CN114257213B - Adjustable band-pass tuning frequency-selecting circuit, electrically tunable filter and adjusting method thereof - Google Patents

Abstract

The invention discloses an adjustable band-pass tuning frequency-selecting circuit, an electric tuning filter and an adjusting method thereof, which relate to the technical field of microwave communication and solve the technical problems of large volume and complex structure of the existing circuit, wherein the adjustable band-pass tuning frequency-selecting circuit comprises two resonant circuits which are cascaded in a symmetrical distribution mode, the resonant circuits comprise three inductors and a group of varactors, one of the inductors is grounded through the varactors, and the input end of the varactors is connected with control voltage; the inductances of the two resonant circuits are coupled to each other via a varactor diode to ground. The invention has the advantages of simple integral structure, easy processing, small volume, low cost and debugging function: the coupling coefficient can be adjusted by adjusting the distance of the coupling inductor, so that bandwidth adjustment is completed, adjustment convenience is improved, and debugging difficulty is reduced; the center frequency can be adjusted by changing the inductance values of the inductor L3 and the inductor L4; the in-band insertion loss and standing wave can be adjusted by changing the inductance L2 and the inductance L5.

Description

An adjustable bandpass tuning frequency selection circuit, an electrically adjustable filter and an adjustment method thereof

Technical field

The present invention relates to the technical field of microwave communication, and more specifically to the technical field of an adjustable bandpass tuning frequency selection circuit, an electrically adjustable filter and an adjustment method thereof.

Background technique

In the increasingly crowded spectrum resources and complex electromagnetic environment, high-performance filters, as an effective means to improve the situation, have received continued attention and research from researchers. Traditional broadband communication systems use multi-channel filters and radio frequency switches to form the front end of the radio frequency system as an anti-interference technical means. This method has problems such as large volume, pass bandwidth, weak anti-interference ability, general linearity, and high cost. The ESC filter can change the capacitance value with a narrow bandwidth by changing the state of the capacitor chip, or changing the DC bias of the varactor diode, and then changes the resonant frequency to continuously scan and cover a wider frequency band. At the same time, the electronically controlled filter can effectively reduce the size and improve the anti-interference ability. Therefore, the electronically controlled filter is a filter with excellent performance today.

The ESC filter is a new type of radio frequency bandpass filter. It is an indispensable key component in modern digital communications and frequency hopping technology. Traditional electronically controlled filters are circuit models designed based on Chebyshev or resonant coupling theory. Chebyshev filters and elliptical filters each have their own advantages and disadvantages: Chebyshev filters have out-of-band suppression, but their filtering performance is not as good as elliptical filters; elliptical filters are not widely used in ESC filtering due to their narrow tuning range. At the same time, ESC filters generally use LC or microstrip technology to implement circuits. However, multiple capacitors need to be used in the LC circuit, and the number of capacitors needs to be selected through switches for tuning, which results in the electronically controlled filter made in this way being larger. Although the volume of the electronically controlled filter produced using the microstrip line process is significantly smaller, it cannot be debugged in actual use, has poor applicability, and has a long production cycle and high cost.

Contents of the invention

The purpose of the present invention is to: in order to solve the above technical problems, the present invention provides an inductively coupled zero-point following LC ESC filter with a simple structure, compact size, high frequency selectivity and easy debugging.

In order to achieve the above object, the present invention specifically adopts the following technical solutions:

In a first aspect, the present invention discloses an adjustable bandpass tuning frequency selection circuit. The bandpass tuning frequency selection circuit includes two resonant circuits cascaded in a symmetrically distributed manner. One of the resonant circuits includes one end and The inductor L1 connected to the signal input end, the inductor L2 and the inductor L3 with one end connected to the other end of the inductor L1 and the other end both grounded, and the inductor L3 is connected in series with the varactor diode D2 and then grounded, and the varactor diode D2 The input terminal is connected to the control voltage V _IN2 ;

The other resonant circuit includes an inductor L6 with one end connected to the signal output end, an inductor L4 and an inductor L5 with one end connected to the other end of the inductor L6 and the other ends both grounded, and the inductor L4 is connected in series with a varactor diode D3 and then grounded. , the input terminal of the varactor diode D3 is connected to the control voltage V _IN3 ;

The inductor L3 and the inductor L4 are coupled.

Furthermore, an adjustable bandpass tuning frequency selection circuit also includes a varactor diode D1 connected in parallel with the inductor L1, and the input end of the varactor diode D1 is connected to the control voltage V _IN1 .

Further, the inductor is a high-Q inductor.

Further, the control voltage V _IN1 and the control voltage V _IN2 are the same variable voltage source.

In a second aspect, the present invention discloses an adjustable electrically adjustable filter, specifically an inductively coupled zero-point adjustable electrically adjustable filter suitable for microwave planar circuits. The bandpass tuning of the electrically adjustable filter The frequency selection circuit is the bandpass tuning frequency selection circuit described in any one of the above.

In a third aspect, the present invention discloses an adjustment method for an adjustable bandpass tuning frequency selection circuit. The adjustment method is implemented based on the following circuit: an adjustable bandpass tuning frequency selection circuit. The bandpass tuning frequency selection circuit is The frequency selection circuit includes two resonant circuits cascaded in a symmetrical distribution manner. One of the resonant circuits includes an inductor L1 with one end connected to the signal input end, an inductor L2 with one end connected to the other end of the inductor L1 and both ends grounded. Inductor L3, and the inductor L3 is connected in series with a varactor diode D2 and then connected to the ground. The input end of the varactor diode D2 is connected to the control voltage V _IN2 ; the other resonant circuit includes an inductor L6 with one end connected to the signal output end. , inductor L4 and inductor L5 with one end connected to the other end of the inductor L6 and the other end grounded, and the inductor L4 is connected in series with the varactor diode D3 and then grounded, and the input end of the varactor diode D3 is connected to the control voltage V _IN3 ; The inductor L3 and the inductor L4 are coupled.

Methods include the following:

Adjust the in-band insertion loss and standing wave: If the inductance values of inductor L2 and inductor L5 are large, then the standing wave difference is different, then reduce the inductance value of inductor L2 and inductor L5, thereby adjusting the standing wave; if the inductor L2 and inductor L5 inductance If the value is small, if the standing wave difference is at this time, increase the inductance value of the inductor L2 and the inductor L5, thereby adjusting the standing wave;

Adjust the center frequency: increase the inductance value of inductor L3 and inductor L4, the center frequency will decrease; otherwise, the center frequency will increase;

Adjustment bandwidth: Increase the distance between the adjustment inductor L3 and the fourth inductor L4, and the bandwidth will decrease.

In a fourth aspect, the present invention also discloses another adjustment method of an adjustable bandpass tuning frequency selection circuit. The adjustment method is implemented based on the following circuit: an adjustable bandpass tuning frequency selection circuit, the bandpass tuning frequency selection circuit is The pass-tuned frequency selection circuit includes two resonant circuits cascaded in a symmetrical distribution manner. One of the resonant circuits includes an inductor L1 with one end connected to the signal input end, and an inductor with one end connected to the other end of the inductor L1 and the other end connected to the ground. L2 and inductor L3, and the inductor L3 is connected in series with a varactor diode D2 and then connected to the ground. The input end of the varactor diode D2 is connected to the control voltage V _IN2 ; the other resonant circuit includes one end connected to the signal output end. Inductor L6, inductor L4 and inductor L5 with one end connected to the other end of inductor L6 and the other end grounded, and the inductor L4 is connected in series with the varactor diode D3 and then grounded, and the input end of the varactor diode D3 is connected to the control voltage. V _IN3 ; The inductor L3 and the inductor L4 are coupled. It also includes a varactor diode D1 connected in parallel with the inductor L1, and the input end of the varactor diode D1 is connected to the control voltage V _IN1 .

The adjustment method includes the following content: Adjust the in-band insertion loss and standing wave: if the inductance value of the inductor L2 and the inductor L5 is large and the standing wave difference is at this time, reduce the inductance value of the inductor L2 and the inductor L5, thereby adjusting the standing wave. ;If the inductance values of inductor L2 and inductor L5 are small and the standing wave difference is at this time, increase the inductance values of inductor L2 and inductor L5 to adjust the standing wave;

Adjust the center frequency: increase the inductance value of inductor L3 and inductor L4, the center frequency will decrease; otherwise, the center frequency will increase;

Adjustment bandwidth: Increase the distance between the adjustment inductor L3 and the fourth inductor L4, and the bandwidth will decrease;

Adjust the distance between the zero point and the center frequency: adjust the inductance value of the inductor L1 and/or change the voltage value of the control voltage V _IN1 of the variable diode D1, thereby changing the zero point position.

The beneficial effects of the present invention are as follows:

1. This invention is based on varactor diodes and inductors, that is, using unconventional LC inductors to build circuits. It not only has a simple overall structure, is easy to process, is small in size, low in cost, and has debugging functions;

2. The present invention is convenient for debugging: the present invention utilizes inductor spatial coupling, and the coupling coefficient can be adjusted by adjusting the distance of the coupling inductor, thereby completing bandwidth adjustment, improving the convenience of adjustment, and reducing the difficulty of debugging; and by changing the inductance L3 and the inductance The L4 inductance value can adjust the center frequency; changing the inductor L2 and inductor L5 can adjust the in-band insertion loss and standing wave;

3. The center frequency and bandwidth of the adjustable bandpass tuning frequency selection circuit designed in the present invention are easy to control. By controlling the size of the control voltage and then the capacitance value of the varactor diode, the center frequency can be controlled, and the coupling inductance can be adjusted The bandwidth can be controlled by the distance; most importantly, based on the zero-point adjustable feature, the zero-point position can be adjusted to make the zero-point closer to the center frequency, and the out-of-band suppression is better;

4. The adjustable bandpass tuning frequency selection circuit designed by the present invention introduces an adjustable zero point outside the band, which improves the out-of-band suppression. If there is no D1, only the center point is adjustable and no zero point is adjustable; the introduction of the varactor diode D1 can quickly lower the frequency and reach the lowest position quickly, forming a resonance pit, that is, the zero point; and the filter introduces a resonance pit, then It can improve the out-of-band suppression, that is, the amplitude height on the right side of the zero point is lower than the amplitude height on the left side; and improving the out-of-band suppression can filter out unnecessary waves, and the filtering effect is better.

Description of the drawings

Figure 1 is a schematic structural diagram of a bandpass tuning frequency selection circuit;

Figure 2 is an amplitude-frequency response diagram of the first implementation in Embodiment 3;

Figure 3 is an amplitude-frequency response diagram of the second implementation in Embodiment 3;

Figure 4 is an amplitude-frequency response diagram of the third implementation in Embodiment 3.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, rather than all embodiments. The components of the embodiments of the invention generally described and illustrated in the figures herein may be arranged and designed in a variety of different configurations.

Therefore, the following detailed description of the embodiments of the invention provided in the appended drawings is not intended to limit the scope of the claimed invention, but rather to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

Example 1

As shown in Figure 1, this embodiment provides an adjustable bandpass tuning frequency selection circuit. The bandpass tuning frequency selection circuit includes two resonant circuits cascaded in a symmetrically distributed manner. One of the resonant circuits includes Inductor L1 with one end connected to the signal input end, inductor L2 and inductor L3 with one end connected to the other end of the inductor L1 and the other end both grounded, and the inductor L3 is connected in series with the varactor diode D2 and then grounded. The input terminal of D2 is connected to the control voltage V _IN2 ;

The other resonant circuit includes an inductor L6 with one end connected to the signal output end, an inductor L4 and an inductor L5 with one end connected to the other end of the inductor L6 and the other ends both grounded, and the inductor L4 is connected in series with a varactor diode D3 and then grounded. , the input terminal of the varactor diode D3 is connected to the control voltage V _IN3 ;

The inductor L3 and the inductor L4 are coupled.

As shown in Figure 1, this embodiment includes six inductors: inductor L1, inductor L2, inductor L3, inductor L4, inductor L5 and inductor L6; and two sets of adjustable varactor diodes: varactor diode D2 and varactor diode D3. . This embodiment mainly consists of the above components to form two symmetrically distributed tuning circuits in this embodiment. One end of inductor L1 is connected to the input end, and the other end is connected between inductor L2 and inductor L3; one end of inductor L2 is connected between inductor L1 and inductor L3, and the other end is connected to ground; one end of inductor L3 is connected between inductor L1 and inductor L2. The other end is grounded; and inductor L3 and inductor L4 are inductively coupled; one end of inductor L4 is grounded, and one end is connected between inductor L5 and inductor L6; one end of inductor L5 is connected between inductor L4 and inductor L6, and one end is grounded; one end of inductor L6 is connected One end of the output terminal is connected between the inductor L4 and the inductor L5. The second group of varactor diodes D2 is connected to the ground terminal of the inductor L3, and the third group of varactor diodes D3 is connected to the ground terminal of one end of the inductor L4.

Since in this embodiment, the bandpass tuning frequency selection circuit uses two cascaded resonant circuits in a symmetrical distribution manner, in order to make the poles of the two symmetrical circuits consistent, that is, to make the transmission poles of the two resonant circuits coincide, it is necessary to follow Implementation is as follows: among the six inductors, the inductance values of inductor L1 and inductor L6 are consistent, the inductance values of inductor L2 and inductor L5 are consistent, and the inductance values of inductor L3 and inductor L4 are consistent; among the two sets of varactor diodes: The output values of the capacitive diode D2 and the varactor diode D3 are consistent. Specifically, they can be controlled by the same control voltage so that their output values are consistent. In this way, through the above settings, the two resonant circuits can be made symmetrical in structure and parameters, so that the transmission poles of the two resonant circuits can overlap, that is, they have a peak; at the same time, the waveform obtained in this way is smooth and the filtering effect is good.

In this embodiment, by changing the inductor L2 and the inductor L5, the in-band insertion loss and standing wave can be adjusted. Specifically, the second inductor adjusts the standing wave according to the actual situation: if the inductance value of the inductor L2 and the inductor L5 is large and the standing wave difference is at this time, the inductance value of the inductor L2 and the inductor L5 is reduced, so that the standing wave can be adjusted well; If the inductance values of the inductor L2 and the inductor L5 are small and the standing wave difference is present, the inductance values of the inductor L2 and the inductor L5 can be increased, so that the standing wave can be adjusted well. That is, the adjustment of inductor L2 and inductor L5 needs to be determined based on the specific situation.

Changing the inductance values of inductor L3 and inductor L4 can control the capacitance value of the varactor diode, thereby adjusting the center frequency. Specifically, as the inductance value of inductor L3 and inductor L4 increases, the center frequency will decrease accordingly; during implementation, the required center frequency can be set according to the desired frequency of the signal.

Changing the distance between inductor L3 and inductor L4 can change the coupling coefficient k, thereby adjusting the bandwidth. Specifically, as the coupling distance increases, the bandwidth decreases. Within a certain bandwidth range, the narrower the bandwidth, the better the out-of-band suppression.

It can be seen from the above that the present invention has the following beneficial effects:

1. This invention is based on varactor diodes and inductors, that is, using unconventional LC inductors to build circuits. It not only has a simple overall structure and is easy to process, but also has small size, low cost, and has debugging function.

2. The present invention is convenient for debugging: the present invention utilizes inductor spatial coupling, and the coupling coefficient can be adjusted by adjusting the distance of the coupling inductor, thereby completing bandwidth adjustment, improving the convenience of adjustment, and reducing the difficulty of debugging; and by changing the inductance L3 and the inductance The L4 inductance value can adjust the center frequency; changing the inductor L2 and inductor L5 can adjust the in-band insertion loss and standing wave.

Example 2

This embodiment provides an adjustable bandpass tuning frequency selection circuit. The bandpass tuning frequency selection circuit includes two resonant circuits cascaded in a symmetrically distributed manner. One of the resonant circuits includes one end connected to a signal input end. Inductor L1, inductor L2 and inductor L3, one end of which is connected to the other end of inductor L1 and the other end of which is grounded, and the inductor L3 is connected in series with a varactor diode D2 and then grounded, and the input end of the varactor diode D2 is connected Control voltage V _IN2 ;

The other resonant circuit includes an inductor L6 with one end connected to the signal output end, an inductor L4 and an inductor L5 with one end connected to the other end of the inductor L6 and the other ends both grounded, and the inductor L4 is connected in series with a varactor diode D3 and then grounded. , the input terminal of the varactor diode D3 is connected to the control voltage V _IN3 ;

The inductor L3 and the inductor L4 are coupled.

An adjustable bandpass tuning frequency selection circuit also includes a varactor diode D1 connected in parallel with the inductor L1, and the input end of the varactor diode D1 is connected to the control voltage V _IN1 .

This embodiment is based on Embodiment 1 and adds a set of varactor diodes D1 connected in parallel with the inductor L1. At the same time, preferably, the inductor is a high-Q inductor. Preferably, the control voltage V _IN1 and the control voltage V _IN2 are the same variable voltage source.

In this embodiment, changing the inductance value of the inductor L1 can change the resonant frequency of the varactor diode D1 and the inductor L1, and then change the position of the resonance pit, that is, change the position of the zero point, so as to achieve the purpose of adjustable zero point. Specifically, by adjusting the inductance value of the inductor L1, the adjustable zero point is closer to the center frequency and the out-of-band suppression is better.

At the same time, in this embodiment, the position of the zero point can also be changed by adjusting the control voltage V _IN1 , that is, changing the size of the control voltage connected to the input end of the varactor diode D1 .

Based on the above two zero point adjustment methods, the present invention combines them, that is, by changing the inductance value of the inductor L1 and adjusting the value of the control voltage V _IN1 , the position of the zero point is changed, which not only achieves the purpose of zero point adjustment, but also achieves the purpose of zero point adjustment through multiple Comprehensive adjustment in this way will provide better adjustment effect.

In summary, the center frequency and bandwidth of the adjustable bandpass tuning frequency selection circuit designed in the present invention are easy to control. By controlling the size of the control voltage and then the capacitance value of the varactor diode, the center frequency can be controlled, and the coupling can be adjusted The distance of the inductor controls the bandwidth. The most important thing is that based on the zero-point adjustable feature, the zero-point position can be adjusted to make the zero-point closer to the center frequency and have better out-of-band suppression.

The adjustable bandpass tuning frequency selection circuit designed by the present invention introduces an adjustable zero point outside the band (the point where the signal does not pass, that is, the varactor diode D1 in Figure 1), which improves the out-of-band suppression. If there is no D1, only the center point is adjustable and no zero point is adjustable. The introduction of the varactor diode D1 can quickly lower the frequency and reach the lowest level quickly, forming a resonance pit, that is, the zero point; and the introduction of the resonance pit by the filter can improve the out-of-band suppression, that is, the amplitude on the right side of the zero point is lower than the left The amplitude height on the side; and improving the out-of-band suppression can filter out unnecessary waves, and the filtering effect is better.

Example 3

The invention discloses an adjustable electrically adjustable filter, specifically an inductively coupled zero-point adjustable electrically adjustable filter suitable for microwave planar circuits. The bandpass tuning frequency selection circuit of the electrically adjustable filter is The bandpass tuning frequency selection circuit described in Embodiment 1 or Embodiment 2 above.

Adjust the inductance value of each inductor so that the center frequency of the bandpass tuning frequency selection circuit reaches the range of 200-400MHz. By adjusting the voltage value of the adjustment control voltage V _IN1 connected to the varactor diode D1, the zero point variable is generated. Specifically, the amplitude-frequency response curves of the electrically adjustable filter with adjustable center frequency are shown in Figures 2, 3, and 4. In the figure, DB(S2,1) is the signal transmission characteristic curve, and the curves VSWR(1) and VSWR(2) are the signal port reflection curves. The value on the left side of the amplitude-frequency response curve is the insertion loss, and the value on the right side is the standing wave ratio.

The first implementation: when the capacitance value of the varactor diode D1 is 9pF, the capacitance value of the varactor diode D2 and the varactor diode D3 is 60pF (the capacitance is adjusted by adjusting the corresponding control voltage. The greater the control voltage, the greater the varactor value. The smaller the capacitance of the diode), as shown in Figure 2, at this time, the center frequency of the ESC filter is 200MHz, the in-band insertion loss is -1.2dB, the 3dB bandwidth is 7.9MHz, and the zero point is located at 258MHz.

The second implementation situation: When the capacitance value of the varactor diode D1 is 5.2pF, the capacitance value of the varactor diode D2 and the varactor diode D3 is 27.4pF. As can be seen from Figure 3, the center frequency of the ESC filter is 300Hz, within the band. The insertion loss is -1.4dB, the 3dB bandwidth is 8.5MHz, and the zero point is at 355MHz.

The third implementation situation: When the capacitance value of the varactor diode D1 is 2.5pF, the capacitance value of the varactor diode D2 and the varactor diode D3 is 15.6pF. As can be seen from Figure 4, the center frequency of the ESC filter is 400MHz, within the band The insertion loss is -1.5dB, the 3dB bandwidth is 9MHz, and the zero point is located at 482.6MHz.

It can be seen from the above that the smaller the capacitance value, the larger the center frequency, and the distance between the zero point and the center frequency first decreases and then increases.

Example 4

The invention discloses an adjustment method for an adjustable bandpass tuning frequency selection circuit. The adjustment method is implemented based on the circuit in Embodiment 1.

Methods include the following:

Adjust the in-band insertion loss and standing wave: If the inductance values of inductor L2 and inductor L5 are large, then the standing wave difference is different, then reduce the inductance value of inductor L2 and inductor L5, thereby adjusting the standing wave; if the inductor L2 and inductor L5 inductance If the value is small, if the standing wave difference is at this time, increase the inductance value of the inductor L2 and the inductor L5, thereby adjusting the standing wave;

Adjust the center frequency: increase the inductance value of inductor L3 and inductor L4, the center frequency will decrease; otherwise, the center frequency will increase;

Adjustment bandwidth: Increase the distance between the adjustment inductor L3 and the fourth inductor L4, and the bandwidth will decrease.

Example 5

The invention also discloses another adjustment method of an adjustable bandpass tuning frequency selection circuit, which is implemented based on the circuit in Embodiment 2.

The adjustment method includes the following content: Adjust the in-band insertion loss and standing wave: if the inductance value of the inductor L2 and the inductor L5 is large and the standing wave difference is at this time, reduce the inductance value of the inductor L2 and the inductor L5, thereby adjusting the standing wave. ;If the inductance values of inductor L2 and inductor L5 are small and the standing wave difference is at this time, increase the inductance values of inductor L2 and inductor L5 to adjust the standing wave;

Adjust the center frequency: increase the inductance value of inductor L3 and inductor L4, the center frequency will decrease; otherwise, the center frequency will increase;

Adjustment bandwidth: Increase the distance between the adjustment inductor L3 and the fourth inductor L4, and the bandwidth will decrease;

Adjust the distance between the zero point and the center frequency: adjust the inductance value of the inductor L1 and/or change the voltage value of the control voltage V _IN1 of the variable diode D1, thereby changing the zero point position.

The above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention shall be included in the protection of the present invention. within the range.

Claims (6)

1. The utility model provides an adjustable band-pass tuning frequency-selecting circuit, its characterized in that, band-pass tuning frequency-selecting circuit includes two resonant circuits that concatenate with symmetrical distribution mode, one of them resonant circuit includes inductance L1 that one end is connected with the signal input, inductance L2 and inductance L3 that one end all is connected with inductance L1 other end and the other end all is grounded, just concatenate varactor D2 of inductance L3 is grounded behind, varactor D2's input access control voltage V _IN2 ；

The other resonant circuit comprises an inductor L6 with one end connected with the signal output end, an inductor L4 and an inductor L5 with one end connected with the other end of the inductor L6 and the other end grounded, wherein the inductor L4 is connected with a varactor diode D3 in series and then grounded, and the input end of the varactor diode D3 is connected into the circuitControl voltage V _IN3 ；

The inductor L3 is coupled with the inductor L4;

the transformer also comprises a varactor D1 connected in parallel with the inductance L1, wherein the input end of the varactor D1 is connected with the voltage V _IN1 。

2. The tunable bandpass tuning frequency selective circuit of claim 1 wherein the inductor is a high Q inductor.

3. An adjustable bandpass tuning frequency selective circuit according to claim 1, wherein said control voltage V _IN1 And control voltage V _IN2 Is the same variable voltage source.

4. An adjustable electrically tunable filter, wherein the bandpass tuning frequency selection circuit of the electrically tunable filter is the bandpass tuning frequency selection circuit of any one of claims 1 to 3.

5. A method for adjusting an adjustable band-pass tuning frequency-selecting circuit, wherein the adjusting method is based on the band-pass tuning frequency-selecting circuit as claimed in any one of claims 1, 2 and 3, and the adjusting method comprises:

adjusting in-band interpolation loss and standing waves: if the inductance values of the inductor L2 and the inductor L5 are larger, the standing wave difference is generated at the moment, and the inductance values of the inductor L2 and the inductor L5 are reduced, so that the standing wave is adjusted; if the inductance values of the inductor L2 and the inductor L5 are smaller, the standing wave difference is generated at the moment, the inductance values of the inductor L2 and the inductor L5 are increased, and thus standing waves are adjusted;

adjusting the center frequency: increasing the inductance value of the inductor L3 and the inductor L4, and reducing the center frequency; otherwise, the center frequency increases;

adjusting the bandwidth: the distance between the adjusting inductance L3 and the fourth inductance L4 is increased, and the bandwidth is reduced.

6. A method for adjusting an adjustable band-pass tuning frequency-selecting circuit, wherein the adjusting method is based on the band-pass tuning frequency-selecting circuit according to claim 1, and the adjusting method comprises:

adjusting in-band interpolation loss and standing waves: if the inductance values of the inductor L2 and the inductor L5 are larger, the standing wave difference is generated at the moment, and the inductance values of the inductor L2 and the inductor L5 are reduced, so that the standing wave is adjusted; if the inductance values of the inductor L2 and the inductor L5 are smaller, the standing wave difference is generated at the moment, the inductance values of the inductor L2 and the inductor L5 are increased, and thus standing waves are adjusted;

adjusting the center frequency: increasing the inductance value of the inductor L3 and the inductor L4, and reducing the center frequency; otherwise, the center frequency increases;

adjusting the bandwidth: the distance between the adjusting inductor L3 and the fourth inductor L4 is increased, and the bandwidth is reduced;

distance between zero and center frequency is adjusted: adjusting the inductance value of the inductance L1 and/or changing the control voltage V of the variable diode D1 _IN1 Thereby changing the zero point position.