct a new parallel resonant frequency while increasing the parallel resonant frequency by connecting a parallel inductor to the series body acoustic wave resonator, and can edit the transmission zero points on the left and right sides of the filter at the same time, thereby improving the far-end out-of-band suppression of the left and right bands of the prototype filter. The present invention can also construct a new series resonant frequency while reducing the series resonant frequency by connecting a series inductor to the parallel body acoustic wave resonator, and can also edit the transmission zero points on the left and right sides of the filter at the same time. The far-end out-of-band suppression of the left and right bands of the prototype filter is improved through the improvements in the above two aspects. It should be noted that out-of-band suppression at the far end is to generate a new resonant frequency point in the required frequency band by adjusting the inductance value to generate a new zero point, which is different from the control method of adjusting the inductance value to move the resonant frequency point in the required frequency band when performing out-of-band suppression at the near end.

[0019] Furthermore, based on the high out-of-band suppression circuit architecture and taking the far-end suppression frequency as the target, the areas of the series BAW resonator and the parallel BAW resonator, and the corresponding inductance values ​​of the parallel inductor and the series inductor are calculated by EDA software.

[0020] Compared with the prior art, the present invention has the following beneficial effects: In the prior art, a new transmission zero is generated by controlling a BAW resonator, a capacitor or an inductor, and the generated new transmission zero is in most cases in the adjacent band of the passband. The present invention changes the electromechanical coupling coefficient of the BAW resonator by expanding a capacitor or an inductor outside the BAW resonator, thereby realizing the editability of the electromechanical coupling coefficient, so that the electromechanical coupling coefficient of the BAW resonator matches the bandwidth and roll-off required by the filter, thereby achieving the improvement of the far-end out-of-band suppression and the steep drop in the adjacent band.

[0021] The invention is based on a large-bandwidth prototype ladder bulk acoustic wave filter, and the series-parallel body acoustic wave resonator on the series-parallel arm is expanded with a passive device capacitor and inductor of a specific capacitance and inductance value, so as to realize controllable editing of the transmission zero point of the near passband frequency band, and replace the large-bandwidth part of the prototype ladder bulk acoustic wave filter with a high steep drop; at the same time, based on the large-bandwidth prototype ladder bulk acoustic wave filter, the series-parallel body acoustic wave resonator on the series-parallel arm is expanded with a passive device inductor of a specific inductance value, and a new transmission zero point at the far end outside the passband of the filter is constructed, so as to realize high out-of-band suppression of a specific frequency band at the far end of the passband. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1a A schematic diagram of a ladder bulk acoustic wave filter circuit provided by a specific embodiment of the present invention; Figure 1b The impedance characteristic curve diagram of the ladder bulk acoustic wave filter circuit provided by the specific embodiment of the present invention; Figure 1c The S-parameter simulation diagram of the ladder bulk acoustic wave filter circuit provided by the specific embodiment of the present invention; Figure 2a A schematic diagram of a high steep drop circuit provided by a specific embodiment of the present invention; Figure 2b The impedance characteristic curve diagram of the high steep drop circuit provided by the specific embodiment of the present invention; Figure 2c The S parameter simulation diagram of the high steep drop circuit provided by the specific embodiment of the present invention; Figure 3 An equivalent BVD model diagram provided for a specific embodiment of the present invention; Figure 4a A schematic diagram of a high out-of-band suppression circuit provided by a specific embodiment of the present invention; Figure 4b The impedance characteristic curve diagram of the high out-of-band suppression circuit provided by the specific embodiment of the present invention; Figure 4c The S parameter simulation diagram of the high out-of-band suppression circuit provided by the specific embodiment of the present invention; Figure 5 A schematic diagram of the externally extended capacitance and inductance values ​​of a resonator provided in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention is described in detail below in conjunction with specific embodiments and drawings. For the convenience of description, the structures shown in the drawings are only the relevant parts of the present invention, rather than the entire structure.

[0024] In order to achieve high steep drop and high out-of-band suppression, a specific embodiment of the present invention provides a high steep drop and high out-of-band suppression bulk acoustic wave filter circuit structure, including a high steep drop circuit and a high out-of-band suppression circuit, the output end of the high steep drop circuit is connected in series with the high out-of-band suppression circuit; the high steep drop circuit is a passive device externally expanded on the ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros near the passband; the high out-of-band suppression circuit is an inductor externally expanded on the ladder bulk acoustic wave filter circuit to construct a new transmission zero in a specific frequency band at the far end of the passband to achieve high out-of-band suppression.

[0025] like Figure 1a As shown, the ladder bulk acoustic wave filter 103 circuit provided by the specific embodiment of the present invention includes a series arm and a plurality of parallel arms; the series arm includes a plurality of series bulk acoustic wave resonators 101 connected in series in sequence; the parallel arm is connected between the series arm and the ground line, and at least one series bulk acoustic wave resonator 101 is spaced between two adjacent parallel arms, and the parallel arm includes a parallel bulk acoustic wave resonator 102. The series bulk acoustic wave resonator on the series arm and the parallel bulk acoustic wave resonator on the parallel arm are both constructed and simulated using the Mason model, and their impedance characteristic curves include a series resonant frequency point fs and a parallel resonant frequency point fp.

[0026] When constructing the ladder bulk acoustic wave filter 103 in the specific embodiment of the present invention, the impedance curve of the series bulk acoustic wave resonator on the series arm is placed at the low frequency end by adjusting the electrode film thickness, and the impedance curve of the parallel bulk acoustic wave resonator on the parallel arm is placed at the high frequency end by adjusting the electrode film thickness. At this time, the impedance characteristic curve of the series-parallel bulk acoustic wave resonator is as follows: Figure 1b As shown, curve a is the impedance curve of the parallel body acoustic wave resonator, and curve b is the impedance curve of the series body acoustic wave resonator. It can be seen from the impedance characteristic curve that when the frequency is at f1, i.e., the series resonance frequency of the parallel body acoustic wave resonator, the impedance of the parallel body acoustic wave resonator on the parallel arm is the minimum value, which is equivalent to a short circuit. At this time, the impedance of the series body acoustic wave resonator on the series arm is very large, constituting the transmission zero point of the filter transmission curve; when the frequency is at f2, i.e., the parallel resonance frequency of the parallel body acoustic wave resonator and the series resonance frequency of the series body acoustic wave resonator, the impedance value of the series body acoustic wave resonator on the series arm is the minimum, and the impedance of the parallel body acoustic wave resonator on the parallel arm is the maximum, constituting the transmission pole of the filter transmission curve; when the frequency is at f3, i.e., the parallel resonance frequency of the series body acoustic wave resonator, the impedance value of the series body acoustic wave resonator on the series arm is the maximum, and at this time, the impedance of the parallel body acoustic wave resonator on the parallel arm is relatively small, constituting another transmission zero point of the filter transmission curve, and the corresponding curve is as follows: Figure 1cAs shown, curve 1 is the transmission curve of the prototype large bandwidth bulk acoustic wave filter S21, and curve 2 is the impedance curve of the series-parallel resonator. It can be seen that the bandwidth of the prototype filter is closely related to the series-parallel resonant frequency difference fps of the resonator. The specific embodiment of the present invention uses a bulk acoustic wave resonator with a high scandium doping ratio AlN as the piezoelectric layer as a filter constituent unit, which has a higher electromechanical coupling coefficient. The series-parallel resonant frequency difference fps of the resonator is large, which can provide a larger limit bandwidth for the constructed prototype bulk acoustic wave filter and provide a higher possibility for improving the roll-off performance of the external passive device capacitor or inductor.

[0027] The passband for satellite network communication is in the N78 band, with a passband of 3.44-3.66 GHz, a bandwidth of 220 MHz, and a transition band of 40 MHz. Figure 2c The roll-off suppression below -35dB shown in the figure provides extremely high near-band suppression and selectivity for satellite communications. In order to achieve its extremely narrow transition band and high roll-off performance, based on the ladder bulk acoustic wave filter circuit provided in the specific embodiment of the present invention, the capacitor or inductor of the passive device is expanded to obtain a high steep drop circuit, so that the zero point of the near-passband frequency band of the prototype filter can be edited, thereby partially replacing the wide bandwidth performance of the ladder bulk acoustic wave filter circuit with an extremely narrow transition band and high roll-off performance.

[0028] The forms of the externally expanded passive device provided in the specific embodiments of the present invention include: (1) a series capacitor of the series BAW resonator on the series arm; (2) a parallel capacitor of the series BAW resonator on the series arm; (3) a series inductor of the series BAW resonator on the series arm; (4) a parallel inductor of the series BAW resonator on the series arm; (5) a series capacitor of the parallel BAW resonator on the parallel arm; (6) a parallel capacitor of the parallel BAW resonator on the parallel arm; (7) a series inductor of the parallel BAW resonator on the parallel arm; (8) a parallel inductor of the parallel BAW resonator on the parallel arm.

[0029] First, the changes in the resonant frequency and impedance characteristics of a single BAW resonator when different passive devices are expanded in different expansion forms are analyzed. Figure 2a The figure shows the resonant frequency and impedance characteristic changes of a single BAW resonator with the inductor and capacitor of the passive devices expanded in series and parallel respectively.

[0030] like Figure 2a As shown, curve a is the impedance characteristic curve of the resonant frequency change when the resonator is connected in series with a capacitor. The series resonant frequency fs will shift to the right and increase due to the series capacitor, and series capacitors of different capacitance values ​​will cause different degrees of increase in the series resonant frequency.

[0031] Curve b is an impedance characteristic curve of the change in resonant frequency when the resonator is connected in parallel with a capacitor. The parallel resonant frequency fp will shift to the left and decrease due to the parallel capacitor, and parallel capacitors with different capacitance values ​​will cause different degrees of reduction in parallel resonant frequency.

[0032] Curve c is an impedance characteristic curve of the change in resonant frequency when the resonator is connected in parallel with the inductor. The parallel resonant frequency fp will shift to the right and increase due to the parallel inductor, and parallel capacitors with different inductance values ​​will cause the parallel resonant frequency to increase to different degrees.

[0033] Curve d is an impedance characteristic curve of the change in resonant frequency when the resonator is connected in series with an inductor. The series resonant frequency fs will shift to the left and decrease due to the series inductor, and series inductors of different inductance values ​​will cause different degrees of reduction in the series resonant frequency.

[0034] In order to conveniently demonstrate the influence of the capacitance and inductance of the external passive device on the resonant frequency of the resonator, the BVD (Butterworth-Van Dyke) model can be used to roughly estimate the capacitance and inductance of the external passive device. For easier calculation, two common equivalent BVD models are used, such as Figure 3 As shown, the parallel equivalent BVD model is the dynamic inductance of FBAR, is the dynamic capacitance of FBAR, is the static capacitance of FBAR; in the series equivalent BVD model is the dynamic inductance of FBAR, is the dynamic capacitance of FBAR, is the static capacitance of FBAR; the parameter conversion relationship of the series-parallel equivalent BVD model is:

[0035] The formula for calculating the series-parallel resonant frequency is:

[0036] in, for , for .

[0037] It can also be inferred that the calculation formula for the external capacitance and inductance of the resonator is as follows: Figure 5 As shown, according to Figure 5 The formula shown can roughly calculate the approximate capacitance and inductance values ​​of the required external extended capacitor and inductor components.

[0038] The specific implementation of high roll-off performance is as follows: Figure 2b As shown in the figure, it is a schematic diagram of externally expanding passive components in the ladder bulk acoustic wave filter circuit to achieve high roll-off and extremely narrow transition band performance. Figure 2b Only the external passive device structure of the prototype filter to achieve narrow transition band and high roll-off performance is shown, not the complete structure of the high-performance satellite filter.

[0039] In order to achieve an extremely narrow transition band and high roll-off performance on the right side of the filter, Figure 2b Capacitor C201 is connected in parallel to the series bulk acoustic wave resonator 202, and capacitor C203 is connected in parallel to the series bulk acoustic wave resonator 204, and the parallel resonant frequency fp of the series bulk acoustic wave resonator 202 and the series bulk acoustic wave resonator 204 is reduced. According to the influence of the above-mentioned series-parallel bulk acoustic wave resonators on the transmission zero point of the filter, it can be known that the parallel resonant frequency of the series bulk acoustic wave resonator on the series arm is reduced, resulting in the left shift of the right transmission zero point of the constructed filter. At the same time, in order to achieve higher roll-off performance, the film thickness of the series bulk acoustic wave resonator electrode on the series arm of the ladder bulk acoustic wave filter is appropriately increased to further reduce the parallel resonant frequency of the series bulk acoustic wave resonator on the series arm. However, in order to ensure the insertion loss at the edge of the passband and the smoothness of the transition band, the series bulk acoustic wave resonators on the series arms cannot be adjusted to the same parallel resonant frequency at the same time, and the transmission zero point of the right band of the filter cannot coincide with the same frequency, and a step-by-step right shift is required, which is Figure 2c The right-side transition band form a shown in . The stepped right-side roll-off performance of the filter is achieved by different transmission zeros with similar frequencies, that is, the series BAW resonator on the series arm needs to have a stepped parallel resonant frequency, which requires the series BAW resonators of different areas on the series arm to be connected in parallel with external capacitors of specific capacitance. The specific estimation refers to the above formula, such as Figure 2c As shown, curve 3 is the transmission curve of the prototype BAW filter S21 with an extremely narrow transition band after the external capacitor is expanded, and curve 4 is the transmission curve of the prototype large-bandwidth BAW filter S21.

[0040] In order to achieve an extremely narrow transition band and high roll-off performance on the left side of the filter, Figure 2b The capacitor C204 is connected in series to the parallel body acoustic wave resonator 205, and the capacitor C206 is connected in series to the parallel body acoustic wave resonator 208, and the series resonance frequency fs of the parallel body acoustic wave resonator 205 and the parallel body acoustic wave resonator 208 increases. According to the influence of the above-mentioned series-parallel body acoustic wave resonators on the transmission zero point of the filter, it can be known that the series resonance frequency of the parallel body acoustic wave resonator on the parallel arm increases, resulting in the right shift of the left transmission zero point of the constructed filter. At the same time, in order to achieve higher roll-off performance, the film thickness of the parallel body acoustic wave resonator electrode on the parallel arm of the Ladder body acoustic wave filter circuit is appropriately reduced, and the series resonance frequency of the parallel body acoustic wave resonator on the parallel arm is further increased. However, similar to the above-mentioned right-side band roll-off, in order to ensure the insertion loss at the edge of the passband and the smoothness of the transition band, the transmission zero point of the right band of the filter cannot coincide with the same frequency, and also needs to be stepped to the left, that is, Figure 2cThe left side with transition zone form b is shown in .

[0041] In the process of realizing the extremely narrow transition band and high roll-off, the electrode thickness of the series body acoustic wave resonator on the series arm is increased, and the electrode thickness of the parallel body acoustic wave resonator on the parallel arm is reduced, respectively, the parallel resonance frequency of the series body acoustic wave resonator is reduced, and the series resonance frequency of the parallel body acoustic wave resonator is increased, but the modification of the film thickness will not change a certain resonance frequency of the resonator alone, and both the series and parallel resonance frequencies will be affected. This causes the series resonance frequency fs of the series body acoustic wave resonator and the parallel resonance frequency fp of the parallel body acoustic wave resonator, which were originally at basically the same frequency point, to be misaligned, causing the center of the filter passband to collapse. For this reason, it is necessary to connect a series capacitor C202 or an inductor L201 to the series body acoustic wave resonator, and a parallel capacitor C205 or an inductor L203 to the parallel body acoustic wave resonator, so that the series and parallel resonance frequencies of the series-parallel body acoustic wave resonators are realigned to ensure that the filter passband is smooth and the ripple is reduced.

[0042] When optimizing the performance of BAW filters, it is necessary to coordinately optimize the area, film thickness and capacitance and inductance of the series-parallel BAW resonator and its external passive components, and adopt an overall optimization method followed by detailed optimization. For high steep drop performance, the optimization process includes: (1) Manually adjust the film thickness of the series-parallel BAW resonator, and shift the zero points on the left and right sides of the large-bandwidth prototype Ladder BAW filter to the required high roll-off zero point. The film thickness that completely fits the filter transmission zero point and the required steep drop frequency is the maximum film thickness of the series BAW resonator and the minimum film thickness of the parallel BAW resonator, respectively. The film thickness of the prototype Ladder filter is used as the minimum film thickness of the series BAW resonator and the maximum film thickness of the parallel BAW resonator, thereby reducing the optimization range of the film thickness and achieving better optimization results; (2) According to the Ladder BAW filter circuit, in order to ensure the insertion loss at the edge of the passband and the smooth transition band without large peaks, the zero points of the left and right transition bands need to be distributed in a stepped manner. Therefore, the series-parallel BAW resonator needs to be The external capacitance value of the acoustic wave resonator is also set to a stepped value range; (3) In order to ensure the out-of-band suppression at the far end of the passband, it is necessary to roughly calculate the inductance value range of the external inductor of the resonator according to the formula to obtain the impedance curve of a single resonator at this time, and adjust the inductance and resonator area value S0 at the same time until the newly constructed transmission zero point falls within the 100MHz range of the far-end suppression frequency band. When optimizing the filter later, the resonator area value at this time is used as the center, and the resonator area optimization range is set to S0 plus or minus 15%, that is, 0.85S0~1.15S0. The new transmission zero point constructed by the resonator of other external inductors must be outside the above 100MHz range to prevent the transmission zero points constructed by the resonators of the two external inductors from overlapping, resulting in the failure to fully meet the suppression requirements of the specific frequency band at the far end.

[0043] In order to suppress the frequency interference of part of the L and S bands (1.9 GHz-2.1 GHz) to the satellite network, the specific embodiment of the present invention needs to achieve extremely high out-of-band suppression of -60 dB and below in the suppression band. However, this frequency band is far from the passband of the ladder bulk acoustic wave filter provided by the specific embodiment of the present invention, and the out-of-band suppression requirement of this frequency band is extremely high, which cannot be achieved by conventionally optimizing the area of ​​the series-parallel bulk acoustic wave resonator. Therefore, it is achieved by adopting externally expanded passive devices such as capacitors and inductors mentioned in the high steep drop circuit.

[0044] The ability of the externally expanded passive device capacitance or inductance used in the high steep drop circuit provided in the specific embodiment of the present invention to edit the series and parallel resonant frequency of the resonator is limited, and is only suitable for fine-tuning within a range of 200 MHz based on the original resonant frequency. If ultra-high out-of-band suppression is to be achieved at the far end, it is obviously necessary to construct a new filter transmission zero point, which requires externally expanded passive devices with a new capacitance and inductance range.

[0045] By analogy with the single BAW resonator external expansion passive device of the high steep drop circuit provided in the specific embodiment of the present invention, the capacitance and inductance of the external expansion capacitor and inductor are changed, the original form of the resonator impedance characteristic curve is changed, and a new resonant frequency fs or fp is constructed. The resonator impedance characteristic curve is as follows: Figure 4a shown.

[0046] In order to meet the performance requirements of far-end out-of-band suppression, it is necessary to construct new far-end out-of-band transmission zeros on the left and right sides of the passband in the ladder bulk acoustic wave filter circuit. Since the transmission zeros of the ladder filter are respectively realized by the series resonant frequency of the parallel bulk acoustic wave resonator on the parallel arm and the parallel resonant frequency of the series bulk acoustic wave resonator on the series arm. Therefore, the newly constructed filter far-end out-of-band transmission zero requires the series bulk acoustic wave resonator on the series arm to construct a new parallel resonant frequency, and the parallel bulk acoustic wave resonator on the parallel arm to construct a new series resonant frequency, which is Figure 4a From (I), we can see that connecting a parallel inductor to the series BAW resonator can increase the parallel resonant frequency and construct a new parallel resonant frequency. It is also possible to edit the transmission zeros on both sides of the filter at the same time and improve the far-end out-of-band suppression of the left and right bands of the prototype filter.

[0047] Depend on Figure 4a From (II), it can be seen that the specific embodiment of the present invention can construct a new series resonant frequency while reducing the series resonant frequency by connecting a series inductor to the parallel body acoustic wave resonator, and can also simultaneously edit the transmission zeros on the left and right sides of the filter to improve the far-end out-of-band suppression of the left and right bands of the prototype filter.

[0048] like Figure 4bAs shown, in order to achieve high out-of-band suppression performance at a specific frequency at the far end, a high out-of-band suppression circuit is obtained by expanding the passive components on the ladder bulk acoustic wave filter circuit. Among them, the parallel bulk acoustic wave resonator 306 and the series inductor L303 on the parallel arm construct a new series resonant frequency, and the parallel bulk acoustic wave resonator 308 and the series inductor L304 construct a new series resonant frequency, thereby constructing a new left transmission zero on the basis of the prototype filter; the parallel inductors L301 and L302 on the series arm construct a new parallel resonant frequency, thereby constructing a new right transmission zero on the basis of the prototype filter. The resonator area and inductance value where the external inductor is located are optimized to align the newly constructed filter transmission zero with the desired far-end out-of-band suppression frequency band, thereby achieving the following. Figure 4c As shown, curve 5 is the transmission curve of the bulk acoustic wave filter S21 with high steep drop and high out-of-band suppression after the prototype bulk acoustic wave filter is externally expanded with capacitors and inductors. Curve 6 is the transmission curve of the bulk acoustic wave filter S21 with an extremely narrow transition band after the prototype bulk acoustic wave filter is externally expanded with capacitors. High out-of-band suppression at the far end of the band is achieved based on the high steep drop circuit.

[0049] The satellite BAW filter implemented by the filter structure and optimization method provided by the specific embodiment of the present invention has extremely excellent steep drop performance, and achieves out-of-band suppression of -35dB or above in the 40MHz transition band; at the same time, it has very high out-of-band suppression performance in specific frequency bands at the far end of the passband, and the out-of-band suppression of part of the S band at 2GHz away from the passband can reach -60dB or above, which can meet the strict requirements of StarNet satellite communications on filters.

Claims

1. A high steep drop and high out-of-band suppression bulk acoustic wave filter circuit structure, characterized in that: It comprises a high steep drop circuit and a high out-of-band suppression circuit, wherein the output end of the high steep drop circuit is connected in series with the high out-of-band suppression circuit; The high steep drop circuit is a passive device that is externally expanded on the ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros near the passband; The high out-of-band suppression circuit is to expand the inductance on the ladder bulk acoustic wave filter circuit to construct a new transmission zero point in a specific frequency band at the far end of the passband to achieve high out-of-band suppression.

2. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 1, characterized in that: The ladder bulk acoustic wave filter circuit includes a series arm and a plurality of parallel arms; Wherein, the series arm comprises a plurality of series bulk acoustic wave resonators connected in series in sequence; The parallel arm is connected between the series arm and the ground line, at least one series BAW resonator is spaced between two adjacent parallel arms, and the parallel arm includes a parallel BAW resonator.

3. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 2, characterized in that: By adjusting the electrode film thickness of the series resonator on the series arm, the impedance curve of the series resonator on the series arm is placed at the low frequency end; By adjusting the electrode film thickness of the parallel bulk acoustic wave resonator on the parallel arm, the impedance curve of the parallel bulk acoustic wave resonator on the parallel arm is placed at the high frequency end.

4. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 3, characterized in that: The series bulk acoustic wave resonator and the parallel bulk acoustic wave resonator both use AlN as the piezoelectric layer.

5. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 1, characterized in that: The high steep drop circuit is a passive device externally expanded on the ladder bulk acoustic wave filter circuit, wherein the method of externally expanding the passive device includes connecting a passive device in series or in parallel on the series bulk acoustic wave resonator on the series arm, and / or connecting a passive device in series or in parallel on the parallel bulk acoustic wave resonator on the parallel arm, and the passive device is a capacitor or an inductor.

6. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 5, characterized in that: The high steep drop circuit includes connecting a capacitor in parallel to at least one series BAW resonator, and the transmission zero point of the right band of the ladder BAW filter circuit is shifted to the right in a step-by-step manner by setting the thickness of the series BAW resonator and the capacitance of the parallel capacitor, and connecting a capacitor in series to at least one parallel BAW resonator, and the transmission zero point of the left band of the ladder BAW filter circuit is shifted to the left in a step-by-step manner by setting the thickness of the parallel BAW resonator and the capacitance of the series capacitor.

7. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 6, characterized in that: The high steep drop circuit also includes a capacitor and / or an inductor connected in series with the series BAW resonator, and a capacitor and / or an inductor connected in parallel with the parallel BAW resonator, so that the series resonant frequency of the series BAW resonator and the parallel resonant frequency of the parallel BAW resonator at the same frequency point in the passband are realigned.

8. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 1, characterized in that: The high out-of-band suppression circuit is to connect an inductor in parallel to at least one series BAW resonator in the series arm of the ladder BAW filter circuit to construct a new transmission zero point, and / or to connect an inductor in series to at least one parallel BAW resonator in the parallel arm to construct a new transmission zero point, thereby achieving high out-of-band suppression.

9. The high steep drop and high out-of-band suppression bulk acoustic wave filter circuit according to claim 8, characterized in that: Taking the far-end external suppression frequency as the target, the areas of the series BAW resonator and the parallel BAW resonator, and the corresponding inductance values ​​of the parallel inductor and the series inductor are calculated by EDA software based on the high out-of-band suppression circuit architecture.

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