阅读说明：本技术 一种电调微波有源滤波器 (Electrically tunable microwave active filter ) 是由 曾运华 于 2020-07-31 设计创作，主要内容包括：本发明公开了一种由输入端(1)、输出端(2)、n个损耗补偿型电调有源谐振器、n+1个耦合电容器级联而成的电调微波有源滤波器,其中心频率、通带宽度可通过各有源谐振器和耦合电容器的直流偏置电压进行调整,由于采用了损耗补偿电路,其带内插入损耗较小；本发明可用砷化镓、硅等集成电路工艺实现。(The invention discloses an electrically tunable microwave active filter formed by cascading an input end (1), an output end (2), n loss compensation type electrically tunable active resonators and n +1 coupling capacitors, wherein the center frequency and the passband width can be adjusted by the direct current bias voltage of each active resonator and the coupling capacitor; the invention can be realized by gallium arsenide, silicon and other integrated circuit processes.)

1. An electrically tunable microwave active filter is formed by cascading an input end (1), an output end (2), n loss compensation type electrically tunable active resonators (n is a positive integer larger than 1 and smaller than 10) and n +1 coupling capacitors, namely the electrically tunable microwave active filter is formed by cascading an input end (1), an output end (2), an input coupling capacitor (3), a first inter-stage coupling capacitor (4), a second inter-stage coupling capacitor (5), an n-1 inter-stage coupling capacitor (6), an output coupling capacitor (7), a first active resonator (8), a second active resonator (9), a third active resonator (10) and an n active resonator (11); the connection relationship is as follows: the input end (1) is electrically connected with the input coupling capacitor (3), the other end of the input coupling capacitor (3) is electrically connected with the first active resonator (8) and the first inter-stage coupling capacitor (4), the other end of the first inter-stage coupling capacitor (4) is electrically connected with the second active resonator (9) and the second inter-stage coupling capacitor (5), the other end of the second inter-stage coupling capacitor (5) is electrically connected with the third active resonator (10) and the next inter-stage coupling capacitor, and so on, the n-1 th coupling capacitor (6) is electrically connected with the nth active resonator (11) and the output coupling capacitor (7), and the grounding ends of the first active resonator (8), the second active resonator (9), the third active resonator (10) and the nth active resonator (11) are grounded; in order to generate a pair of transmission zeros, a cross-coupling microstrip line (12) which is electrically connected with the open end of the ground via hole of the two non-adjacent active resonators which are far away from each other, such as the open end of the ground via hole (16) of the first active resonator and the open end of the ground via hole (28) of the nth active resonator is also arranged; the signal coupling between the input end (1) and the output end (2) of the filter and the active resonator and between the two adjacent active resonators is realized by coupling capacitors respectively, so the filter belongs to a capacitive coupling type coupled resonator filter.

2. The filter of claim 1, wherein the loss compensation type electrically tunable active resonator such as the first active resonator (8), the second active resonator (9), the third active resonator (10), the n-th active resonator (11), etc. is composed of a microstrip diamond coil, a ground via, a compensation amplifier, and a variable capacitor in the same connection relationship, taking the first active resonator (8) as an example, it is composed of a first microstrip diamond coil (15), a first active resonator ground via (16), a first compensation amplifier (13), and a first variable capacitor (14), the open end of the first microstrip diamond coil (15) is electrically connected to the input end of the first compensation amplifier (13) and one end of the first variable capacitor (14), the ground end of the first microstrip diamond coil (15) is electrically connected to the first active resonator ground via (16), a first microstrip diamond-shaped coil (15) and a first active resonator grounding via hole (16) form a parallel resonance body of the first active resonator (8), a tap of the first microstrip diamond-shaped coil (15) is electrically connected with the output end of the first compensation amplifier (13), and the other end of the first variable capacitor (14) is grounded; the action of each compensation amplifier enables each active resonator to generate a negative resistance to offset loss resistance in the active resonator and the front and rear coupling capacitors of the active resonator; all the active resonators produce a negative resistance, as a result, the inherent losses of all the elements are compensated, the quality factor, i.e. the no-load Q value, of each active resonator is greatly improved, accordingly, the technical indexes of the insertion loss, the rectangular characteristic and the like of the filter are also greatly improved, and the insertion loss and the rectangular characteristic of the filter can be changed by changing the bias voltage of the compensating amplifier in each active resonator.

3. The filter of claim 1, wherein the diagonal of the microstrip diamond coil passing through the ground terminal is a longitudinal diagonal, the other diagonal is a transverse diagonal, the microstrip diamond coils in all the active resonators, namely the first microstrip diamond coil (15), the second microstrip diamond coil (19), the third microstrip diamond coil (23), the third microstrip diamond coil, and the nth microstrip diamond coil (27) are arranged into a linear array, and are spaced at proper or slightly larger intervals, and the transverse diagonals of all the microstrip diamond coils are on the same straight line or close to the same straight line, so that the inductive coupling of the two diagonally arranged microstrip diamond coils is weakest under the condition of the same spacing, the influence on the performance of the filter is minimum, and the weak inductive coupling can be counteracted as much as possible by increasing the capacitance of each coupling capacitor moderately, so that the influence on the performance of the filter is further reduced; in order to reduce the capacitance which needs to be increased by the first interstage coupling capacitor (4) and the (n-1) th interstage coupling capacitor (6) for counteracting the inductive coupling, a first active resonator (8), a second active resonator (9), an nth active resonator (11) and the active resonators in front of the nth active resonator are arranged in a mutually reverse mode, namely the adjacent sides of the two resonators are close-to-open sides, and the adjacent sides are close-to-ground sides; when the filter has a higher order, i.e. a larger number of active resonators, e.g. more than five, trimming of the coupling capacitors is not sufficient to counteract the detrimental effect of the inductive coupling of the coil, the filter may be divided into left and right sections having no more than four active resonators, separated by metal partitions, which are as thin as possible, in order to avoid too much influence on the filter performance.

4. The filter of claim 1, wherein: the variable capacitor in each active resonator consists of a field effect transistor with short circuit of a drain and a source or a varactor and a plurality of bias elements, and the change of the bias voltage of the variable capacitor in each active resonator is equivalent to the change of the resonance capacitance in each active resonator, so that the resonance frequency of each active resonator is changed, and further the center frequency of the filter is changed, and therefore the center frequency of the filter can be adjusted by the direct current voltage through the variable capacitor; each coupling capacitor consists of a field effect transistor with short-circuited source or a pair of field effect transistors with short-circuited source or a varactor diode or a pair of varactor diodes connected in series, a plurality of biasing elements and a plurality of microstrip lines, and the capacitance of each coupling capacitor, namely an input coupling capacitor (3), a first stage coupling capacitor (4), a second stage coupling capacitor (5), an n-1 stage coupling capacitor (6) and an output coupling capacitor (7) can be changed by changing the biasing voltage of each coupling capacitor, so that the coupling strength of each stage is changed to be consistent with the characteristics of the pass band width of the filter and the like, and the pass band width of the filter is mainly adjusted by the biasing voltage of each coupling capacitor.

5. The loss compensation type electrically tunable active resonator according to claim 2, further comprising a means for introducing cross-inductive coupling between the two active resonators, which is a long high-resistance microstrip line (12) connected in series to the open end of the ground via hole of the two active resonators, and an inductive coupling loop is formed by the ground via hole of the two active resonators, so that inductive coupling is generated between the two active resonators.

Technical Field

The invention relates to the field of microwave coupled resonator filters, in particular to a coupled resonator filter consisting of a plurality of loss compensation type electrically-tunable active resonators and a plurality of coupling capacitors.

Background

Most of the existing microwave coupling resonator filters, such as axial cavity filters, three-dimensional spiral filters, planar spiral filters, dielectric filters and the like, are passive filters with unadjustable center frequency and passband width or only adjustable mechanically, so that the application range of a single filter is greatly reduced.

Disclosure of Invention

The invention aims to overcome and solve the defects and technical problems that the center frequency and the passband width of the existing coupling resonator filter can not be adjusted electrically, and provides an electrically tunable microwave active filter (hereinafter referred to as 'filter') with the center frequency and the passband width both adjusted by direct current voltage, which is formed by cascading an input end, an output end, n loss compensation type electrically tunable active resonators (hereinafter referred to as 'active resonators') (n is a positive integer larger than 1 and smaller than 10) and n +1 coupling capacitors, wherein the connection relationship is as follows: the input end of the first active resonator is electrically connected with the input coupling capacitor, the other end of the first active resonator is electrically connected with the first inter-stage coupling capacitor, the other end of the first inter-stage coupling capacitor is electrically connected with the second active resonator and the second inter-stage coupling capacitor, the other end of the second inter-stage coupling capacitor is electrically connected with the third active resonator and the next inter-stage coupling capacitor, and so on, the nth-1 inter-stage coupling capacitor is electrically connected with the nth active resonator and the output coupling capacitor, and the grounding ends of the first active resonator, the second active resonator, the third active resonator, the. In order to generate a pair of transmission zeros, a cross-coupled microstrip line electrically connected to the open ends of the ground vias of two non-adjacent active resonators which are far apart, such as the open end of the ground via of the first active resonator and the open end of the ground via of the nth active resonator, is also arranged; the signal coupling between the input end and the output end of the filter and the active resonators and between two adjacent active resonators is realized by coupling capacitors respectively, so the filter belongs to a capacitive coupling type coupled resonator filter.

The loss compensation type electrically-adjusted active resonator consists of a micro-strip diamond coil, a grounding via hole, a compensation amplifier and a variable capacitor, wherein the micro-strip diamond coil and the grounding via hole form a parallel resonance body of the active resonator, the micro-strip diamond coil comprises an open end, a grounding end and a tap, the grounding end of the micro-strip diamond coil is electrically connected with the grounding via hole, and the open end of the micro-strip diamond coil is electrically connected with the compensation amplifier and the variable capacitor.

The diagonal line of the micro-strip diamond-shaped coil passing through the grounding end of the micro-strip diamond-shaped coil is taken as a longitudinal diagonal line, the other diagonal line is taken as a transverse diagonal line, all the micro-strip diamond-shaped coils in the filter are arranged into a linear array, and the transverse diagonal lines of all the micro-strip diamond-shaped coils are on the same straight line or close to the same straight line, so that the inductive coupling of the two micro-strip diamond-shaped coils arranged diagonally is the weakest under the condition of the same interval, the influence on the performance of the filter is the smallest, and the weak inductive coupling can be counteracted by properly increasing the capacitance of each coupling capacitor, so that; when the filter has a higher order, i.e. a larger number of active resonators, e.g. more than five, trimming of the coupling capacitors is not sufficient to counteract the detrimental effect of the inductive coupling of the coil, the filter may be divided into left and right sections having no more than four active resonators, separated by metal partitions, which are as thin as possible, in order to avoid too much influence on the filter performance. The capacitive coupling resonator filter has the advantages that the requirement of the capacitive coupling strength between the first two active resonators and the requirement of the capacitive coupling strength between the last two active resonators of the capacitive coupling resonator filter are larger than the requirement of the capacitive coupling strength between the middle stages by taking the input end as a starting point, so that the required coupling capacitance is larger, in order to reduce the capacitance required to be increased by the coupling capacitor between the first two active resonators and the coupling capacitor between the last two active resonators due to the cancellation of the inductive coupling, the first two active resonators and the last two active resonators are arranged in an opposite mode, namely adjacent sides of the two resonators are near-open sides, and the separated sides are near-ground sides.

The insertion loss of the filter is determined by the loss resistances of the coupling capacitors and the active resonators, and the compensation amplifiers are used for generating a negative resistance in the active resonators to offset the loss resistances; all the compensation amplifiers are single-stage or multi-stage amplifiers, and the final stage of the compensation amplifiers is an emitter follower or a source follower, namely a common collector amplifying circuit or a common drain amplifying circuit; the microwave signal shunted from the input end of the compensation amplifier is amplified by the compensation amplifier to form compensation current, and the compensation current is injected into the coil by a tap of the microstrip diamond coil, so that the coil is in a negative resistance state, and the total loss of the active resonator and the coupling capacitor adjacent to the active resonator is compensated; all the active resonators generate a negative resistance, so that the inherent loss of all the elements is compensated, the quality factor, namely the no-load Q value, of each active resonator is greatly improved, and accordingly, the technical indexes of the filter, such as the insertion loss, the rectangular characteristic and the like, are also greatly improved; the insertion loss and the rectangular characteristic of the filter can be changed by changing the bias voltage of the compensation amplifier in each active resonator.

The center frequency of the filter is mainly determined by the resonant frequency of each active resonator, the variable capacitor in each active resonator is composed of a field effect transistor with short circuit between drain and source or a variable capacitance diode and a plurality of bias elements, and changing the bias voltage of the variable capacitor in each active resonator is equivalent to changing the resonant capacitance in each active resonator, so that the resonant frequency of each active resonator is changed, and further the center frequency of the filter is changed, and therefore the center frequency of the filter can be adjusted by the direct current voltage through the variable capacitor.

The pass band width of the filter is mainly determined by the coupling strength of each stage mainly determined by each coupling capacitor; each coupling capacitor is composed of a field effect tube with a short-circuited drain source or a pair of field effect tubes with a short-circuited drain source in series or a varactor or a pair of varactor in series and a plurality of biasing elements and a plurality of sections of microstrip lines, so that the capacitance of each coupling capacitor can be changed by changing the biasing voltage of each coupling capacitor, thereby changing the coupling strength of each stage to be consistent with the characteristics of the pass band width and the like of the filter, and the pass band width of the filter is mainly adjusted by the biasing voltage of each coupling capacitor.

In order to obtain the transmission zero point of the filter, usually it is necessary to artificially introduce cross-inductive coupling between some two non-adjacent active resonators which are far apart, therefore, the invention also provides a device for introducing cross-inductive coupling between two active resonators, which is a long section of high-resistance microstrip line, i.e. a cross-coupled microstrip line, which is connected in series with the open end of the ground via hole of the two active resonators, and an inductive coupling loop is formed by the ground via holes of the two active resonators, so that the two active resonators generate inductive coupling, and the transmission characteristic of the filter has the required transmission zero point.

The negative resistance of the active resonator has frequency-variable characteristics, so that the in-band unevenness of the filter is large; a small resistor is connected in series with the base electrode or the grid electrode of each compensating amplifier amplifying tube, so that the in-band flatness of the filter can be improved, and the cost is that the power consumption of the whole filter is slightly increased.

In order to reduce the complexity and design difficulty of the filter, the filter may adopt a symmetrical structure, such as a six-active resonator filter, and the filter may be divided into two halves symmetrically with respect to a central plane, and circuit structures, elements and parameters of the left half and the right half are completely the same, but are arranged in opposite directions.

The invention has the beneficial effects that: the center frequency and the pass band width of the provided filter are adjustable; the provided filter can improve the electronic countermeasure level of the radar, and can greatly improve the performances of mobile phones, communication equipment, communication systems, instrument equipment, wireless routers, Internet of things equipment and the like.

The invention is further described with reference to the following figures and detailed description.

Description of the drawings:

fig. 1 is a schematic block diagram of an electrically tunable microwave active filter according to the present invention;

FIG. 2 is a schematic block diagram of an embodiment of an electrically tunable microwave active filter according to the present invention;

FIG. 3 is a schematic diagram of the left half of the circuit of the present invention;

FIG. 4 is a diagram of the shape and size of a microstrip diamond coil used in an embodiment of the present invention;

FIG. 5 is a graph showing the response of the filter to the transmission and reflection frequencies when the center frequency is adjusted to 10GHz according to an embodiment of the invention;

FIG. 6 is a graph showing the response of the transmission and reflection frequencies of the filter when the center frequency is adjusted to 11GHz according to an embodiment of the invention;

the specific implementation mode is as follows:

as shown in fig. 1, an electrically tunable microwave active filter is formed by cascading an input terminal 1, an output terminal 2, n loss compensation type electrically tunable active resonators (n is a positive integer greater than 1 and less than 10), and n +1 coupling capacitors, that is, an input terminal 1, an output terminal 2, an input coupling capacitor 3, a first inter-stage coupling capacitor 4, a second inter-stage coupling capacitor 5, an n-1 inter-stage coupling capacitor 6, an output coupling capacitor 7, a first active resonator 8, a second active resonator 9, a third active resonator 10, and an n active resonator 11 in the sequence shown in the figure, wherein the third to n-1 inter-stage coupling capacitors and the fourth to n-1 active resonators are omitted in the figure; the connection relationship is as follows: the input end 1 is electrically connected with an input coupling capacitor 3, the other end of the input coupling capacitor 3 is electrically connected with a first active resonator 8 and a first inter-stage coupling capacitor 4, the other end of the first inter-stage coupling capacitor 4 is electrically connected with a second active resonator 9 and a second inter-stage coupling capacitor 5, the other end of the second inter-stage coupling capacitor 5 is electrically connected with a third active resonator 10 and a next inter-stage coupling capacitor (omitted in the figure), and so on, the nth-1 inter-stage coupling capacitor 6 is electrically connected with an nth active resonator 11 and an output coupling capacitor 7, and the grounding ends of the first active resonator 8, the second active resonator 9, the third active resonator 10 and the nth active resonator 11 are grounded; in order to generate a pair of transmission zeros, a cross-coupled microstrip line 12 electrically connected to the open ends of two distant non-adjacent active resonator ground vias, such as the open end of the first active resonator ground via 16 and the open end of the nth active resonator ground via 28, is further provided; the signal coupling between the input end 1 and the output end 2 of the filter and the active resonators and between two adjacent active resonators is realized by coupling capacitors respectively, so the filter belongs to a capacitive coupling type coupled resonator filter.

The loss compensation type electrically-adjusted active resonator is a first active resonator 8, a second active resonator 9, a third active resonator 10, an nth active resonator 11 and the like shown in figure 1, which respectively comprise a microstrip diamond coil, a ground via hole, a compensation amplifier and a variable capacitor in the same connection relation, taking the first active resonator 8 as an example, the first active resonator comprises a first microstrip diamond coil 15, a first active resonator ground via hole 16, a first compensation amplifier 13 and a first variable capacitor 14, the open end of the first microstrip diamond coil 15 is electrically connected with the input end of the first compensation amplifier 13 and one end of the first variable capacitor 14, the ground end of the first microstrip diamond coil 15 is electrically connected with the first active resonator ground via hole 16, the first microstrip diamond coil 15 and the first active resonator ground via hole 16 form a parallel body of the first active resonator 8, a tap of the first microstrip diamond-shaped coil 15 is electrically connected with an output end of the first compensation amplifier 13, and the other end of the first variable capacitor 14 is grounded; taking the nth active resonator 11 as an example, the nth active resonator 11 is composed of an nth microstrip diamond coil 27, an nth active resonator ground via 28, an nth compensation amplifier 26 and an nth variable capacitor 25, wherein an open end of the nth microstrip diamond coil 27 is electrically connected with an input end of the nth compensation amplifier 26 and one end of the nth variable capacitor 25, the connection point is the open end of the nth active resonator 11, a ground end of the nth microstrip diamond coil 27 is electrically connected with the nth active resonator ground via 28, the nth microstrip diamond coil 27 and the nth active resonator ground via 28 form a parallel resonator of the nth active resonator 11, a tap of the nth microstrip diamond coil 27 is electrically connected with an output end of the nth compensation amplifier 26, and the other end of the nth variable capacitor 25 is grounded.

Taking the diagonal line of the microstrip diamond-shaped coil passing through the ground end of the microstrip diamond-shaped coil as a longitudinal diagonal line, and taking the other diagonal line as a transverse diagonal line, as shown in fig. 1, the microstrip diamond-shaped coils in all the active resonators, namely the first microstrip diamond-shaped coil 15, the second microstrip diamond-shaped coil 19, the third microstrip diamond-shaped coil 23, the fourth microstrip diamond-shaped coil 27 and the nth microstrip diamond-shaped coil 27, should be arranged into a linear array, and are spaced at a proper or slightly larger interval from each other, and the transverse diagonal lines of all the microstrip diamond-shaped coils are on the same straight line or close to the same straight line, so that the two microstrip diamond-shaped coils arranged diagonally are weakest in inductive coupling under the condition of the same interval, and have the smallest influence on the filter performance, and the weak inductive coupling can be counteracted as; when the filter has a higher order, i.e. a larger number of active resonators, e.g. more than five, trimming of the coupling capacitors is not sufficient to counteract the detrimental effect of the inductive coupling of the coil, the filter may be divided into left and right sections having no more than four active resonators, separated by metal partitions, which are as thin as possible, in order to avoid too much influence on the filter performance.

The capacitive coupling resonator filter requires a capacitive coupling strength between the first two active resonators and a capacitive coupling strength between the last two active resonators to be greater than the capacitive coupling strength between the middle stages, i.e., the capacitance of the first inter-stage coupling capacitor 4 and the capacitance of the (n-1) th inter-stage coupling capacitor 6 are greater than the capacitance of the other inter-stage coupling capacitors in fig. 1. in order to reduce the capacitance of the first inter-stage coupling capacitor 4 and the capacitance of the (n-1) th inter-stage coupling capacitor 6 which needs to be increased for canceling the inductive coupling, the first active resonator 8, the second active resonator 9, the (n) th active resonator 11 and the preceding active resonators should be arranged in opposite directions to each other, i.e., adjacent sides of the two resonators are near-open sides, and the opposite sides are near-ground sides.

The insertion loss of the filter is determined by the loss resistances of the coupling capacitors and the active resonators, and the compensation amplifiers are used for generating a negative resistance in the active resonators to offset the loss resistances; taking the first active resonator 8 as an example, the first compensation amplifier 13 is a single-stage or multi-stage amplifier, the final stage of the first compensation amplifier is an emitter follower or a source follower, i.e. a common collector amplification circuit or a common drain amplification circuit, the microwave signal shunted from the input end of the first compensation amplifier 13 is amplified by the first compensation amplifier 13 to form a compensation current, and then the compensation current is injected into the coil by the tap of the first microstrip diamond-shaped coil 15, so that the coil presents a negative resistance state, thereby compensating the total loss of the first active resonator 8, the input coupling capacitor 3 adjacent to the first active resonator and the first inter-stage coupling capacitor 4; all the active resonators generate a negative resistance, so that the inherent loss of all the elements is compensated, the quality factor, namely the no-load Q value, of each active resonator is greatly improved, and accordingly, the technical indexes of the filter, such as the insertion loss, the rectangular characteristic and the like, are also greatly improved; the insertion loss and the rectangular characteristic of the filter can be changed by changing the bias voltage of the compensation amplifier in each active resonator.

The center frequency of the filter is mainly determined by the resonant frequency of each active resonator, the variable capacitor in each active resonator consists of a field effect transistor with short circuit of drain and source or a varactor and a plurality of bias elements, and changing the bias voltage of the variable capacitor in each active resonator is equivalent to changing the resonant capacitance in each active resonator, thereby changing the resonant frequency of each active resonator and further changing the center frequency of the filter, so that the center frequency of the filter can be adjusted by the direct current voltage through the variable capacitor; taking the first active resonator 8 as an example, the first variable capacitor 14 is the main capacitor participating in the resonance of the first active resonator 8, and since it consists of a drain-source short-circuited fet or a varactor and several biasing elements, changing its bias voltage changes its capacitance, which is equivalent to changing the total resonant capacitance of the first active resonator 8, thereby changing the resonant frequency of this resonator.

The pass band width of the filter is mainly determined by the coupling strength of each stage mainly determined by each coupling capacitor; the input coupling capacitor 3 determines the coupling strength between the input 1 and the first active resonator 8, the first inter-stage coupling capacitor 4 determines the coupling strength between the first active resonator 8 and the second active resonator 9, the second inter-stage coupling capacitor 5 determines the coupling strength between the second active resonator 9 and the third active resonator 10, and so on, the output coupling capacitor 7 determines the coupling strength between the nth active resonator 11 and the output 2; each coupling capacitor is composed of a field effect transistor with a short drain-source circuit or a pair of field effect transistors with a short drain-source circuit in series or a varactor or a pair of varactor in series and a plurality of biasing elements and a plurality of sections of microstrip lines, so that the capacitance of the input coupling capacitor 3, the first inter-stage coupling capacitor 4, the second inter-stage coupling capacitor 5, the n-1 th inter-stage coupling capacitor 6 and the output coupling capacitor 7 can be changed by changing the biasing voltage thereof, thereby changing the coupling strength of each stage to be consistent with the characteristics of the pass band width and the like of the filter, and the pass band width of the filter is mainly adjusted by the biasing voltage of each coupling capacitor.

In order to obtain the transmission zero point of the filter, usually it is necessary to artificially introduce cross-inductive coupling between some two non-adjacent active resonators which are far apart, therefore, the invention also provides a device for introducing cross-inductive coupling between two active resonators, which is a long section of high-resistance microstrip line, i.e. a cross-coupled microstrip line (12), which is connected in series with the open end of the ground via hole of the two active resonators, and an inductive coupling loop is formed by the ground via holes of the two active resonators, so that the two active resonators generate inductive coupling, and the transmission characteristic of the filter has the required transmission zero point; for example, for an even-order filter with an order greater than or equal to 4, two ends of the cross-coupled microstrip line 12 are electrically connected between the first active resonator ground via 16 and the nth active resonator ground via 28, respectively, to form a pair of transmission zeros, i.e., one transmission zero on each of the low-frequency side and the high-frequency side of the passband.

The negative resistance of the active resonator has frequency-variable characteristics, so that the in-band unevenness of the filter is large; a small resistor is connected in series with the base electrode or the grid electrode of each compensating amplifier amplifying tube, so that the in-band flatness of the filter can be improved, and the cost is that the power consumption of the whole filter is slightly increased.

In order to reduce the complexity and design difficulty of the filter, the filter may adopt a symmetric structure, such as a six-active resonator filter, and may be symmetrically divided into two halves by a central plane, and the circuit structures, elements and parameters of the left half circuit and the right half circuit are completely the same, but the arrangement directions are opposite, fig. 2 shows a symmetric six-active resonator filter, the right half circuit and the left half circuit are completely the same, but two elements at symmetric positions are arranged in opposite directions, so the elements of the right half circuit adopt the same numbers as those on the left side.