Circulator based on film bulk acoustic wave filter

Abstract

The invention discloses a circulator based on a film bulk acoustic wave filter, which comprises two single-ended circulators with the same structure, ports of the two single-ended circulators are interconnected to form a differential structure, and each single-ended circulator comprises a three-port nonreciprocal network, the three-port nonreciprocal network comprises two linear time-invariant networks formed by thin film bulk acoustic wave filters and three pairs of radio frequency switches used for modulating the two linear time-invariant networks. Radio frequency switch modulation is utilized to replace the existing variable capacitance diode and switched capacitor modulation; a differential circuit structure is adopted to design the circulator based on the FBAR filter. The working frequency, the modulation ratio and the isolation of the circulator are effectively improved, the insertion loss of the circulator is reduced, and the planarization and the integration of the device are realized.

Description

Technical Field

[0001] The invention belongs to the field of communication technology, and in particular relates to a circulator based on a thin film bulk acoustic wave filter. Background Art

[0002] Circulators are important components in current wireless communication systems. Traditional circulators are generally ferrite circulators, which require permanent magnets to provide a DC bias magnetic field. The permanent magnets are large in size and will increase with the increase in the operating frequency of the device, resulting in the circulator being too large in size and mass. Ferrite materials are incompatible with semiconductor processes, which is not conducive to device integration and has the disadvantages of large size, high cost, and difficulty in integration.

[0003] Existing new circulators such as diode-modulated circulators, switched capacitor-modulated circulators, and RF switch + film bulk acoustic wave (FBAR) filter circulators have been proposed to improve the disadvantages of large size, high cost, and difficult integration. However, diode-modulated circulators are affected by nonlinear factors, have low modulation ratios, and poor isolation performance; switched capacitor-modulated circulators generally have a center frequency below 1 GHz, a low operating frequency, and poor insertion loss due to the use of switched capacitors; and although general RF switch + film bulk acoustic wave (FBAR) filter circulators can operate at frequencies above 1 GHz, their isolation and insertion loss need to be further optimized.

[0004] Therefore, there is an urgent need for a circulator that can simultaneously meet the performance requirements of high frequency, high isolation, and low power consumption. Summary of the invention

[0005] The purpose of the present invention is to provide a circulator based on thin film bulk acoustic wave filters to form two linear time invariant (LTI) networks, using radio frequency switch modulation to replace the existing varactor diode and switch capacitor modulation; using a differential circuit structure to design a circulator based on an FBAR filter. The circulator's operating frequency, modulation ratio and isolation are effectively improved, the circulator's insertion loss is reduced, and the device's planarization and integration are achieved.

[0006] In order to achieve the above purpose, the technical solution adopted is: On the one hand, the present invention proposes a circulator based on a thin film bulk acoustic wave filter, including two single-ended circulators with the same structure, the ports of the two single-ended circulators are interconnected to form a differential structure, each single-ended circulator includes a three-port non-reciprocal network, the three-port non-reciprocal network includes two linear time-invariant networks formed by thin film bulk acoustic wave filters and three pairs of radio frequency switches for modulating the two linear time-invariant networks.

[0007] The inventive concept of this application is: With the development of communication technology, higher performance requirements are put forward for circulators. In addition to being small in size, light in weight, and easy to integrate into various compact devices, they are also required to be able to operate in the 2.45GHz WIFI frequency band, while also ensuring high isolation and low loss. Existing circulators include diode-modulated circulators, switched capacitor-modulated circulators, RF switches + film bulk acoustic wave (FBAR) filter circulators, etc. These new circulators have improved some performances, but they cannot simultaneously meet the performance requirements of high frequency, high isolation, low power consumption, etc.

[0008] Therefore, this application aims to solve the problem that the dielectric filter has a low operating frequency and is difficult to achieve a fast roll-off, and the surface acoustic wave (SAW) filter has a low operating frequency and is difficult to support the application of the circulator in the WIFI frequency band. The circulator designed with FBAR filter has the advantages of small size, integration, and high frequency. The circulator implemented with FBAR filter can be applied to 2.45GHz operating frequency and has a high Q value, which improves the performance indicators of the circulator.

[0009] In order to meet the requirements of high isolation and low loss at the same time, a linear time-invariant (LTI) network is constructed using an RF switch, and a differential circuit is used. The differential circuit structure is used to suppress the zero drift of the device and enhance the signal, offset the influence of the intermodulation product, thereby effectively reducing the insertion loss of the circulator and improving the isolation to a certain extent. In addition, the use of RF switch modulation does not require an additional bias network, which not only makes the modulation ratio easy to control, but also reduces the insertion loss.

[0010] Therefore, it can be seen that the circulator designed by combining the RF switch and the FBAR filter can have higher operating frequency, better reliability, higher modulation ratio and other performances at the same time.

[0011] In some specific embodiments, each three-port non-reciprocal network includes a first linear time-invariant network, a second linear time-invariant network, a first pair of RF switches, a second pair of RF switches, and a third pair of RF switches. The first linear time-invariant network and the second linear time-invariant network each include a first port, a second port, and a third port. One end of each RF switch in the first pair of RF switches is respectively connected to the first port of the first linear time-invariant network and the first port of the second linear time-invariant network, and the other end is connected to the output port Port1. One end of each RF switch in the second pair of RF switches is respectively connected to the second port of the first linear time-invariant network and the second port of the second linear time-invariant network, and the other end is connected to the output port Port2. One end of each RF switch in the third pair of RF switches is respectively connected to the third port of the first linear time-invariant network and the third port of the second linear time-invariant network, and the other end is connected to the output port Port3.

[0012] In some specific embodiments, the first linear time-invariant network and the second linear time-invariant network each include three identical first FBAR filters, second FBAR filters, and third FBAR filters, the first FBAR filters, the second FBAR filters, and the third FBAR filters are connected end to end to form a triangle, and the connection between every two FBAR filters is used as an output port. A modulation signal is applied by three pairs of RF switches to construct a circulator three-port non-reciprocal network.

[0013] In some specific implementations, the first FBAR filter, the second FBAR filter, and the third FBAR filter are all ladder filters.

[0014] In some specific implementations, each FBAR filter adopts a four-step ladder filter structure.

[0015] In some specific embodiments, each ladder filter structure is composed of a parallel FBAR and a series FBAR, and the resonant area ratio of the parallel FBAR to the series FBAR is C ps The value range is 1~4.

[0016] In some specific embodiments, the parallel FBAR and the series FBAR both use cavity FBAR. The cavity structure is etched between the support layer and the substrate, and a cavity is formed on the silicon surface to achieve total reflection of the sound wave. It has the characteristics of high Q value, high stability, and integration.

[0017] In some specific embodiments, the structure of the cavity FBAR is from top to bottom: top electrode, piezoelectric layer, bottom electrode, support layer and substrate, and a cavity is formed between the support layer and the substrate. The cavity structure is etched between the support layer and the substrate, and a cavity is formed on the silicon surface to achieve total reflection of the sound wave. It has the characteristics of high Q value, high stability and integration.

[0018] In some specific implementations, the piezoelectric layer is made of aluminum nitride film, the top electrode and the bottom electrode are made of molybdenum, and the support layer is made of silicon dioxide.

[0019] When selecting materials for various parts of FBAR in the 2.45GHz frequency band, various influencing factors need to be considered. First, aluminum nitride (AlN) is selected as the piezoelectric film material because of its excellent properties such as longitudinal wave sound velocity and dielectric loss. Then molybdenum (Mo) is used as the electrode material. Its advantage is that its higher acoustic impedance can better confine the sound energy to the piezoelectric area. Since the role of the support layer is to enhance the mechanical stability of the FBAR component, a low-stress material between the bottom electrode and the substrate is usually selected, and silicon dioxide (SiO 2) to ensure that the FBAR filter maintains stability and durability during operation. At the same time, the higher heat transfer efficiency of silicon dioxide is also beneficial to improve the power capacity of the device. Therefore, it was determined that the piezoelectric film of the FBAR filter is aluminum nitride, the electrode material is molybdenum, and the support layer is silicon dioxide.

[0020] The resonant frequency is adjusted by optimizing the thickness of each FBAR film layer, and the center frequency and bandwidth of the FBAR are optimized by changing the thickness of the electrodes and piezoelectric layers of the series and parallel FBARs. In terms of process implementation, the electrode thickness of the bottom layer of the series and parallel FBARs based on the circulator non-reciprocal network design of the FBAR filter is often kept consistent and should not be too thin to prevent the quality factor of the FBAR from being reduced due to increased ohmic losses.

[0021] The beneficial effects of the present invention compared with the prior art are: 1. Since RF switch modulation does not require an additional bias network, the modulation ratio is easy to control and the energy loss is smaller, the present invention uses RF switch modulation to replace the existing varactor diode and switch capacitor modulation, and adopts FBAR filter to achieve the 2.45GHz operating frequency of the circulator; by combining RF switch and FBAR filter, the designed circulator has higher operating frequency, better reliability, higher modulation ratio and other performances; 2. A circulator based on FBAR filter is designed with a differential circuit structure. The operating frequency, modulation ratio and isolation of the circulator are effectively improved, the insertion loss of the circulator is reduced, and the device is planarized and integrated. The differential circuit structure is often used to suppress the zero drift of the device and enhance the signal. This design uses a circulator design with a differential circuit structure to offset the influence of intermodulation products, thereby effectively reducing the insertion loss of the circulator and improving the isolation to a certain extent.

[0022] 3. The circulator designed by using FBAR filter based on cavity-type bulk acoustic wave resonator (high Q value) can achieve high-frequency performance and meet the high-frequency requirements of WIFI operating frequency 2.45GHz; it has high isolation and low loss, with an isolation of 18.5dB, meeting the requirements of full-duplex communication. The differential structure is used to eliminate the influence of intermodulation products at the RF port, and the insertion loss is reduced to 2.4dB. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a circulator based on a thin film bulk acoustic wave filter provided in an embodiment of the present invention; Figure 2 A schematic diagram of the structure of a single-ended circulator provided in an embodiment of the present invention; Figure 3 A schematic diagram of the structure of an FBAR filter provided in an embodiment of the present invention; Figure 4 A schematic diagram of the structure of the FBAR provided in an embodiment of the present invention; Figure 5 A schematic diagram of a circulator test device based on an FBAR filter provided in an embodiment of the present invention; Figure 6 A schematic diagram showing the effect of different resonant region areas on the port transmission coefficient S21 provided in an embodiment of the present invention; Figure 7 Different filter orders and area ratios C provided in the embodiment of the present invention ps Schematic diagram of the impact on out-of-band suppression; Figure 8 A schematic diagram of how the modulation frequency varies with isolation at different duty cycles provided by an embodiment of the present invention; Fig. 9 A schematic diagram of the influence of duty cycle and isolation provided in an embodiment of the present invention; Fig.10 A schematic diagram of modulation frequency-isolation / insertion loss provided by an embodiment of the present invention; Fig.11 A schematic diagram showing the influence of order-isolation / insertion loss provided by an embodiment of the present invention; Fig.12 A schematic diagram of simulation results without applying modulation provided by an embodiment of the present invention; Fig.13 A schematic diagram of a simulation result of applying a modulation signal provided in an embodiment of the present invention; Fig.14 A schematic diagram of performance simulation of a differential circulator provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] In order to reflect the creativity and novelty of the present invention, the present invention will be described below in conjunction with the accompanying drawings and specific embodiments, but the embodiments of the present invention are not limited thereto.

[0025] Circulators are important components in current wireless communication systems. Traditional ferrite circulators have the disadvantages of being large, costly, and difficult to integrate. Although some new circulators have improved these disadvantages, they still cannot simultaneously meet the performance requirements of circulators such as high frequency, high isolation, and low power consumption. Therefore, the designed circulator needs to be optimized to meet the following performance requirements: 1. High frequency: To meet the high frequency requirements of modern communication and radar systems, a circulator operating in the WIFI frequency band of about 2.45 GHz is designed; the circulator is required to be small in size, light in weight, easy to integrate and apply to various compact devices.

[0026] 2. High isolation: to prevent signal interference and ensure a clear signal path, thereby improving the transmission performance and reliability of communication systems and radars.

[0027] 3. Low power consumption: ensure signal strength, improve system efficiency and performance, reduce power loss, and extend the service life of the circulator.

[0028] In order to achieve the above purpose, the present application designs a circulator based on a thin film bulk acoustic wave filter, and the specific design method and structure are as follows: Example 1 like Figure 1 As shown, this embodiment proposes a circulator based on a thin film bulk acoustic wave filter, including two single-ended circulators 1 and single-ended circulators 2 with the same structure, and the single-ended circulators 1 and the single-ended circulators 2 both include output port Port1, output port Port2 and output port Port3. It can be seen that the output port Port1 of the single-ended circulator 1 and the output port Port1 of the single-ended circulator 2 are connected together, the output port Port2 of the single-ended circulator 1 and the output port Port2 of the single-ended circulator 2 are connected together, and the output port Port3 of the single-ended circulator 1 and the output port Port3 of the single-ended circulator 2 are connected together. The ports of the single-ended circulator 1 and the single-ended circulator 2 are interconnected to form a differential structure. The differential circuit of the circulator is composed of two identical single-ended circulators connected in a symmetrical form, and a modulation signal with different phases is applied to the single end. By applying a modulation signal with a phase difference of 120° on the single-ended circulator, a dynamic balance of the intermodulation product is achieved. The input signal in the circuit only has a specific frequency fundamental frequency component and a first-order harmonic intermodulation component, but no higher-order intermodulation components, so it can be optimized through a differential circuit. At the single-ended circulator RF interface, the intermodulation products appear as out-of-phase current modes, which cancel each other out through node addition.

[0029] Each single-ended circulator includes a three-port non-reciprocal network, which includes two linear time-invariant networks formed by thin film bulk acoustic wave filters and three pairs of RF switches for modulating the two linear time-invariant networks. Prioritize, since the RF switch plays a key role in the modulation of the circulator, the single-pole double-throw switch AS179-92LF is selected on the basis of meeting the requirements of this design, that is, each pair of RF switches is represented by a single-pole double-throw switch.

[0030] The non-reciprocal circuit of the circulator mainly includes two sets of linear time-invariant filter networks composed of three 2.45GHz FBAR filters and three pairs of RF switches. The carrier signal is input from the input end of the RF switch S1, modulated by the control end of the switch, and then output to the FBAR filter network. RF switches S2 and S3 input two other sets of square wave signals with different phases for modulation. Since modulation signals with different phases are introduced in the whole process, the reciprocity of the circulator is broken.

[0031] like Figure 1As shown, the three-port non-reciprocal network of the single-ended circulator 1 includes a first linear time-invariant network LTI1, a second linear time-invariant network LTI2, a first pair of RF switches (S 1 , )、The second pair of RF switches (S 2 , ), the third pair of RF switches (S 3 , ), the first linear time-invariant network and the second linear time-invariant network each include a first port 1, a second port 2 and a third port 3, the conducting end of the RF switch S1 is respectively connected to the first port 1 of the first linear time-invariant network LTI1, the switch end is connected to the RF switch The switch end is used as the output port Port1, and the RF switch The conducting end of the RF switch S2 is connected to the first port 2 of the first linear time-invariant network LTI1, and the switch end is connected to the RF switch The switch end is used as the output port Port2, and the RF switch The conducting end of the RF switch S is connected to the second port 2 of the second linear time-invariant network LTI2; 3 The conducting end is connected to the third port 3 of the first linear time-invariant network LTI1, and the switch end is connected to the RF switch The switch end is used as the output port Port3, and the RF switch The conduction end of is connected to the third port 3 of the second linear time-invariant network LTI2.

[0032] Since the structures of the single-ended circulator 1 and the single-ended circulator 2 are the same, the structures of the third linear time-invariant network LTI3 and the fourth linear time-invariant network LTI4 included in the three-port non-reciprocal network of the single-ended circulator 2 are the same as the connection structure of the single-ended circulator 1, and are not described here. Since the structures of the four linear time-invariant networks are the same, the specific structure of the single-ended circulator 1 is described below by taking the single-ended circulator 1 as an example.

[0033] like Figure 2 As shown, the first linear time-invariant network LTI1 and the second linear time-invariant network LTI2 both include three identical first FBAR filters F1, second FBAR filters F2 and third FBAR filters F3 having two-port networks. The first FBAR filters F1, the second FBAR filters F2 and the third FBAR filters F3 are connected end to end to form a triangle. The ports of the first FBAR filter F1 and the second FBAR filter F2 are connected as the first port 1, the ports of the first FBAR filter F1 and the third FBAR filter F3 are connected as the second port 2, and the ports of the second FBAR filter F2 and the third FBAR filter F3 are connected as the third port 3.

[0034] from Figure 2 It can be seen that after modulation by the RF switch, a linear time-invariant network is connected to the circulator each time, and the input signal is transmitted from Port1 to Port2, and then from Port2 to Port3 output, strictly following the characteristics of the circulator's unidirectional transmission of high-frequency signals, thereby achieving signal isolation. In order to verify that the circulator designed using the RF switch and the FBAR filter can achieve this non-reciprocal characteristic, a feasibility analysis is conducted. First, a qualitative analysis is performed from the perspective of the frequency domain. The ideal circulator scattering matrix is ​​expressed as: The above formula shows that the input signal at Port1 is strictly transmitted to Port2 without any reflection or leakage to Port3. The input signal at Port2 or Port3 is transmitted to Port3 and Port1 respectively. Since the amplitude and frequency of the three ports are equal and the phase difference is 120°, Port1 can be represented by the common mode state (0), Port2 by the right-hand mode (+), and Port3 by the left-hand mode (-). When no modulation is applied, the left-hand mode frequency = the right-hand mode frequency; after the modulation signal is applied, the left-hand mode frequency ≠ the right-hand mode frequency. When the circulator is working normally, the RF carrier frequency W is modulated by the RF switch. RF Convert to sideband W RF ±W 0 Due to the modulation of the RF switch, the signal of Port1 is transmitted to Port2 and Port3 in three ways. Specifically, part of the RF power of Port1 can be directly transmitted to Port2 and Port3 without frequency conversion, and part of the power is up / down converted to the intermodulation (IM) frequency through switch S1 under the influence of the RF switch modulation, and then down / up converted to the RF carrier frequency (W) by switches S2 and S3. RF ±W 0 ). Due to the different phases of the modulated signals, the phases of the signals in the other two transmission paths except the normal path change, resulting in different frequency components received by the two ports, thus breaking the reciprocity and realizing the circulator function.

[0035] Since multiple adjacent passband signals are often transmitted simultaneously during the operation of the circulator, there are high requirements for the suppression of adjacent stopbands. The grid type cannot achieve fast roll-off to meet the out-of-band suppression of adjacent stopbands. Specifically, when designing the FBAR filter, the first FBAR filter, the second FBAR filter and the third FBAR filter all use ladder filters because the trapezoidal FBAR filter can make up for the problem that the grid filter does not have a transmission zero point, slow roll-off and poor rectangular coefficient performance. In this example, a fourth-order ladder filter is adopted to achieve higher out-of-band suppression (>25dB) of adjacent stopbands. Ultimately, a BAW filter with a center frequency of 2.45GHz and a bandwidth of 100MHz can be realized. More specifically, the composition of the fourth-order ladder filter can adopt a combination of L-type and π-type designs, such as Figure 3 As shown, the order of the filter is determined by the number of series-parallel connections, and the fourth-order ladder filter can be composed of a first-order L-type and a second-order π-type to form a fourth-order filter. Optionally, the fourth-order ladder filter can also adopt a fourth-order T-type filter.

[0036] like Figure 4 As shown, in this application, both parallel FBAR and series FBAR use cavity FBAR. Specifically, the structure of the cavity FBAR is from top to bottom: top electrode, piezoelectric layer, bottom electrode, support layer and substrate, and a cavity is formed between the support layer and the substrate. The cavity structure is etched between the support layer and the substrate, and a cavity is formed on the silicon surface to achieve total reflection of sound waves. It has the characteristics of high Q value, high stability and integration.

[0037] When selecting materials for each part of FBAR, various influencing factors need to be considered. First, aluminum nitride (AlN) is selected as the piezoelectric film material because of its excellent properties such as longitudinal wave sound velocity and dielectric loss. Then molybdenum (Mo) is used as the electrode material. Its advantage is that its higher acoustic impedance can limit the mechanical energy within the piezoelectric layer. Since the role of the support layer is to enhance the mechanical stability of the FBAR component, a low-stress material between the bottom electrode and the substrate is usually selected, and silicon dioxide (SiO 2) to ensure that the FBAR filter maintains stability and durability during operation. Therefore, it is determined that the piezoelectric film of the FBAR filter is aluminum nitride, the electrode material is molybdenum, and the supporting layer is silicon dioxide. In some specific embodiments, the piezoelectric layer uses aluminum nitride film, the materials of the top electrode and the bottom electrode are molybdenum, and the supporting layer uses silicon dioxide. And in terms of process implementation, the bottom layer of the series and parallel FBARs is based on the circulator non-reciprocal network design of the FBAR filter. The thickness of the electrode of the two is often consistent. The thickness of the supporting layer, bottom electrode and piezoelectric layer of the series-parallel FBAR in the circuit is the same. The center frequency of the filter of 2.45GHz is achieved by changing the thickness of the top electrode of the series-parallel resonator. The electrode thickness should not be too thin to prevent the quality factor of the FBAR from being reduced due to increased ohmic loss.

[0038] In order to better illustrate that the circulator designed by the above scheme in this embodiment has the performance of high frequency, high isolation and low loss, a circuit is built as follows Figure 5 The circulator test device based on the FBAR filter shown is used to verify and test various performances of the circulator.

[0039] According to the above design, when the filter design adopts FBAR design filter, a trapezoidal circuit structure is adopted; the filter insertion loss and out-of-band suppression performance are comprehensively considered, such as Figure 6-Figure 7 As shown in Figure 1, the influence of the resonant area ratio of the series-parallel FBAR of the filter on the output coefficient S21. If only the order of the filter is changed, such as Figure 6-Figure 7 It can be seen that when the resonator area ratio of the parallel FBAR to the series FBAR (C ps ) remains constant, the out-of-band suppression and the increase of the filter order show a synchronous improvement law. On the contrary, when the filter order is constant, the out-of-band suppression and C ps The relationship between them is linearly positively correlated. ps As the order of the filter increases, the increase in out-of-band suppression increases. Considering the influence of insertion loss, C ps It should not be excessively increased. In fact, although the two methods have little difference in total area, from the perspective of achieving the target out-of-band suppression, increasing the filter order is more effective. Therefore, in this embodiment, the FBAR filter is optimized, C ps Set it in the range of 1 to 4. Within this range, every increase of one order will increase the out-of-band suppression by about 10dB.

[0040] Therefore, preferably, when the resonant area of ​​the parallel FBAR is larger than that of the series FBAR, ps Take 4; the resonance area of ​​the series FBAR is 100x100μm 2 At this time, the center frequency of the filter is 2.45GHz and the bandwidth is about 100MHz.

[0041] After obtaining the above filter, when continuing to optimize the circulator design, it is necessary to study the influence of the FBAR filter's duty cycle, modulation frequency, order and other indicators on the circulator's isolation and insertion loss, and then realize the optimal design of the circulator. Figure 8-Figure 9 As shown in the figure, according to the change law of modulation frequency with isolation under different duty ratios, the isolation is optimal when it is 0.5, and the duty ratio has little effect on insertion loss; Fig.10 It can be seen that when the modulation frequency is 2.45MHz, the isolation is the largest and the insertion loss is the smallest. At this time, the circulator performance is optimal. When designing the filter order, the change law of the filter order, isolation and loss is taken into consideration, such as Fig.11 It can be seen that the higher the order of the filter, the higher the isolation and the higher the insertion loss; through the above analysis, it is determined that the circulator performance is good when the modulation frequency is 2.45MHz and the switching duty cycle is 0.5, and the change of the filter order will also affect the performance indicators of the circulator, so the analysis of the filter order is essential. When the circulator performance changes with the filter order from 1 to 5, the circulator isolation shows an upward trend with the increase of the order, and the insertion loss also increases, but the amplitude of the change (the slope of the broken line) is different; the amplitude of the change of the circulator insertion loss increases after the filter order is 3rd order, which means that the insertion loss increases faster and faster, while the amplitude of the change of the isolation decreases, which means that the isolation increases slower and slower. Under the premise of meeting the lowest possible insertion loss, a third-order filter was selected as the simulation basis of the circulator based on the FBAR filter after comprehensive consideration. However, when a third-order filter is used, the isolation is 17.5dB and the insertion loss is 16.8dB (urgently needs to be reduced). Therefore, this application reduces the insertion loss to 2.4dB by using a differential structure to eliminate the influence of the intermodulation products of the RF port.

[0042] In order to verify the non-reciprocity achieved by modulation using RF switches, such as Fig.12 As shown in Figure 1, when no modulation is applied, the input power of Port 1 is evenly distributed to Port 2 and Port 3, so S21 and S31 coincide. However, after applying a 2.45MHz modulation signal, as shown in Figure 1, Fig.13 As shown, S21 and S31 no longer coincide, resulting in non-reciprocity.

[0043] When two LTI networks are used to design the circulator, the isolation is 17.5dB, but the insertion loss is 16.8dB, which is too high and needs to be improved. After the differential design is adopted in this application, Fig.14 As shown in the figure, all performances have been improved, the intermodulation products generated at the RF port have been eliminated, the isolation has been slightly increased to 18.5dB, and the insertion loss has been significantly reduced to 2.4dB.

[0044] The performance comparison between the circulator designed in this application and the existing circulator is shown in the following table: It can be seen that the present application adopts a circulator design of "RF switch and FBAR filter". The RF switch modulation does not require an additional bias network, the modulation ratio is easy to control, and the energy loss is smaller. The FBAR filter can achieve a 2.45GHz operating frequency of the circulator. The circulator designed by combining the RF switch and the FBAR filter has higher operating frequency, better reliability, higher modulation ratio and other performance. In addition, the present application adopts a circulator design with a differential circuit structure, which uses the differential circuit structure to suppress the zero drift of the device and enhance the signal, offset the influence of the intermodulation product, thereby effectively reducing the insertion loss of the circulator, and at the same time improving the isolation to a certain extent.

[0045] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A circulator based on a thin film bulk acoustic wave filter, characterized in that: It includes two single-ended circulators with the same structure. The ports of the two single-ended circulators are interconnected to form a differential structure. Each single-ended circulator includes a three-port non-reciprocal network. The three-port non-reciprocal network includes two linear time-invariant networks formed by thin film bulk acoustic wave filters and three pairs of radio frequency switches for modulating the two linear time-invariant networks.

2. A circulator based on a thin film bulk acoustic wave filter according to claim 1, characterized in that: Each three-port non-reciprocal network includes a first linear time-invariant network, a second linear time-invariant network, a first pair of RF switches, a second pair of RF switches, and a third pair of RF switches. The first linear time-invariant network and the second linear time-invariant network both include a first port, a second port, and a third port. One end of each RF switch in the first pair of RF switches is respectively connected to the first port of the first linear time-invariant network and the first port of the second linear time-invariant network, and the other end is connected to the output port Port1. One end of each RF switch in the second pair of RF switches is respectively connected to the second port of the first linear time-invariant network and the second port of the second linear time-invariant network, and the other end is connected to the output port Port2. One end of each RF switch in the third pair of RF switches is respectively connected to the third port of the first linear time-invariant network and the third port of the second linear time-invariant network, and the other end is connected to the output port Port3.

3. A circulator based on a thin film bulk acoustic wave filter according to claim 2, characterized in that: The first linear time-invariant network and the second linear time-invariant network both include three identical first FBAR filters, second FBAR filters and third FBAR filters, the first FBAR filters, the second FBAR filters and the third FBAR filters are interconnected end to end, and the connection ports of every two FBAR filters serve as output ports.

4. A circulator based on a thin film bulk acoustic wave filter according to claim 3, characterized in that: The first FBAR filter, the second FBAR filter and the third FBAR filter all adopt ladder filters.

5. The circulator based on the thin film bulk acoustic wave filter according to claim 3, characterized in that: Each FBAR filter adopts a four-step ladder filter structure.

6. The circulator based on the thin film bulk acoustic wave filter according to claim 5, characterized in that: Each ladder filter structure is composed of a parallel FBAR and a series FBAR, and the resonant area ratio of the parallel FBAR to the series FBAR is C ps The value range is 1~4.

7. A circulator based on a thin film bulk acoustic wave filter according to claim 6, characterized in that: The parallel FBAR and the series FBAR both adopt cavity type FBAR.

8. The circulator based on the thin film bulk acoustic wave filter according to claim 7, characterized in that: The structure of the cavity-type FBAR is, from top to bottom, a top electrode, a piezoelectric layer, a bottom electrode, a support layer and a substrate, with a cavity formed between the support layer and the substrate.

9. The circulator based on the thin film bulk acoustic wave filter according to claim 8, characterized in that: The piezoelectric layer is made of aluminum nitride film, the materials of the top electrode and the bottom electrode are molybdenum, and the supporting layer is made of silicon dioxide.

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