RU2793624C1 - Switchable two-band filter on surface acoustic waves - Google Patents

Abstract

FIELD: communication systems.

SUBSTANCE: adjustable frequency filtering of signals for communication systems. Invention is related in particular to switchable filters based on surface acoustic waves (SAW), with operating frequency in the range of 10-2500 MHz regulated by voltage. The switchable two-band filter contains four parallel acoustic channels formed by two input interdigital converters (IDTs), two microstrip directional couplers (MSDCs), and two output IDTs with the ability to switch each pair of acoustic channels with its independent vanadium dioxide film switch. The width of the vanadium dioxide film is no more than two periods of the input IDT, and the thickness of the vanadium dioxide film is 200 nm.

EFFECT: increase of the difference in attenuation values between the acoustic channels up to 70 dB, lowering of control voltage to 0.1 V and reduction of attenuation due to SAW propagation with increasing frequency.

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Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The invention relates to acoustoelectronics, namely to adjustable signal frequency filtering for communication systems, in particular, to switchable filters on surface acoustic waves, the operating frequency of which in the range of 10 MHz - 2500 MHz is regulated by voltage.

BACKGROUND OF THE INVENTION

Currently, cellular communication systems use surface acoustic wave (SAW) frequency filters with a fixed center frequency. Such filters are used in cell phones, television transmitters and receivers. However, these filters are usually fixed frequency filters with no tuning. For practical applications, miniature narrow-band filters with a voltage-adjustable center frequency are needed. The main problem is that adding bands to mobile devices has become complicated and not cost effective. Achieving this filtering performance for mobile devices requires a 20 percent tuning range with less than 3 dB insertion loss for voltage applications from 3.5V to 30V. can cover many frequency ranges. Currently, these interface modules are built using switched filter banks based on a large number of discrete filters and multiplexing switches at the input and output, which leads to an increase in module size, cost and power consumption. There is a need for an adaptive filtering design where frequency channels can be added or removed while maintaining the fewest required elements.

For modern mobile communication systems, a necessary requirement is the support of operation in several frequency bands, achieved through the implementation of tunable input stages. To create such cascades, it is necessary to develop high-frequency (HF) tunable and switchable filters based on surface acoustic waves (SAWs) capable of changing the bandwidth and center frequency, having high selectivity and low insertion loss. In addition, such a filter should have small overall dimensions and low power consumption.

Switchable SAW filters are known. For example, in the patent (US 6459345 - 2002-10-01, IPC H03H 9/64) [1] a tunable surfactant filter is presented, the characteristics of which can be programmed. It contains a piezoelectric substrate, on the surface of which there are input and output unidirectional three-phase interdigital transducers (IDT), and the groups of electrodes of each of the phases are connected to the generator (or receiver) through controlled phase shifters and amplifiers. This allows you to change the phase and amplitude characteristics of the filter by changing the phase and gain of the amplifiers and program the necessary characteristics in advance.

A tunable filter based on frequency selective matrices is known (RU 2121214, IPC H03H 9/72, publ. 1998-10-27) [2]. These matrices represent an n-channel SAW filter in which different channels are switched using switches. In this case, the input IDTs of each channel have a small aperture, and the output IDTs are common to all channels and have an aperture at least n times larger. This leads to a significant increase in insertion loss, since the SAW emitted by the transmitting IDT fall only on a small part of the receiving IDT, which leads to SAW reradiation by this IDT in places where there is no SAW from the transmitting IDT, which is a disadvantage of such a tunable filter.

Tunable surface acoustic wave resonators and SAW filters with digital to analog converters are known. ; H03H9/64; H03M1/66, publ. 05/26/2020) [3], where external elements for filter restructuring are excluded. A constant bias voltage is applied to the IDT electrodes located on the semiconductor layer, which leads to a shift in the central frequency of the IDT, and, consequently, to a shift in the frequency of the SAW resonator. In this case, there are no amplifiers, phase shifters and switches in the tunable filter. However, for the manufacture of such filters, multilayer structures are required in order to form regions under each IDT electrode, where the SAW speed will change under the action of the voltage applied to the electrodes, which complicates the technology for manufacturing such filters, since it requires precise alignment of semiconductor layers with IDT electrodes, the dimensions of which may be less than 1 µm.

Known multi-channel surface acoustic wave filter device with voltage controlled tunable frequency response, patent US 7821360, IPC H03H 9/64, publ. 26.10. 2010) [4], in in which the input and output IDTs are several partial IDTs connected in parallel with different periods, calculated in such a way that the frequency response of such a filter is comb. Partial IDTs are located on the semiconductor layer. There is a bottom electrode on the bottom surface of the substrate. By applying a voltage between the bottom electrode and one of the IDT combs, it is possible to change the SAW velocity in the layer, thereby changing the central frequency of the partial IDTs. Moreover, the voltage applied to the input and output IDT can be different. This leads to the fact that the central frequencies of partial IDTs located in one acoustic channel may differ, which leads to a change in the attenuation in the amplitude-frequency characteristic in this acoustic channel. Here, the layer in which the SAW velocity changes under the action of voltage is not located under each IDT electrode, but the IDT is deposited directly on this layer. In this filter, you can change the attenuation between different channels up to 50-55 dB. complicates the manufacturing technology of such filters, since it requires precise alignment of semiconductor layers with IDT electrodes, the dimensions of which can be less than 1 μm, complicates the manufacturing technology of such filters

Another approach for obtaining tunable filters is the use of resonator SAW filters, which use additional capacitances, by changing which you can change the amplitude-frequency characteristics of the filter (V. A. Arzhanov, I. V. Veremeev. Basic approaches to the development of modern tunable and switchable high-frequency SAW filters, OMSK SCIENTIFIC VESTNIK No. 1 (127) 2014) [5]. The basic structure of a tunable filter is exactly the same as that of a regular ladder filter. Varicaps are connected to SAW resonators in series and parallel branches of the filter. Varicaps connected in parallel with SAW resonators shift their antiresonant frequencies, while varicaps connected in series shift their resonant frequencies. Thus, the position of the upper and lower limits of the filter bandwidth is adjusted independently. To achieve a large tuning range in this type of filter, it is necessary to create a broadband SAW resonator. Thus, it is possible to obtain attenuation differences in the tunable channels by only 7-10 dB.

For further miniaturization of such filters, see (Arash Fouladi Azarnaminy, Junwen Jiang, Raafat R. Mansour, Switched Dual-Band SAW Filter Using Vanadium Oxide Switches. Published 7 June 2021, Physics 2021 IEEE MTT-S International Microwave Symposium (IMS) [6 ] for filter tuning on SAW resonators, not variable voltage-controlled capacitances (varicaps) are used, but vanadium oxide films, the resistance of which changes sharply at temperatures above the phase transition temperature.This resistance varies from 235 ohms to 0.8 ohms. Switching occurs by heating The main advantage of using VO2 switches over MEMS or solid state switches is that VO2 switches can be monolithically integrated with SAW filters on a single chip. losses, and the disadvantage is that it is possible to obtain a attenuation difference in tunable channels of no more than 7 - 10 dB.

This drawback can be eliminated by creating a periodic structure between the IDTs in the form of a directional coupler, the period of which differs from the SAW length, so that the SAW reflection band from such a periodic structure would be outside the filter bandwidth.

So, for example, a two-channel tunable filter on surface acoustic waves (SU 915225, IPC H03H 9/64, publ. 03/23/1982) [7], taken as a prototype of the present invention, as the closest in technical essence, contains a piezoelectric It has one input and two output interdigital transducers forming two parallel acoustic channels, and a multi-strip directional coupler located between the input and output transducers. In order to simplify the design and improve reliability, control electrodes connected to a voltage source are located on both sides of the multistrip directional coupler, and a semiconductor film is applied to the surface of the multistrip directional coupler and control electrodes. The output IDTs have different center frequencies, and the total bandwidth of the output IDTs does not exceed the bandwidth of the input IDT. The SAW is excited by the input transducer and, propagating along the piezoelectric sound duct, enters the multi-strip directional coupler. The pitch and the number of pins of the multistrip directional coupler are chosen in such a way that in the absence of a control voltage, all the energy emitted by the input converter is reradiated to the output converter. The voltage applied to the control electrodes creates a drift of charge carriers in the direction of propagation of the acoustic wave, while the effective electromechanical coupling coefficient is a function of the electric field strength in the semiconductor film. The use of lithium niobate and an indium antimonide semiconductor film with a charge carrier mobility of 1250 cm2/V s and a thickness of 0.5 μm as a piezoelectric acoustic conductor makes it possible to obtain complete signal switching between channels when the control voltage changes from 0 to 60 V.

The disadvantage of this design is a rather high voltage (up to 60 V), which must be applied to the control electrodes. In addition, the difference in attenuation in different acoustic channels does not exceed 20 dB, even if the effective electromechanical coupling coefficient changes by a factor of 10. In this case, complete switching occurs only in one acoustic channel, and in the other acoustic channel - in a frequency band smaller than the filter bandwidth, in this case, signals with the same amplitudes can be in both bands. In addition, since the semiconductor film is 0.5 µm thick, it can introduce significant SAW propagation attenuation with increasing frequency, which means that the operating frequencies of such a tunable filter are limited.

The objective of the present invention is to create a switchable two-band filter on surface acoustic waves that eliminates these disadvantages by achieving new technical results.

The technical result of the present invention is to increase the difference in attenuation values between the acoustic channels up to 70 dB, lower the control voltage to 0.1 V and reduce the attenuation due to SAW propagation with increasing frequency.

The specified technical result is achieved by the fact that the switchable two-band filter on surface acoustic waves (SAW) contains a piezoelectric sound duct, on the working surface of which there are the first input and two output interdigital transducers (IDT) with different central frequencies and between them the first multistrip directional coupler ( MPNO) interacting with the film switch of acoustic channels, forming the first and second parallel acoustic channels.

According to the invention, the filter additionally contains a second input IDT connected in series with the first input IDT, and a second MPNO, while the first MPNO and the second MPNO are located on the same axis perpendicular to the SAW propagation direction, forming additional third and fourth parallel acoustic channels, while each MPNO is made in the form of an interdigital system (IAS) and has an electrical contact in the middle part with an acoustic channel switch made in the form of a transverse strip of a vanadium dioxide film with a sublayer of titanium dioxide formed on the working surface of the sound duct, the tires of each IAAS are connected through a restrictive resistor to the control voltage source, the bus of the first input IDT is connected to the filter input through a matching inductance, all IDTs are made unidirectional with internal reflectors, the period of the first input IDT is equal to the period of the first output IDT, and the period of the second input IDT is equal to the period of the second output IDT and the periods of these pairs The IDTs correspond to the center frequencies of the lower and upper passbands of the switchable filter.

In the preferred embodiments of the filter:

- the first VSS MPNO is from 0.5 to 1.5 periods of the input IDT of the first acoustic channel;

- the period of the second IDT MPNO is from 0.5 to 1.5 periods of the input IDT of the third acoustic channel;

- the width of the vanadium dioxide film is no more than two periods of the input IDT;

- the thickness of the vanadium dioxide film is 200 nm.

The implementation of four parallel acoustic channels formed by two input IDTs, two MPNOs and two output IDTs with the ability to switch each pair of acoustic channels with its independent vanadium dioxide film switch ensures attenuation between channels of at least 70 dB.

Since the first IDT, the IDT is from 0.5 to 1.5 periods of the input IDT of the first acoustic channel, which ensures that there are no SAW reflections from the coupler in the passband of the first input IDT and the first output IDT.

Since the period of the second IDT MPNO is from 0.5 to 1.5 periods of the input IDT of the third acoustic channel, which ensures the absence of SAW reflections from the coupler in the passband of the second input IDT and the second output IDT .

The use of acoustic channel switches made of a vanadium dioxide film provides a decrease in the control voltage to 0.1 V. Since the vanadium dioxide film is made in the form of a narrow strip with a width of no more than two periods of the input IDT, the SAWs practically do not cross it, which eliminates the increase in SAW attenuation with increasing operating frequencies of the filter. The thickness of the vanadium dioxide film is 200 nm and was chosen experimentally in order to exclude the short circuit of the electrodes of the HSS MPNO to ensure the passage of SAW from one acoustic channel to another in the absence of voltage on the buses of the HSS MPNO.

LIST OF GRAPHIC FIGURES

The claimed invention is illustrated by the figures of the drawings.

Fig. 1 - Switchable two-band filter on surface acoustic waves, top view.

Fig. 2 - Switchable two-band filter on surface acoustic waves, top view, where horizontal lines show acoustic channels 1, 2, 3, 4.

Fig. 3 - Frequency response (AFC) of a switchable two-band SAW filter, measured in the absence of a control voltage on the MPNO, where 1 is the frequency response of the filter in the low-frequency band, 2 is the frequency response of the filter in the high-frequency band, 3 is the frequency response of the MPNO in the second or in fourth acoustic channels, 4 - frequency response of MPNO in the first or third acoustic channels.

Fig. 4 - Frequency response (AFC) of a switchable two-band SAW filter, measured under the following conditions: the coupler in the low-frequency band is closed (on the VShS4 buses, the control voltage is U = 0.1 V), and the coupler in the high-frequency band is open (on the VShS 5 buses control voltage U=0 V), where 1 is the AFC of the filter in the low-frequency band, 2 is the AFC of the filter in the high-frequency band, 3 is the AFC of the MPNO in the second acoustic channel, 4 is the AFC of the MPNO in the first acoustic channel.

Fig. Fig. 5. Frequency response (AFC) of a switchable two-band SAW filter, measured under the following conditions: the coupler in the high-frequency band is closed (on the VShS MPNO 5 buses, the control voltage is U = 0.1 V), the coupler in the low-frequency band is open (on the VShS MPNO 4 buses control voltage U=0 V), where 1 is the frequency response of the filter in a lower frequency band, 2 is the frequency response of the filter in the high frequency band, 3 is the frequency response of the MPNO in the first acoustic channel, 4 is the frequency response of the MPNO in the second acoustic channel

. Fig. Fig. 6. The amplitude-frequency characteristics (AFC) of a switchable two-band SAW filter, measured under the following conditions: the coupler in the low-frequency band is closed (on the VShS MPNO 4 buses, the control voltage is U = 0.1 V), and the coupler in the high-frequency band is closed (on the VShS MPNO 4 buses 5 control voltage U = 0.1 V), where 1 is the frequency response of the filter in a lower frequency band, 2 is the frequency response of the filter in the high frequency band, 3 is the frequency response of the MPNO in the first or third acoustic channels, 4 is the frequency response of the MPNO in the second and fourth acoustic channels.

The switchable two-band filter on surface acoustic waves (figure 1) contains a piezoelectric sound guide 1 of a rectangular shape, on the working surface of which on one side there are formed connected in series the first input IDT 2 and the second input IDT 3 having the same period, two multi-strip directional couplers MPNO 4 and MPNO 5 in the form of interdigital structures (ISWs), located on the same axis, perpendicular to the direction of propagation of the surfactant. VShS MPNO 4 has an electrical contact in the middle part with an acoustic channel switch made in the form of a transverse strip of a film of vanadium dioxide 6 with a sublayer of titanium dioxide formed on the working surface of the sound duct 1. VSHS MPNO 5 has an electrical contact in the middle part with another switch of acoustic channels , made in the form of a transverse strip of vanadium dioxide film 7 with a sublayer of titanium dioxide, formed on the working surface of the sound duct 1. On the sound duct 1, two output interdigital transducers VDT 8 and VDT 9 with different center frequencies are also formed. VShS MPNO 4 and VShS MPNO 5 are located between the input VShP 2, VShP 3 and the output VShP 8, VSW 9. Buses VShS MPNO 4 are connected through a limiting resistor 10 to the control voltage source 11, and the bus VSHS MPNO 5 is connected through a limiting resistor 12 to the source control voltage 13. The first VSS MPNO 4, interacting with the film switch 6 acoustic channels, forms the first and second parallel acoustic channels. The first acoustic channel contains VST 2 and VSS MPNO 4, the second acoustic channel - VST 8 and VSS MPNO 4. The second VSS MPNO 5, interacting with the film switch 7 acoustic channels, forms the second and third parallel acoustic channels. The third acoustic channel contains the IDT 3 and VSS MPNO 5, and the fourth acoustic channel - IDT 8 and VSS MPNO 5 (figure 2). The bus of the first input IDT 2 is connected to the filter input through a matching inductance 14, all IDTs are made unidirectional with internal reflectors, the first input IDT 2 and the first output IDT 8 have the same period of the interdigital structure. The period of the interdigital structure of the first IDT MPNO 4 is from 0.5 to 1.5 periods of the IDT 2 of the first acoustic channel, the period of the second IDS MPNO 5 is from 0.5 to 1.5 periods of the input IDT 3 of the third acoustic channel. The width of the vanadium dioxide film 4, 7 is no more than two periods of the input IDT 2 and IDT 3, and the thickness of the vanadium dioxide film is 200 nm and is selected in such a way as to ensure the passage of the SAW from one acoustic channel to another in the absence of voltage on the buses of the IDT MPNO.

The period of the first input IDT 2 is the same as the period of the first input IDT 8 and differs from the same periods of the second input IDT 3 and the second output IDT 9. These periods are determined by the center frequencies of the lower and upper passbands of the switchable filter.

Switchable two-band filter on surface acoustic waves works as follows. The signal applied to the filter input is fed to the input IDT 2 and IDT 3, connected in series through the inductance 14 to the filter input to compensate for the influence of the impedance of the IDT 2 or IDT 3, if the input signal is in one of the filter bands, which leads to a decrease in insertion attenuation, so as the voltage drop across the capacitive component of the IDT, whose operating frequencies are outside the frequency band of the input signal and whose impedance has a capacitive character. This voltage drop will be partially compensated by the voltage drop across the inductance, which will increase the voltage across the IDT, whose operating frequencies are in the input signal bandwidth. Next, the SAW, excited by the input IDT 2 and IDT 3, enter the acoustic channels 1 and 3 and enter the VSS MPNO 4 and 5 (figure 1, 2), which transfer the SAW to the acoustic channels 2 and 4. If the bus VSS MPNO 4 , 5, the voltage is not applied, necessary to ensure that these VSSs do not transfer SAWs from one acoustic channel to another, they transfer SAWs to another acoustic channel and SAWs enter the output VSHV 8 and VSHP 9 and signals appear at the filter outputs (Fig. 3, graphs 1, 2).

If the control voltage from the voltage source 11 is applied to the VSS MPNO 4, then the vanadium dioxide film 6 heats up and during the phase transition its resistance drops sharply and the VSS MPNO 4 does not transfer the SAW from the acoustic channel 1 to the acoustic channel 2 and the signal level drops at the output of the VDT 8 by 70 dB. At the same time, the film on the VSS MPNO 7 does not heat up, since the voltage source 13 does not supply the required voltage to the tires of the VSS MPNO 5 and the VSS MPNO 5 transfers the SAW from the acoustic channel 3 to the acoustic channel 4 and a signal appears at the output of the VSHS MPNO 9 (Fig.4, graph 2). In FIG. 4 marked: graph 3 - frequency response of the MPNO in the second acoustic channel, graph 4 - frequency response of the MPNO in the first acoustic channel, the frequency response of the MPNO in the third and fourth acoustic channels are similar to the frequency response of the VSS MPNO in the second and first acoustic channels, if the voltage is U = 0.1 V applied to the VSS MPNO 5, and there is no voltage on the buses of the VSS MPNO 4.

If the voltage necessary for the VSS MPNO to transfer the SAW from one acoustic channel to another is supplied from the voltage source 13 to the VSS MPNO 5, then it will not transfer the SAW from the acoustic channel 3 to the acoustic channel 4. the required voltage is applied from the source 11 and the signal appears at the output of the IDT 8, and at the output of the IDT 9 the signal is suppressed by 70 dB. This happens in the same way as in the previous case due to the fact that, due to heating, the resistance of the vanadium dioxide film 6 drops sharply and the HSS MPNO 4 does not transfer the surfactant from acoustic channel 3 to acoustic channel 4 (Fig. 5, graph 1). Figure 5 graph 3 - frequency response MPNO in the second acoustic channel, 4 - frequency response MPNO in the first acoustic channel. The frequency response of the MPNO in the third and fourth acoustic channels are similar to the frequency response of the MPNO in the second and first acoustic channels, if the voltage U=0.1 V is applied to the VSS MPNO 4, and there is no voltage on the buses of the VSS MPNO 5. If the control voltages from the sources 11 and 13 are supplied to the VShS MPNO bus, then the VShS MPNO 4 and the VSS MPNO 5 do not transfer SAW from one acoustic channel to another and the signals at the output of the VShP 8 and 9 fall by 70 dB (Fig.6, graphs 1 , 2). Unlike the prototype, the coupler is controlled not on the basis of a change in the electromechanical coupling coefficient due to the applied control voltage, but due to the fact that the vanadium dioxide film changes its resistance in the region of the phase transition, which leads to the short circuit of the coupler electrodes, which, at a certain resistance of the film, stops transfer SAW from one acoustic channel to another and transfers almost completely SAW to another acoustic channel if the film resistance is greater than a certain value. The voltage control of sources 11 and 13 is performed by the operator, or by a special control device according to a given algorithm, depending on the problem being solved, in which frequency channels can be added.

Let us show that the SAW passing from one acoustic channel to another with the help of the ISS MPNO depends on the resistance of the vanadium dioxide film. The SAW incident on such a coupler can be represented as symmetric and asymmetric modes [8, 9]. In this case, the asymmetric mode does not interact with it, since it induces opposite currents of equal magnitude in them, which are mutually compensated. The symmetric mode perceives these IDTs as a sequence of partial IDTs with an aperture equal to the sum of IDT apertures in different acoustic channels. In this case, at the output of this IDT, the SAW experiences an additional phase shift, which can be determined from the SAW transmission coefficient under the IDT. Then, at the output of the partial IDT, the SAW receives an additional phase shift, provided that Ga, Ва<< ωC _T ,

where Ga is the active component of the IDT conductivity,

f ₀ is the central frequency of the IDT (acoustic synchronism frequency), C _T is the capacitance of the partial IDT, N is the number of pairs of electrodes of the partial IDT,

k ² is the square of the electromechanical coupling coefficient, ω=2πf .

If there are M such partial IDTs, then the total phase shift of the symmetric mode on the IDT will be:

Then the frequency response of such a coupler at the output of the channel, where there is no input IDT, looks like:

And at the output of the channel, where the input IDT SAW is located, they are shifted by 90 ^° relative to the other channel, and the frequency response of the coupler has the form [10]:

Obviously, at φtot = π, the symmetric and asymmetric modes will be in antiphase and the SAW will completely transfer from one acoustic channel _to another, since sin (π/2) = 1. At the central frequency of the IDT X = 0, and the function ( sin X)/X=1. Then for 100% SAW transition from one acoustic channel to another NM=π ² /(4 k ² )= N ₀ M ₀ . In this case, at the center frequency in the channel where the input IDT is located, the SAW will not pass to the receiving IDT, and in another acoustic channel, the SAW emitted by the input IDT will fall on the receiving IDT. If now a load in the form of conductivity Y is connected to one partial IDT, then the phase shift of the symmetric mode on it will have the form:

). Тогда при фиксированном числе парциальных ВШП (M), когда имеется 100% переход ПАВ из одного акустического канала в другой при нулевой нагрузке (Y=0) может получится так , что ПАВ почти не будет переходить в другой акустический канал, если Y>>

. При условии, что Y<<

, нагрузка Y не будет влиять на фазу φ и ответвитель будет переводить ПАВ из одного акустического канала в другой. В этом случае:It follows from this expression that the phase shift will depend on the magnitude of the load and will be the smaller, the greater this load is the capacitive conductance of the partial IDT (

). Then, for a fixed number of partial IDTs ( M ), when there is a 100% SAW transition from one acoustic channel to another at zero load ( Y = 0), it can turn out that the SAW will almost not go to another acoustic channel if Y>>

. Provided that Y<<

, the load Y will not affect the phase φ and the coupler will transfer the SAW from one acoustic channel to another. In this case:

Where

φ≈0 и ПАВ проходят через ответвитель, не переходя в параллельный акустический канал, так как в этом случае H(f) ≈0, а H1(f) ≈1, а при Y<<

H1(f) ≈0, а H(f) ≈1 и ПАВ переходят в другой акустически канал, при условии NM=π²/(4k²). Очевидно, что при изменении Y от Y<

до Y>>

будет происходит плавный переход ПАВ из одного акустического канала в другой. Если положить N=1/2, то парциальный ВШП будет иметь всего 2 электрода и его относительная полоса пропускания будет более 100%. Это позволит выбрать период ВШС таким образом, чтобы длина ПАВ достаточно сильно отличалась бы от периода ВШС МПНО. Тогда ПАВ, не будет отражаться от такого ответвителя, так как его период будет отличаться от длины ПАВ. Вместе с тем из-за того, что относительная полоса пропускания парциального ВШП более 100%, частота ПАВ будет всегда находиться в его полосе пропускания и при определенном числе секций M будет происходить переход ПАВ из одного акустического канала в другой. Таким образом, каждый парциальный ВШП ответвителя в виде ВШС будет состоять и двух электродов. При этом получается, что у каждого такого ВШП соседние электроды можно заменить одним общим той же ширины, что и остальные. Тогда последовательность парциальных ВШП будет представлять собой последовательность полосков с периодом, который отличается от периода ПАВ, чтобы ПАВ не отражались от ответвителя в рабочей полосе частот переключаемого фильтра. Если теперь через один полоски соединить с шинами, которые будут располагаться как сверху, так и снизу последовательности полосков мы получим ВШС (ВШС 6 или ВШС 7 на фиг.1), которая как и ВШП будет иметь рабочие частоты, находящиеся вне рабочих частот входных и выходных ВШП перестраиваемого фильтра, как показано на фиг.1, так как период ВШС МПНО 4 и ВШС МПНО 5 составляет 0,5 – 1,5 периодов входных ВШП 2 и 3. При этом электрические сигналы, наводимые ПАВ в электродах ВШС будут складываться на шинах ВШС не синфазно, что приведет к тому, что напряжение между шинами ВШС будет близко к нулю. Это будет означать, что при подсоединении к шинам ВШС нагрузки она не будет оказывать никакого влияния на распространение ПАВ в акустических каналах. From these expressions, it follows thatY>>

φ≈0 and SAW pass through the coupler without passing into a parallel acoustic channel, since in this caseH(f) ≈0, AH1(f) ≈1, and whenY<<

H1(f) ≈0, AH(f) ≈1 and surfactants go to another acoustic channel, provided NM=π²/(4k²). Obviously, when changingY fromY<

beforeY>>

there will be a smooth transition of SAW from one acoustic channel to another. If we putN=1/2, then the partial IDT will have only 2 electrodes and its relative bandwidth will be more than 100%. This will allow one to choose the period of the SSW in such a way that the length of the SAW would be sufficiently different from the period of the VSW of the MONO. Then the SAW will not be reflected from such a coupler, since its period will differ from the length of the SAW. At the same time, due to the fact that the relative bandwidth of the partial IDT is more than 100%, the SAW frequency will always be in its bandwidth even for a certain number of sectionsM there will be a transition of SAW from one acoustic channel to another. Thus, each partial IDT coupler in the form of an IDT will also consist of two electrodes. It turns out that for each such IDT, adjacent electrodes can be replaced by one common electrode of the same width as the others. Then the sequence of partial IDTs will be a sequence of strips with a period that differs from the SAW period so that the SAWs are not reflected from the coupler in the operating frequency band of the switched filter. If we now connect through one strip to the buses, which will be located both above and below the sequence of strips, we will get the VSS (VSS 6 or VSS 7 in Fig.1), which, like the IDT, will have operating frequencies that are outside the operating frequencies of the input and output IDT of the tunable filter, as shown in Fig. 1, since the period of the IDT DNA 4 and the IDT DNA 5 is0.5 - 1.5 periods of the input IDTs 2 and 3. In this case, the electrical signals induced by the SAW in the electrodes of the IDT will be added out of phase on the IDT buses, which will lead to the fact that the voltage between the IDT buses will be close to zero. This will mean that when a load is connected to the busbars, it will not have any effect on the propagation of surfactants in acoustic channels.

If now, under the VSS MPNO 4 or VSS MPNO 5, films 8 and 9 of vanadium dioxide are applied in the middle in the form of a narrow strip with a width of 1-2 periods of the IDT 3, then it will be connected to each electrode and the parameters of the SAW passage through the coupler in the form of VSS 4 and VSS 5 with a film 6 and 7 will be determined by expressions 7 – 9, where the load Y = 1/R, R is the resistance of the film between adjacent electrodes of the VSS. In this case, a constant control voltage applied to the VShS MPNO 4 or VShS MPNO 5 buses will be applied to the loads Y, changing their resistance, which will lead to a change in the SAW distribution between the channels. The passage of current through the film between the electrodes of the IHS will cause its heating, which will lead to a change in its resistance due to the phase transition [11], which occurs in films of vanadium dioxide in the region of 50 ^° C - 70 ^° C and, according to expressions 7-9, the surfactant will be redistributed between the acoustic channels (figure 4, 5, graphs 1 and 2). When heated to the phase transition temperature, the film resistance will begin to drop sharply and may decrease by three orders of magnitude compared to the resistance at room temperature, which will lead to a sharp increase in current, which can lead to overheating and destruction of the film. To prevent this from happening, the control voltage source 11 or 13 is connected to the film through limiting resistors 10 or 12, which limits the current through the film when its resistance drops sharply and keeps it from destruction. Since the film is connected to all the VSS electrodes, even if the film resistance between the electrodes is 1000 Ω, the resistance between the VSS MPNO buses will be M times less, which does not exceed 10 Ω. Such resistance of the film is achieved by appropriate selection of its thickness, since the width of the film cannot be more than two periods of the input IDT 2 or input IDT 3. Then, in order to cause the film to heat up to temperatures above the phase transition, a voltage of fractions of a volt will be sufficient, which is much less than control voltage used in the prototype.

Since the period of the IDS MPNO differs from the period of the IDT by 0.5–1.5 times, the central frequency of the VSS MPNO will differ from the central frequency of the IDT by 0.67–2 times . This means that when the SAW passes from channel to channel, when the number of electrodes of the IDT MPNO is equal to the number of electrodes for 100% transmission at the center frequency to the acoustic channel where the output IDT is located, the IDT of the MPNO introduces attenuation of only about 10 dB (see formula (8 ) and Fig. 3, 5, graphs 3, 4), which cannot be considered satisfactory. At the same time, when the load resistance drops during the phase transition by three orders of magnitude, the VSW of the MPNO introduces an attenuation of more than 70 dB in the parallel acoustic channel. Therefore, it was proposed to introduce another pair of acoustic channels (acoustic channels 3 and 4 in Fig. 2) on the surface of the piezoelectric sound duct 1, containing the input IDT 2 and the output IDT 8, having the same period (the same center frequency) and located in 1 and 2 acoustic channels (figure 2). While the input IDT 3 and output IDT 9, located in the 3 and 4 acoustic channels also have the same period, which differs from the periods of the IDT 3 and 8, located in the 1 and 2 acoustic channels. Between the IDT 3 and the IDT 9 is the VSS MPNO 5, which covers 3 and 4 acoustic channels (figure 2). IDT 2 and IDT 8, together with the IDT MPNO 4, form a lower-frequency filter band, and IDT 3 and IDT 9, together with the INR MPNO 5, form a high-frequency filter band.

In this case, it is possible to obtain an isolation between the bands of more than 70 dB, as well as to increase the attenuation of the filter by more than 70 dB (Fig.4, 6, graphs 1 and 2). To reduce the IDT loss due to bidirectionality [11], it was proposed to use unidirectional IDTs with internal reflectors in the filter.

In FIG. 3-6 show the frequency response of the switched filter at various control voltages, where:

The frequency response of the filter at a control voltage of 0 V on the VSS MPNO 4 and 0 V on the VSS MPNO 5 (figure 3, graphs 1 and 2);

The frequency response of the filter at a control voltage of 0.1 V on the VSS MPNO 4 and 0 V on the VSS MPNO 5 (figure 4, graphs 1 and 2);

The frequency response of the filter at a control voltage of 0 V on the VSS MPNO 4 and 0.1 V on the VSS MPNO 5 (figure 5, graphs 1 and 2);

The frequency response of the filter at a control voltage of 0.1 V on the VSS MPNO 4 and 0.1 V on the VSS MPNO 5 (Fig. 6, graphs 1 and 2).

The width of the vanadium dioxide film is 1–2 periods of the input IDT, which is much less than the SAW beam width equal to the IDT aperture, which is tens of its periods. Since the width of the vanadium dioxide films 6, 7 is tens of times smaller than the width of the acoustic beam, the acoustic beam practically does not cross it; as a result, the film cannot increase the attenuation of the SAW with an increase in the operating frequencies of the filter. The thickness of the vanadium dioxide film is equal to 200 nm and was chosen experimentally from the condition of preventing the short circuit of the electrodes of the VSS in order to ensure the passage of the SAW from one acoustic channel to another when voltage is not applied to the VSS busbars.

Execution example.

The switchable SAW filter was made on a piezoelectric sound conductor 1 from a YX/127 ^o cut of lithium niobate. The input IDT 2 and IDT 3, connected in series, have 13 pairs of electrodes, the electrode overlap is 1000 µm, the IDT 2 period is 6.61 µm, and the IDT 3 period is 4.97 µm, and the center frequencies are 600 and 800 MHz, respectively.

Output IDT 8 and IDT 9 have the same aperture, periods of 6.61 μm and 4.97 μm, center frequencies of 600 and 800 MHz, respectively. The number of periods in the output IDT 8 and IDT 9 is equal to 15. The IDS MPNO 4 and the IDS MPNO 5 have a period of 4.4 μm, which corresponds to a center frequency of 900 MHz and the ratio of the periods of the IDT and the IDS is 1.3. The number of IDS electrodes is 100. All IDTs and IDSs are made of 200 nm aluminum film. Films of vanadium dioxide in the form of strips 200 nm thick and 4.4 μm wide with an adhesive sublayer of titanium oxide were deposited on the surface of the piezoelectric acoustic conductor 1 by pulsed laser deposition through a mask formed on the surface of the acoustic conductor. The laser pulse energy density on the target surface was 2 J/ ^cm2 for both layers. The TiO ₂ film was deposited in 2000 (~65 nm) pulses at T=640 ^o C and P(O ₂ )=3*10 ^-2 mbar. The VO ₂ film was deposited in 4000 (~130 nm) pulses at T=560 ^o C and P(O ₂ )=2*10 ^-2 mbar, after which the mask was removed and IDT and IDT were fabricated. In this case, the resistance of the VSS MPNO measured on the tires of the VSS at room temperature is 10 Ohm, which corresponds to the resistance R = 1000 Ohm and Y = 0.001 cm, i.e. As follows from formula (7), the film, in the absence of voltage on the VSS busbars, provides 100% SAW transition from one acoustic channel to another. When current is passed through the film, it is heated to a temperature above the phase transition temperature. In this case, the resistance on the VShS MPNO buses drops to 0.1 Ohm, which corresponds to R=10 Ohms and Y=0.1 cm. 1) a voltage of 0.1 V is applied, which is much less than the control voltages used in the prototype. Limiting resistances are equal to 1 Ohm. Then the current through the film is 0.0091 A. At this current, the film is heated and a phase transition occurs in it at a temperature of 60 ^° C. In this case, the entire voltage drop is applied to the limiting resistances 10 or 12, which are 10 times greater than the resistance of the VSS MPNO, and the current is limited and becomes equal to 0.091 A. At this current, the film rapidly heats up, since it does not have time to give off heat to the substrate on which it is located, and a phase transition occurs in it at a temperature of 60 ^° C. In this case, the entire drop voltage is applied to limiting resistances 10 or 12, which are 10 times greater than the resistance of the VSS, and the current is limited and becomes equal to 0.091 A, i.e. heat generation increases 100 times (current increases 10 times), which is sufficient to maintain film temperature is higher than the phase transition temperature. In this case, as follows from formulas (7) and (8), the VSS MPNO cannot transfer SAW from one acoustic channel to another, and the attenuation in the frequency band provided by the input and output IDT and the VSS MPNO, to the buses of which the voltage is applied, is more than 70 dB. The capabilities of modern technology for manufacturing periodic VSSs limit the frequency range of the SAW filter to no more than 2500 MHz.

Thus, the following technical characteristics of the switchable SAW filter were obtained:

The center frequency of the low-frequency band is 600 MHz.

Low Bandwidth - 50 MHz

High band center frequency - 800 MHz

High frequency bandwidth - 60 MHz

Insertion loss - 4 dB

Isolation between channels when one band is turned on - 70 dB

Dynamic range - 70 dB

Control voltage - 0.1 V

The developed switchable SAW devices can be successfully used as a preselector for a VHF communication radio receiver, a wide-range scanning receiver, and a multichannel receiver in radio monitoring systems.

Information sources:

1. US 6459345, IPC ⁷ H03H 9/64, publ. 01.10.2002.

2. RU 2121214, IPC H03H 9/64 publ. 07/27/1996.

3. US 10666227, IPC H03H 9/64, H01L 41/107, H03H 9/105, publ. 01/30/2020.

4. US 7821360, IPC H03H 9/64, publ. 03/04/2010.

5. V. A. Arzhanov, I. V. Veremeev Basic approaches to the development of modern tunable and switchable high-frequency SAF filters, OMSK SCIENTIFIC VESTNIK No. 1 (127) 2014

6. Arash Fouladi Azarnaminy, Junwen Jiang, Raafat R. Mansour, Switched Dual-Band SAW Filter Using Vanadium Oxide Switches. Published 7 June 2021, Physics 2021 IEEE MTT-S International Microwave Symposium (IMS).

7. USSR author's certificate 915225, IPC H03H 9/64, publ. 03/23/1982 - prototype.

8. G. Marshall and E. G. S. Paige, "Novel acoustic-surface-wave directional coupler with diverse applications". // Electronics Lett. − V.7. −1971. P. 460-464.

9. A.S. Bagdasaryan, G.Ya. Karapetyan, Interdigital SAW directional couplers and filters based on them / // Communications. − Issue 4. - 1988. - p. 21.

10. Morgan D. Devices for processing signals on surface acoustic waves, M. "Radio and communication". 1991., formula 5.34 on page 123.

11. M. E. Kutepov, G. Ya. Karapetyan, T. A. Minasyan, V. E. Kaydashev, I. V. Lisnevskaya, K. G. Abdulvakhidov, A. A. Kozmin and E. M. Kaidashev Embedding epitaxial VO2 film with quality metal-insulator transition to SAW devices//J. Adv. Dielect. 12, 2250018 (2022).

Claims (5)

1. A switchable two-band filter based on surface acoustic waves (SAW), containing a piezoelectric sound duct, on the working surface of which there are the first input and two output interdigital transducers (IDTs) with different central frequencies and between them the first multistrip directional coupler (MSCO), interacting with a film switch of acoustic channels, forming the first and second parallel acoustic channels, characterized in that the filter additionally contains a second input IDT connected in series with the first input IDT, and a second MPNO, while the first MPNO and the second MPNO are located on the same axis perpendicular to with respect to the direction of SAW propagation, forming additional third and fourth parallel acoustic channels, each MPNO is made in the form of an interdigital system (ISS) and has an electrical contact in the middle part with an acoustic channel switch made in the form of a transverse strip of vanadium dioxide film with a titanium dioxide sublayer formed on the working surface of the sound duct, the buses of each IDT are connected through a limiting resistor to a control voltage source, the bus of the first input IDT is connected to the filter input through a matching inductance, all IDTs are made unidirectional with internal reflectors, the period of the first input IDT is equal to the period of the first output IDT, and the period of the second input IDT is equal to the period of the second output IDT, and the periods of said pairs of IDTs correspond to the center frequencies of the lower and upper passbands of the switchable filter. 2. Switchable two-band filter according to claim 1, characterized in that the period of the interdigital structure of the first MPNO is from 0.5 to 1.5 periods of the input IDT of the first acoustic channel. 3. A switchable two-band filter according to claim 1, characterized in that the period of the second MPNO is from 0.5 to 1.5 periods of the input IDT of the third acoustic channel. 4. Switchable two-band filter according to claim 1, characterized in that the width of the vanadium dioxide film is no more than two periods of the input IDT. 5. Switchable two-band filter according to claim 1, characterized in that the thickness of the vanadium dioxide film is 200 nm.

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