A bulk acoustic wave filter circuit structure with high steep drop and high out-of-band suppression

Abstract

The present invention discloses a bulk acoustic wave filter circuit structure with high steepness and high out-of-band suppression, which includes a high-steepness circuit and a high out-of-band suppression circuit. The output end of the high-steepness circuit is connected in series with the high out-of-band suppression circuit. The high-steepness circuit is an external expansion of passive devices on a Ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros in the near-passband. The high out-of-band suppression circuit is an external expansion of inductors on a Ladder bulk acoustic wave filter circuit to construct new transmission zeros at specific frequency bands in the far end of the passband to achieve high out-of-band suppression. This circuit can not only achieve high out-of-band suppression but also avoid large spikes in the near-passband.

Description

Technical Field

[0001] The present invention belongs to the technical field of filter circuits, and particularly relates to a bulk acoustic wave filter circuit structure with high steepness and high out-of-band rejection. 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' intelligent mobile terminals have increasingly high requirements for communication quality in extreme situations. However, the conventional communication methods relying on communication base stations cannot meet this demand. Therefore, various difficulties in the satellite communication function of mobile intelligent terminals need to be solved urgently.

[0003] Traditional dedicated satellite phones do not pay too much attention to the size and portability of the devices. The size of the radio frequency filters they use is mostly 10mm * 10mm or more, and in most cases, multiple-stage filters are required to meet the filtering requirements. However, current intelligent mobile terminals are highly integrated and cannot reserve such a large space for the radio frequency front-end of the satellite communication function. Therefore, BAW filters with smaller size and better performance are needed.

[0004] The invention patent application with the publication number CN113411069A discloses a bulk acoustic wave filter device and a method for improving out-of-band rejection. Among them, the bulk acoustic wave filter device includes a substrate, a protection 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 around 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 bonding manner through the metallized vias of the protection cap wafer. Further, the parallel resonant arms in the filter are connected to an inductor on the metal layer and then connected to the ground metal, and adjacent two inductors are separated by the ground metal. The bulk acoustic wave filter device disclosed in this patent application cannot achieve good improvement in out-of-band rejection at the far end of the passband.

[0005] The invention patent application with the publication number CN111200418A discloses a bulk acoustic wave filter and a signal processing device, relating 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-connected bulk acoustic wave resonators connected in sequence; a parallel branch is connected to the connection node between two adjacent series-connected 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 second inductor; there is 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 the inductors affects the calculation of the series inductance value, the newly constructed series resonance frequency cannot be accurately estimated, and such a circuit structure will cause a new parallel resonance frequency to be constructed at the left end of the original series resonance frequency of the parallel bulk acoustic wave resonator, resulting in large spikes in the left transition band of the filter and affecting the filter performance. Summary of the Invention

[0006] The present invention provides a bulk acoustic wave filter circuit structure with high steepness and high out-of-band suppression, which can achieve high out-of-band suppression and avoid large spikes in the near passband.

[0007] A specific embodiment of the present invention provides a bulk acoustic wave filter circuit structure with high steepness and high out-of-band suppression, including a high-steepness circuit and a high out-of-band suppression circuit, and the output end of the high-steepness circuit is connected in series with the high out-of-band suppression circuit;

[0008] The high-steepness circuit is to expand passive devices on the Ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros in the near passband;

[0009] The high out-of-band suppression circuit is to expand inductors on the Ladder bulk acoustic wave filter circuit to construct new transmission zeros in a specific frequency band at the far end of the passband to achieve high out-of-band suppression.

[0010] Further, the Ladder bulk acoustic wave filter circuit includes series arms and multiple parallel arms;

[0011] Among them, the series arms include a plurality of series-connected bulk acoustic wave resonators connected in sequence;

[0012] The parallel arms are connected between the series arms and the ground wire, and at least one series-connected bulk acoustic wave resonator is spaced between two adjacent parallel arms, and the parallel arms include parallel bulk acoustic wave resonators.

[0013] Further, by adjusting the electrode film thickness of the series resonators on the series arm, the impedance curve of the series resonators on the series arm is placed at the low-frequency end;

[0014] By adjusting the electrode film thickness of the parallel bulk acoustic wave resonators on the parallel arm, the impedance curve of the parallel bulk acoustic wave resonators on the parallel arm is placed at the high-frequency end.

[0015] Further, both the series bulk acoustic wave resonator and the parallel bulk acoustic wave resonator use AlN as the piezoelectric layer.

[0016] In the present invention, by placing the impedance curve of the series resonators on the series arm at the low-frequency end and the impedance curve of the parallel bulk acoustic wave resonators at the high-frequency end, and at the same time combining the bulk acoustic wave resonator using high-scandium-doped AlN with a higher electromechanical coupling coefficient as the piezoelectric layer in the present invention, a relatively large series-parallel resonance frequency difference fps of the resonator is achieved, which can provide a larger ultimate bandwidth for the constructed prototype bulk acoustic wave filter.

[0017] Further, the high-steepness circuit is formed by expanding passive devices on the Ladder bulk acoustic wave filter circuit. Among them, the way of expanding passive devices includes connecting passive devices in series or in parallel on the series bulk acoustic wave resonators on the series arm, and / or connecting passive devices in series or in parallel on the parallel bulk acoustic wave resonators on the parallel arm. The passive devices are capacitors or inductors.

[0018] Further, the high-steepness circuit includes connecting a capacitor in parallel on at least one series bulk acoustic wave resonator, and making the transmission zero points on the right sideband of the Ladder bulk acoustic wave filter circuit shift step by step to the right by setting the thickness of the series bulk acoustic wave resonator and the capacitance value of the parallel capacitor, and connecting a capacitor in series on at least one parallel bulk acoustic wave resonator, and making the transmission zero points on the left sideband of the Ladder bulk acoustic wave filter circuit shift step by step to the left by setting the thickness of the parallel bulk acoustic wave resonator and the capacitance value of the series capacitor.

[0019] Compared with the prior art, in the present invention, by connecting a capacitor in parallel on the series bulk acoustic wave resonator, the transmission zero points on the right sideband shift step by step to the right, and by connecting a capacitor in series on the parallel bulk acoustic wave resonator, the transmission zero points on the left sideband shift step by step to the left, thereby achieving an extremely narrow transition band.

[0020] Further, the high-steepness circuit further includes connecting a capacitor and / or an inductor in series on the series bulk acoustic wave resonator, and connecting a capacitor and / or an inductor in parallel on the parallel bulk acoustic wave resonator, so as to realign the series resonance frequency of the series bulk acoustic wave resonator and the parallel resonance frequency of the parallel bulk acoustic wave resonator at the same frequency point in the passband.

[0021] When modifying the film thickness, it is easy to cause the misalignment of the series resonance frequency fs of the series bulk acoustic wave resonator and the parallel resonance frequency fp of the parallel bulk acoustic wave resonator, which are originally basically at the same frequency point, resulting in the collapse of the center of the filter passband. To solve the above problems, the present invention makes the series and parallel resonance frequencies of the series-parallel bulk acoustic wave resonator realign again by connecting a capacitor and / or an inductor in series on the series bulk acoustic wave resonator and connecting a capacitor and / or an inductor in parallel on the parallel bulk acoustic wave resonator, ensuring the flatness of the filter passband and reducing the ripple.

[0022] Further, the high out-of-band rejection circuit is to connect an inductor in parallel on at least one series bulk acoustic wave resonator in the series arm of the Ladder bulk acoustic wave filter circuit to construct a new transmission zero point, and / or connect an inductor in series on at least one parallel bulk acoustic wave resonator in the parallel arm to construct a new transmission zero point, thereby realizing high out-of-band rejection.

[0023] Compared with the prior art that mainly constructs zeros in the transition band, in order to achieve ultra-high out-of-band rejection at the far end and thus construct new filter transmission zeros, the present invention can construct new parallel resonance frequencies while increasing the parallel resonance frequency by connecting an inductor in parallel on the series bulk acoustic wave resonator, and can edit the transmission zeros on both the left and right sides of the filter at the same time, improving the far-end out-of-band rejection of the left and right sides of the prototype filter; the present invention also connects an inductor in series on the parallel bulk acoustic wave resonator to reduce the series resonance frequency and can construct new series resonance frequencies at the same time, and can also edit the transmission zeros on both the left and right sides of the filter. Through the above two aspects of improvement, the far-end out-of-band rejection of the left and right sides of the prototype filter is improved. It should be noted that out-of-band rejection at the far end is achieved by adjusting the inductance value to generate new resonance frequency points in the required frequency band to generate new zeros, which is different from the control method of moving the resonance frequency point in the required frequency band by adjusting the inductance value for out-of-band rejection at the near end.

[0024] Further, based on the high out-of-band rejection circuit architecture with the far-end out-of-band rejection frequency as the target, the areas of the series bulk acoustic wave resonator and the parallel bulk acoustic wave resonator, and the inductance values of the corresponding parallel inductors and series inductors are calculated by EDA software.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] In the prior art, new transmission zeros are generated by controlling the bulk acoustic wave resonator, capacitor or inductor, and most of the generated new transmission zeros are in the adjacent band of the passband. The present invention changes the electromechanical coupling coefficient of the bulk acoustic wave resonator by expanding the capacitor or inductor outside the bulk acoustic wave resonator, realizes the editable electromechanical coupling coefficient, makes the electromechanical coupling coefficient of the bulk acoustic wave resonator match the required bandwidth and roll-off of the filter where it is located, and further realizes the improvement of the far-end out-of-band rejection and the high steepness of the adjacent band.

[0027] Based on the large-bandwidth prototype Ladder bulk acoustic wave filter, passive devices such as capacitors and inductors with specific capacitance and inductance values are externally expanded on the series-parallel arms of the series-parallel bulk acoustic wave resonators to achieve controllable editing of the transmission zeros in the near-passband frequency band, and the large-bandwidth part of the prototype Ladder bulk acoustic wave filter is replaced with a high steepness drop; at the same time, based on the large-bandwidth prototype Ladder bulk acoustic wave filter, passive device inductors with specific inductance values are externally expanded on the series-parallel arms of the series-parallel bulk acoustic wave resonators to construct new transmission zeros at the far end outside the filter passband and achieve high out-of-band suppression in specific frequency bands at the far end of the passband. Description of the Drawings

[0028] Figure 1a Schematic diagram of the Ladder bulk acoustic wave filter circuit provided by a specific embodiment of the present invention;

[0029] Figure 1b Impedance characteristic curve diagram of the Ladder bulk acoustic wave filter circuit provided by a specific embodiment of the present invention;

[0030] Figure 1c S-parameter simulation diagram of the Ladder bulk acoustic wave filter circuit provided by a specific embodiment of the present invention;

[0031] Figure 2a Schematic diagram of the high steepness drop circuit provided by a specific embodiment of the present invention;

[0032] Figure 2b Impedance characteristic curve diagram of the high steepness drop circuit provided by a specific embodiment of the present invention;

[0033] Figure 2c S-parameter simulation diagram of the high steepness drop circuit provided by a specific embodiment of the present invention;

[0034] Figure 3 Equivalent BVD model diagram provided by a specific embodiment of the present invention;

[0035] Figure 4a Schematic diagram of the high out-of-band suppression circuit provided by a specific embodiment of the present invention;

[0036] Figure 4b Impedance characteristic curve diagram of the high out-of-band suppression circuit provided by a specific embodiment of the present invention;

[0037] Figure 4c S-parameter simulation diagram of the high out-of-band suppression circuit provided by a specific embodiment of the present invention;

[0038] Figure 5 Schematic diagram of the capacitance and inductance values externally expanded for the resonator provided by a specific embodiment of the present invention. Detailed Description of the Invention

[0039] The present invention will be described in detail below in conjunction with specific embodiments and the accompanying 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.

[0040] In order to achieve high steepness and high out-of-band suppression, a specific embodiment of the present invention provides a bulk acoustic wave filter circuit structure with high steepness and high out-of-band suppression, including a high-steepness circuit and a high out-of-band suppression circuit. The output end of the high-steepness circuit is connected in series with the high out-of-band suppression circuit; the high-steepness circuit is an external expansion of passive devices on the Ladder bulk acoustic wave filter circuit to achieve a stepped distribution of transmission zeros in the near-passband; the high out-of-band suppression circuit is an external expansion of inductors on the Ladder bulk acoustic wave filter circuit to construct new transmission zeros in a specific frequency band at the far end of the passband to achieve high out-of-band suppression.

[0041] As Figure 1a shown, the Ladder bulk acoustic wave filter 103 circuit provided by a specific embodiment of the present invention includes series arms and a plurality of parallel arms; the series arms include a plurality of series-connected series bulk acoustic wave resonators 101; the parallel arms are connected between the series arms and the ground wire, and at least one series bulk acoustic wave resonator 101 is spaced between adjacent two parallel arms. The parallel arms include parallel bulk acoustic wave resonators 102. The series bulk acoustic wave resonators on the series arms and the parallel bulk acoustic wave resonators on the parallel arms are both simulated using the Mason model, and their impedance characteristic curves include series resonance frequency points fs and parallel resonance frequency points fp.

[0042] When constructing the Ladder bulk acoustic wave filter 103 in a 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 also placed at the high-frequency end by adjusting the electrode film thickness. At this time, the impedance characteristic curves of the series and parallel bulk acoustic wave resonators are as Figure 1b shown, where curve a is the impedance curve of the parallel bulk acoustic wave resonator, and b is the impedance curve of the series bulk acoustic wave resonator. It can be seen from the impedance characteristic curves that when the frequency is at f1, that is, the series resonance frequency point of the parallel bulk acoustic wave resonator, the impedance of the parallel bulk acoustic wave resonator on the parallel arm is the minimum value, equivalent to a short circuit, while at this time the impedance of the series bulk acoustic wave resonator on the series arm is very large, constituting a transmission zero of the filter transmission curve; when the frequency is at f2, that is, the parallel resonance frequency point of the parallel bulk acoustic wave resonator and the series resonance frequency point of the series bulk acoustic wave resonator, the impedance value of the series bulk acoustic wave resonator on the series arm is the minimum, and the impedance of the parallel bulk acoustic wave resonator on the parallel arm is the maximum, constituting a transmission pole of the filter transmission curve; when the frequency is at f3, that is, the parallel resonance frequency point of the series bulk acoustic wave resonator, the impedance value of the series bulk acoustic wave resonator on the series arm is the maximum, while at this time the impedance of the parallel bulk acoustic wave resonator on the parallel arm is relatively small, constituting another transmission zero of the filter transmission curve, and the corresponding curve is as Figure 1cAs shown, curve 1 is the S21 transmission curve of the prototype wide-band bulk acoustic wave filter, and curve 2 is the impedance curve of the series-parallel resonator. It can be seen from this that the bandwidth of the prototype filter is closely related to the series-parallel resonance frequency difference fps of the resonator. In the specific embodiment of the present invention, a bulk acoustic wave resonator using high-scandium-doped AlN as the piezoelectric layer is used as the filter building unit, which has a higher electromechanical coupling coefficient. The series-parallel resonance frequency difference fps of the resonator is relatively large, which can provide a larger ultimate bandwidth for the constructed prototype bulk acoustic wave filter and a higher possibility for expanding the capacitance or inductance of external passive devices to improve the roll-off performance.

[0043] The passband applicable to satellite network communication belongs to the N78 band, the passband is 3.44 - 3.66 GHz, the bandwidth is 220 MHz, and the roll-off suppression below -35 dB can be achieved within the transition band of 40 MHz as shown in Figure 2c This high roll-off performance provides extremely high near-band suppression and selectivity for satellite communication. 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, external passive device capacitance or inductance is expanded to obtain a high-steepness circuit, realizing the zero-point editability of the prototype filter in the near-passband frequency band, thereby partially replacing the large-bandwidth performance of the Ladder bulk acoustic wave filter circuit with an extremely narrow transition band and high roll-off performance.

[0044] The forms of the external passive devices provided in the specific embodiment of the present invention include: (1) a series capacitor of a series bulk acoustic wave resonator on the series arm; (2) a parallel capacitor of a series bulk acoustic wave resonator on the series arm; (3) a series inductor of a series bulk acoustic wave resonator on the series arm; (4) a parallel inductor of a series bulk acoustic wave resonator on the series arm; (5) a series capacitor of a parallel bulk acoustic wave resonator on the parallel arm; (6) a parallel capacitor of a parallel bulk acoustic wave resonator on the parallel arm; (7) a series inductor of a parallel bulk acoustic wave resonator on the parallel arm; (8) a parallel inductor of a parallel bulk acoustic wave resonator on the parallel arm.

[0045] First, analyze the changes in the resonance frequency and impedance characteristics of a single bulk acoustic wave resonator when different passive devices are externally expanded in different forms. As shown in Figure 2a are the resonance frequencies and impedance characteristic changes of a single bulk acoustic wave resonator when passive device inductors and capacitors are externally expanded in series and parallel forms respectively.

[0046] As shown in Figure 2a Among them, curve a is the impedance characteristic curve of the resonance frequency change when the resonator is in series with a capacitor. Its series resonance frequency fs will shift to the right and increase due to the series capacitor, and series capacitors with different capacitance values will cause different degrees of increase in the series resonance frequency.

[0047] Curve b is the impedance characteristic curve of the change in the resonant frequency when the resonator is shunted with a capacitor. Its parallel resonant frequency fp will shift to the left and decrease due to the shunt capacitor, and shunt capacitors with different capacitance values will result in different degrees of decrease in the parallel resonant frequency.

[0048] Curve c is the impedance characteristic curve of the change in the resonant frequency when the resonator is shunted with an inductor. Its parallel resonant frequency fp will shift to the right and increase due to the shunt inductor, and shunt inductors with different inductance values will result in different degrees of increase in the parallel resonant frequency.

[0049] Curve d is the impedance characteristic curve of the change in the resonant frequency when the resonator is series-connected with an inductor. Its series resonant frequency fs will shift to the left and decrease due to the series inductor, and series inductors with different inductance values will result in different degrees of decrease in the series resonant frequency.

[0050] To conveniently show the influence of the values of externally extended passive device capacitors and inductors on the resonant frequency of the resonator, the BVD (Butterworth-Van Dyke) model can be used to roughly estimate the values of the externally extended capacitors and inductors. For more convenient calculation, two common equivalent BVD models are used, as Figure 3 shown, where in the parallel equivalent BVD model is the dynamic inductance of the FBAR, is the dynamic capacitance of the FBAR, is the static capacitance of the FBAR; in the series equivalent BVD model is the dynamic inductance of the FBAR, is the dynamic capacitance of the FBAR, is the static capacitance of the FBAR; the parameter conversion relationship of the series-parallel equivalent BVD model is:

[0051]

[0052] From this, the calculation formulas for the series and parallel resonant frequencies can be obtained as:

[0053]

[0054] Among them, is , is .

[0055] From this, it can also be inferred that the calculation formula for the values of the externally extended capacitors and inductors of the resonator, as Figure 5 shown, according to the formula shown in Figure 5 , the approximate capacitance and inductance values of the required externally extended capacitor and inductor components can be roughly calculated.

[0056] The specific implementation of the high roll-off performance is as follows: As Figure 2bAs shown, it is a schematic diagram of externally expanding passive devices on a Ladder bulk acoustic wave filter circuit to achieve high roll-off and extremely narrow transition band performance. Figure 2b Only the structure of the externally expanded passive devices 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.

[0057] To achieve extremely narrow transition band and high roll-off performance in the right sideband of the filter, Figure 2b Capacitor C201 is connected in parallel with series bulk acoustic wave resonator 202, and capacitor C203 is connected in parallel with series bulk acoustic wave resonator 204. The parallel resonance frequency fp of series bulk acoustic wave resonator 202 and series bulk acoustic wave resonator 204 decreases. According to the influence of the series and parallel bulk acoustic wave resonators on the transmission zeros of the filter as described above, the decrease in the parallel resonance frequency of the series bulk acoustic wave resonator on the series arm causes the right transmission zero of the constructed filter to shift to the left. At the same time, to achieve higher roll-off performance, the electrode film thickness of the series bulk acoustic wave resonator on the series arm of the Ladder bulk acoustic wave filter is appropriately increased to further reduce the parallel resonance frequency of the series bulk acoustic wave resonator on the series arm. However, to ensure the insertion loss at the passband edge and the smoothness of the transition band, the series bulk acoustic wave resonators on the series arm cannot be adjusted to the same parallel resonance frequency simultaneously, and the transmission zeros in the right sideband of the filter do not coincide at the same frequency and need to shift step by step to the right, which is the Figure 2c right sideband transition band form a shown in. The step-by-step right sideband roll-off performance of the filter is achieved by different transmission zeros with similar frequencies, that is, the series bulk acoustic wave resonators on the series arm need to have a step-by-step parallel resonance frequency, which requires the series bulk acoustic wave resonators with different areas on the series arm to be connected in parallel with externally expanded capacitors with specific capacitance values. The specific estimation can refer to the above formula, as Figure 2c shown. Curve 3 is the S21 transmission curve of the bulk acoustic wave filter with an extremely narrow transition band after externally expanding the capacitor of the prototype bulk acoustic wave filter, and curve 4 is the S21 transmission curve of the prototype large-bandwidth bulk acoustic wave filter.

[0058] To achieve extremely narrow transition band and high roll-off performance in the left sideband of the filter, Figure 2bThe capacitor C204 is connected in series with the parallel bulk acoustic wave resonator 205, and the capacitor C206 is connected in series with the parallel bulk acoustic wave resonator 208. The series resonance frequency fs of the parallel bulk acoustic wave resonator 205 and the parallel bulk acoustic wave resonator 208 increases. According to the influence of the series-parallel bulk acoustic wave resonator pair on the transmission zero point of the filter, an increase in the series resonance frequency of the parallel bulk acoustic wave resonator on the parallel arm causes the left transmission zero point of the constructed filter to shift to the right. At the same time, to achieve higher roll-off performance, the electrode film thickness of the parallel bulk acoustic wave resonator on the parallel arm of the Ladder bulk acoustic wave filter circuit is appropriately reduced to further increase the series resonance frequency of the parallel bulk acoustic wave resonator on the parallel arm. However, similar to the realization of the right-side band roll-off mentioned above, to ensure the insertion loss at the passband edge and the smoothness of the transition band, the transmission zero points of the right-side band of the filter cannot coincide at the same frequency and also need to shift left step by step, that is Figure 2c the left-side band transition band form b shown in

[0059] During the above realization of an extremely narrow transition band and high roll-off, the electrode thickness of the series bulk acoustic wave resonator on the series arm is increased, and the electrode thickness of the parallel bulk acoustic wave resonator on the parallel arm is reduced, respectively reducing the parallel resonance frequency of the series bulk acoustic wave resonator and increasing the series resonance frequency of the parallel bulk acoustic wave resonator. However, the modification of the film thickness does not independently change a certain resonance frequency of the resonator, and both the series and parallel resonance frequencies are affected. This causes the series resonance frequency fs of the series bulk acoustic wave resonator and the parallel resonance frequency fp of the parallel bulk acoustic wave resonator, which were originally basically at the same frequency point, to be misaligned, resulting in the collapse of the center of the filter passband. Therefore, a series capacitor C202 or an inductor L201 needs to be connected in series with the series bulk acoustic wave resonator, and a parallel capacitor C205 or an inductor L203 needs to be connected in parallel with the parallel bulk acoustic wave resonator to realign the series and parallel resonance frequencies of the series-parallel bulk acoustic wave resonator, ensure the flatness of the filter passband, and reduce the ripple.

[0060] When optimizing the performance of a bulk acoustic wave filter, it is necessary to synergistically optimize the areas of series and parallel bulk acoustic wave resonators, the film thickness, and the capacitance and inductance values of the external passive devices. An optimization method of first overall and then refinement is adopted. For high steepness performance, the optimization process includes: (1) Manually adjusting the film thickness of series and parallel bulk acoustic wave resonators to shift the zeros on both sides of the large bandwidth prototype Ladder bulk acoustic wave filter to the desired high roll-off zeros. The film thicknesses at which the transmission zeros of the filter perfectly fit the required steepness frequencies are the maximum film thickness of the series bulk acoustic wave resonator and the minimum film thickness of the parallel bulk acoustic wave resonator respectively. Taking the film thickness of the prototype Ladder filter as the minimum film thickness of the series bulk acoustic wave resonator and the maximum film thickness of the parallel bulk acoustic wave resonator can reduce the optimization range of the film thickness and achieve a better optimization effect; (2) According to the Ladder bulk acoustic wave filter circuit, to ensure the insertion loss at the edge of the passband and the smoothness of the transition band without large spikes, the zeros in the left and right transition bands need to be distributed step by step. Therefore, the capacitance values of the external capacitors of the series and parallel bulk acoustic wave resonators also need to be set in a stepwise value range; (3) To ensure the out-of-band rejection at the far end of the passband, it is necessary to roughly calculate the range of the inductance value of the external inductor of the resonator according to the formula first, obtain the impedance curve of a single resonator at this time, and adjust the inductor and the resonator area value S0 simultaneously until the newly constructed transmission zero falls within the 100 MHz range of the far-end rejection frequency band. When optimizing the filter subsequently, with the resonator area value at this time as the center, the optimization range of the resonator area is set within plus or minus 15% of S0, that is, 0.85S0 to 1.15S0. The newly constructed transmission zeros of the resonators with other external inductors need to be outside the above 100 MHz range to prevent the coincidence of the transmission zeros constructed by the resonators with two external inductors, resulting in the inability to fully meet the rejection requirements in a specific frequency band at the far end.

[0061] In a specific embodiment of the present invention, in order to suppress the frequency band interference of some L and S frequency bands (1.9 GHz - 2.1 GHz) on the satellite network, it is necessary to achieve an extremely high out-of-band rejection of -60 dB or lower in this rejection frequency 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 rejection requirement for this frequency band is extremely high, which cannot be achieved by conventionally optimizing the areas of series and parallel bulk acoustic wave resonators. Therefore, external passive device capacitors and inductors similar to those mentioned in the high steepness circuit are used to achieve this.

[0062] The ability of the external passive device capacitance or inductance used in the high steepness circuit provided by the specific embodiment of the present invention to edit the series and parallel resonance frequencies of the resonator is limited and is only applicable to fine-tuning within a range of 200 MHz based on the original resonance frequency. If extremely high out-of-band rejection at the far end is to be achieved, it is obvious that new filter transmission zeros need to be constructed, which requires external passive devices with a new capacitance and inductance value range.

[0063] By analogy with the passive devices externally expanded for a single bulk acoustic wave resonator of the high-steepness drop circuit provided in the specific embodiments of the present invention, the capacitance value and inductance value of the externally expanded capacitor and inductor are changed, and the original form of the impedance characteristic curve of the resonator is changed to construct a new resonance frequency fs or fp. The impedance characteristic curve of the resonator is as Figure 4a shown.

[0064] To meet the performance requirements of far-end out-of-band rejection, it is necessary to construct new far-end out-of-band transmission zeros on both sides of the passband of the Ladder bulk acoustic wave filter circuit. Since the transmission zeros of the ladder filter are respectively realized by the series resonance frequency of the parallel bulk acoustic wave resonator on the parallel arm and the parallel resonance frequency of the series bulk acoustic wave resonator on the series arm. Therefore, for the newly constructed far-end out-of-band transmission zeros of the filter, it is necessary to construct a new parallel resonance frequency for the series bulk acoustic wave resonator on the series arm, and construct a new series resonance frequency for the parallel bulk acoustic wave resonator on the parallel arm. From Figure 4a (I) of it, it can be seen that when an inductor is connected in parallel on the series bulk acoustic wave resonator to increase the parallel resonance frequency, a new parallel resonance frequency can be constructed, and the transmission zeros on both the left and right sides of the filter can be edited simultaneously, improving the far-end out-of-band rejection of the left and right sides of the prototype filter.

[0065] From Figure 4a (II) of it, it can be seen that in the specific embodiments of the present invention, when an inductor is connected in series on the parallel bulk acoustic wave resonator to decrease the series resonance frequency, a new series resonance frequency can be constructed, and the transmission zeros on both the left and right sides of the filter can also be edited simultaneously, improving the far-end out-of-band rejection of the left and right sides of the prototype filter.

[0066] As Figure 4b shown, to achieve high out-of-band rejection performance at a specific far-end frequency, passive devices are externally expanded on the Ladder bulk acoustic wave filter circuit to obtain a high out-of-band rejection circuit. Among them, an inductor L303 is connected in series with the parallel bulk acoustic wave resonator 306 on the parallel arm to construct a new series resonance frequency, and an inductor L304 is connected in series with the parallel bulk acoustic wave resonator 308 to construct a new series resonance 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 resonance frequency, thereby constructing a new right transmission zero on the basis of the prototype filter. Optimize the area of the resonator where the externally expanded inductor is located and the inductance value, so that the newly constructed transmission zeros of the filter are aligned with the required far-end out-of-band rejection frequency band, and achieve as Figure 4c shown. Curve 5 is the S21 transmission curve of the bulk acoustic wave filter that realizes high-steepness drop and high out-of-band rejection after the capacitor and inductor are externally expanded for the prototype bulk acoustic wave filter. Curve 6 is the S21 transmission curve of the bulk acoustic wave filter that realizes an extremely narrow transition band after the capacitor is externally expanded for the prototype bulk acoustic wave filter. High out-of-band rejection at the far end is achieved on the basis of the high-steepness drop circuit.

[0067] The bulk acoustic wave filter for satellites implemented by using the filter structure and optimization method provided in the specific embodiments of the present invention has extremely excellent steep drop performance, achieving out-of-band rejection of -35 dB or more in the 40 MHz transition band; at the same time, it has very high out-of-band rejection performance in a specific frequency band at the far end of the passband, achieving out-of-band rejection of -60 dB or more for some S bands 2 GHz away from the passband, and can meet the strict requirements of satellite network satellite communication for the filter.

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; 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 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; 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 stepped rightward 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 stepped leftward by setting the thickness of the parallel BAW resonator and the capacitance of the series capacitor; The high steep drop circuit further comprises connecting a capacitor and / or an inductor in series with the series BAW resonator, and connecting a capacitor and / or an inductor 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 of the passband are realigned; The high out-of-band suppression circuit is to connect an inductor in parallel to at least one series BAW resonator of the series arm of the ladder BAW filter circuit, thereby constructing a new transmission zero point, and to connect an inductor in series to at least one parallel BAW resonator of the parallel arm, thereby constructing a new transmission zero point, thereby achieving 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: 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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