CN202364184U - Balanced radio frequency electric adjustment band pass filter with constant absolute bandwidth - Google Patents

Abstract

The utility model discloses a balanced radio frequency electric regulation band-pass filter with constant absolute bandwidth. The bandpass filter is composed of a microstrip structure in the upper layer, a dielectric substrate in the middle layer and a ground metal in the lower layer. The upper microstrip structure adopts a balanced circuit, including four half-wavelength resonators, two input feed networks, two output feed networks, two input ports and two output ports; the four half-wavelength resonators are composed of microstrip The first half-wavelength resonator and the third half-wavelength resonator are loaded with capacitance in the middle; the second half-wavelength resonator and the fourth half-wavelength resonator are bent and symmetrically arranged; the entire filter The device structure is mirror-symmetrical. The balanced radio frequency electronically adjustable filter of the utility model realizes absolutely constant bandwidth when the central frequency is tuned, can suppress common mode interference, and can be used as a reconfigurable radio frequency front end of wireless communication.

Description

Balanced RF Electrically Tunable Bandpass Filter with Constant Absolute Bandwidth

technical field

The utility model relates to a balanced radio frequency electric regulation bandpass filter with adjustable center frequency, in particular to a balanced radio frequency electric regulation with constant absolute bandwidth that can be applied in multi-band, wide-band and reconfigurable radio frequency front-end systems bandpass filter. the

Background technique

Modern ultra-wideband radar and wireless communications require high-performance reconfigurable RF front-ends. For example, in the cognitive radio system, in order to fully utilize and integrate various wireless channels and standards, the RF front-end needs to work at different frequencies, which requires a reconfigurable RF front-end with tunable center frequency. As an important part of the reconfigurable RF front-end, RF electronically tuned band-pass filters are receiving increasing attention. In this regard, there have been some research reports, and a variety of different tuning devices have also been used, such as semiconductor varactors, radio frequency micro-electromechanical systems (RF MEMS) capacitors, and ferroelectric thin-film material varactors. the

No matter which tuning device is used, the problems faced by the RF electric tuning bandpass filter mainly include:

(1) For example, when tuning the center frequency of the passband, the absolute bandwidth of the passband will also change accordingly, and in many applications the absolute bandwidth of the wireless channel is constant, so we need to tune the center frequency Keeps the absolute bandwidth of the passband constant.

(2) Interference from environmental noise around the system. The presence of environmental noise leads to a decrease in the performance of the filter, thereby affecting the overall performance of the RF front-end. Therefore, some measures must be taken to suppress environmental noise. the

Aiming at the problem of constant bandwidth when the center frequency is tuned, some methods have been proposed so far. According to "M. Sanchez-Renedo, R. Gomez-Garcia, J. I. Alonso, and C. Briso-Rodriguez, Tunable combline filter with continuous control of center frequency and bandwidth , IEEE Trans. Microw. Theory Tech., vol. 53, no 1, pp. 191-199, Jan, 2005." The analysis provided shows that the constant bandwidth can be satisfied by controlling the coupling coefficient by inserting a medium between the resonators. According to "S. J. Park, and G. M. Rebeiz, Low-loss two-pole tunable filters with three different predefined bandwidth characteristics , IEEE Trans. Microw. Theory Tech., vol. 56, no. 5, pp. 1137-1148, May, 2008 .” The analysis provided shows that the specific bandwidth characteristics can be achieved by using independent electrical coupling and magnetic coupling mechanisms to control the variation of the coupling coefficient. According to "M. A. El-Tanani, and G. M. Rebeiz, High-Performance 1.5-2.5-GHz RF-MEMS Tunable Filters for Wireless Applications , IEEE Trans. Microw. Theory Tech., vol. 58, no. 6, pp. 1629-1637 , Jun, 2010." According to the analysis provided, the electromagnetic hybrid coupling mechanism can also satisfy the constant bandwidth. However, the methods proposed above are all single-port circuits, which are basically powerless to suppress environmental noise.

Balanced structure circuits have a good suppression effect on environmental noise, so balanced circuits are widely used in modern communication systems. Most of the current research focuses on stopband extension, common-mode rejection, broadening the passband, or using differential-mode response to obtain dual-band. According to "J. Shi, and Q. Xue, Balanced Bandpass Filters Using Center-Loaded Half-Wavelength Resonators , IEEE Trans. Microw. Theory Tech., vol. 58, no. 4, pp. 970-977, Apr, 2010. "The analysis provided shows that the way of loading the resistor in the middle can absorb the common mode signal. But the above-mentioned balanced filter design is not frequency adjustable. So far there is no research report on balanced RF electrically tunable filters with absolute bandwidth control and common-mode rejection characteristics.

Utility model content

In order to achieve a constant absolute bandwidth and suppress common-mode signals such as environmental noise, the utility model provides a balanced radio frequency electronically adjustable band-pass filter with a constant absolute bandwidth. The balanced band-pass filter not only Constant absolute bandwidth and good rejection of common-mode signals. the

The technical scheme that the utility model solves its technical problem adopts is:

A balanced RF electronically tuned bandpass filter with constant absolute bandwidth, including the upper microstrip structure, the middle dielectric substrate and the lower ground metal; the upper microstrip structure is attached to the upper surface of the middle dielectric board, and the lower middle dielectric board The surface is grounded metal; the upper microstrip structure includes four half-wavelength resonators, two input feed networks, two output feed networks, two input ports and two output ports, and the two input ports are respectively connected to the two input The feed network is connected, the two output ports are respectively connected to the two output feed networks, the two input feed networks are respectively connected to the first half-wavelength resonator in the form of a tap line, and the first half-wavelength resonator is connected to the second half-wavelength resonator respectively. The second half-wavelength resonator is connected to the fourth half-wavelength resonator, the second half-wavelength resonator and the fourth half-wavelength resonator are respectively coupled to the third half-wavelength resonator, and the third half-wavelength resonator is then tapped. They are respectively connected to the two output feed networks. The middle of the first half-wavelength resonator and the third half-wavelength resonator are loaded with capacitors of different sizes for absorbing common mode signals. Both ends of all the above-mentioned half-wavelength resonators are There are varactor diodes.

In the above-mentioned balanced radio frequency electronically adjustable bandpass filter with constant absolute bandwidth, the first half-wavelength resonator is composed of a first varactor diode, a first microstrip line, a second microstrip line, and a third microstrip line , the fourth microstrip line and the second varactor diode are connected in sequence, the anodes of the first varactor diode and the second varactor diode pass through the intermediate dielectric substrate and are connected to the lower ground metal, and the third half-wavelength resonator and The structure of the first half-wavelength resonator is the same; the second half-wavelength resonator is composed of the third varactor diode, the fifth microstrip line, the sixth microstrip line, the seventh microstrip line, and the fourth varactor diode connected in sequence, The anodes of the third varactor diode and the fourth varactor diode are both connected to the lower ground metal through the intermediate layer dielectric substrate, and the fourth half-wavelength resonator has the same structure as the second half-wavelength resonator and is located between the first half-wavelength resonator and the second half-wavelength resonator. Between the third half-wavelength resonators; the fifth microstrip line of the second half-wavelength resonator and the second microstrip line of the first half-wavelength resonator are arranged in parallel to form an interstage coupling structure; the fifth microstrip line of the second half-wavelength resonator The seven microstrip lines and the tenth microstrip line of the third half-wavelength resonator are arranged in parallel to form an interstage coupling structure; the above-mentioned four half-wavelength resonators are arranged in a symmetrical structure both left and right and up and down; in the two input feed networks The first input feed network of the first input feed network is composed of the first capacitor and the eighth microstrip line connected in sequence, and the other end of the eighth microstrip line is connected to the second microstrip line of the first half-wavelength resonator in the form of a tap line; the second microstrip line The structure of the two-input feed network is the same as that of the first input feed network; the first input port in the two input ports is composed of the ninth microstrip line, and the ninth microstrip line is connected to the first capacitor of the first input feed network. The beginning connection, the second input port has the same structure as the first input port, the two input feed networks have the same structure as the two output feed networks, the two input ports and the two output ports have the same structure; the two input feed networks, The two output feeding networks, the two input ports, the two output ports and the above four half-wavelength resonators are arranged together to form a symmetrical structure both left and right and up and down. the

In the above-mentioned balanced radio frequency electronically tuned bandpass filter with constant absolute bandwidth, the other ends of the capacitors pass through the intermediate dielectric substrate and are connected to the ground metal on the lower layer. the

In the balanced radio frequency electronically adjustable bandpass filter with a constant absolute bandwidth, when the first input port and the second input port input differential mode signals, the entire filter resonates in the first half-wavelength resonator and the third half-wavelength resonator An electrical isolation wall is formed on the line where the midpoint of the device is located. Since the coupling between resonators of this structure is mainly electrical coupling, the first half-wavelength resonator and the third half-wavelength resonator have no current in the middle position, and are connected to the first half-wavelength resonator and the third half-wavelength resonator The capacitance loaded in the middle can be ignored, so under differential mode excitation, the first half-wavelength resonator and the third half-wavelength resonator are equivalent to two identical quarter-wavelength resonators, and the second half-wavelength resonator at the same time The wavelength resonators are coupled to form a band-pass filter structure; when the first input port and the second input port input a common-mode signal, the line where the midpoint of the first half-wavelength resonator and the third half-wavelength resonator of the entire filter is located position to form a magnetic isolation wall. The first half-wavelength resonator and the third half-wavelength resonator have a current flowing in the middle position, and the capacitor loaded between the first half-wavelength resonator and the third half-wavelength resonator has a current flowing through it. The two quarter-wavelength resonators equivalent to the first half-wavelength resonator and the third half-wavelength resonator need to consider the capacitance loaded in the middle. Since the capacitances loaded in the middle of the first half-wavelength resonator and the third half-wavelength resonator are different, the resonant frequencies of the two quarter-wavelength resonators that are actually working equivalently are different, so that the common mode signal cannot pass through , to achieve the effect of suppression. All the input and output feed networks adopt the tapped line feed mode, which can obtain a better external quality factor; the electromagnetic hybrid coupling mechanism in the coupling area can satisfy constant absolute bandwidth. the

For further realizing the purpose of the utility model, the length of the first microstrip line of the balanced radio frequency electrically adjustable bandpass filter with constant absolute bandwidth is 10.2mm, the width is 0.8mm, and the length of the second microstrip line is 18.7mm. The length of the fifth microstrip line is 24.1mm, the width is 0.8mm, the length of the sixth microstrip line is 10.4mm, the length of the eighth microstrip line is 3.3mm, the width is 0.6mm, the first capacitance is 7pF, the first half The capacitance loaded in the middle of the wavelength resonator is 20pF, the capacitance loaded in the middle of the third half-wavelength resonator is 7pF, and the distance between the second microstrip line and the fifth microstrip line is 0.6mm. the

Compared with the prior art, the utility model adopts a new balance structure and a half-wavelength resonator interstage coupling structure, and the absolute bandwidth bandwidth is kept constant when the center frequency is tuned, and the electronically adjustable bandpass filter that can well suppress common-mode interference signals device. Overall, it has the following advantages and effects:

(1) Due to the use of a balanced structure design, the bandpass filter can work normally for differential-mode signals, but has a better suppression effect for common-mode signals, so it has an immune function against interference such as environmental noise. The measured common-mode rejection levels in the embodiments all exceed -23dB.

(2) By setting the input feed network and inter-stage coupling mode, the relative bandwidth or absolute bandwidth can be kept constant when the center frequency is tuned, which can meet different application requirements. the

Description of drawings

Fig. 1 is a schematic diagram of a balanced radio frequency electrically adjustable bandpass filter ABW with a constant absolute bandwidth;

Fig. 2 is the equivalent circuit of ABW differential mode;

Figure 3a is an equivalent quarter-wavelength resonator in the case of ABW differential mode;

Figure 3b is an equivalent half-wavelength resonator in the case of ABW differential mode;

Fig. 4 is the relationship between the Xie vibration frequency, the capacitance value and the length of the microstrip line of the quarter-wavelength resonator in Fig. 3a;

Fig. 5 is the relationship between the Xie vibration frequency, the capacitance value and the length of the microstrip line of the half-wavelength resonator in Fig. 3b;

Fig. 6 is ABW common mode equivalent circuit;

Fig. 7 is the equivalent quarter-wavelength resonator of the first resonator in the case of ABW common mode;

Fig. 8a is the differential mode transmission characteristic curve of ABW;

Figure 8b is the differential mode return loss curve of ABW;

Figure 8c is the common mode transmission characteristic curve of ABW.

specific implementation plan

The utility model will be described in further detail below in conjunction with the accompanying drawings, but the scope of protection claimed by the utility model is not limited to the scope of the following examples. the

As shown in Figure 1, a balanced radio-frequency electronically tuned bandpass filter with constant absolute bandwidth is characterized in that it includes an upper microstrip structure, an intermediate dielectric substrate and a lower ground metal; the upper microstrip structure is attached to the intermediate dielectric The upper surface of the board and the lower surface of the intermediate dielectric board are grounded metal; the upper microstrip structure includes four half-wavelength resonators, two input feed networks, two output feed networks, two input ports and two output ports, The two input ports are respectively connected to the two input feed networks, the two output ports are respectively connected to the two output feed networks, and the two input feed networks are respectively connected to the first half-wavelength resonator in the form of tapped lines (ie not connected to both ends of the first half-wavelength resonator, but to the part between the two ends), the first half-wavelength resonator is connected to the second half-wavelength resonator and the fourth half-wavelength resonator respectively, and the second half-wavelength resonator The second half-wavelength resonator and the fourth half-wavelength resonator are respectively coupled with the third half-wavelength resonator, and the third half-wavelength resonator is respectively connected to the two output feed networks in the form of a tapped line, and the first half-wavelength resonator Capacitors of different sizes are loaded in the middle of the resonator and the third half-wavelength resonator for absorbing common-mode signals; all of the above-mentioned half-wavelength resonators have varactor diodes at both ends. the

The first half-wavelength resonator consists of a first varactor diode 1, a first microstrip line 2, a second microstrip line 3, a third microstrip line 4, a fourth microstrip line 5 and a second varactor diode 6 connected sequentially, the anodes of the first varactor diode 1 and the second varactor diode 6 pass through the intermediate dielectric substrate and are connected to the lower ground metal, the third half-wavelength resonator has the same structure as the first half-wavelength resonator; The second half-wavelength resonator is composed of the third varactor diode 7, the fifth microstrip line 8, the sixth microstrip line 9, the seventh microstrip line 10, and the fourth varactor diode 11 connected in sequence, and the third varactor diode 7 and the anodes of the fourth varactor diode 11 are connected to the lower ground metal through the intermediate dielectric substrate, and the fourth half-wavelength resonator has the same structure as the second half-wavelength resonator and is located between the first half-wavelength resonator and the third half-wavelength resonator. Between the wavelength resonators; the fifth microstrip line 8 of the second half-wavelength resonator and the second microstrip line 3 of the first half-wavelength resonator are arranged in parallel to form an interstage coupling structure; the seventh of the second half-wavelength resonator The microstrip line 10 and the tenth microstrip line 12 of the third half-wavelength resonator are arranged in parallel to form an interstage coupling structure; the above-mentioned four half-wavelength resonators are arranged in a symmetrical structure both left and right, up and down; two input feed networks The first input feed network 17 is formed by connecting the first capacitor 15 and the eighth microstrip line 16 in sequence, and the other end of the eighth microstrip line 16 is connected to the second microstrip of the first half-wavelength resonator in the form of a tapped line. On the strip line 3; the structure of the second input feed network 18 is the same as that of the first input feed network 17; the first input port IN in the two input ports is formed by the ninth microstrip line 21, and the ninth microstrip line 21 Connected to the beginning of the first capacitor 15 of the first input feed network 17, the second input port IN' has the same structure as the first input port IN, the two input feed networks have the same structure as the two output feed networks, and the two input The port and the two output ports have the same structure; two input feed networks, two output feed networks, two input ports, two output ports and the above four half-wavelength resonators are arranged together to form a left-right, up-down symmetrical structure. The middle of the first half-wavelength resonator and the third half-wavelength resonator are loaded with a second capacitor 13 and a third capacitor 14 of different sizes for absorbing common mode signals, and the second capacitor 13 and the third capacitor 14 have One end passes through the intermediate dielectric substrate and connects to the lower ground metal;

The characteristic impedance of the transmission lines of the two input ports and the two output ports is 50Ω.

Adjust the parameters of the filter to make the filter achieve a balance in the entire structure. When the first input port IN and the second input port IN' input a differential mode signal, the whole filter forms an electrical isolation wall at the linear position where the midpoints of the first half-wavelength resonator and the third half-wavelength resonator are located. Since the coupling between resonators of this structure is mainly electrical coupling, the first half-wavelength resonator and the third half-wavelength resonator have no current in the middle position, and are connected to the first half-wavelength resonator and the third half-wavelength resonator The second capacitor 13 and the third capacitor 14 loaded in the middle can be ignored, so under differential mode excitation, the first half-wavelength resonator and the third half-wavelength resonator are equivalent to two quarter-wavelength resonators , while coupling with the second half-wavelength resonator to form a bandpass filter structure; at this time, the equivalent structure of the filter is shown in FIG. 2 . Figures 3a and 3b show the equivalent quarter-wavelength resonator and second half-wavelength resonator for the differential mode case. According to the analysis provided by "A. R. Brown, and G. M. Rebeiz, A varactor-tuned RF filter , IEEE Trans. Microw. Theory Tech., vol. 48, no. 7, pp. 1157-1160, Jul, 2000." In Figure 3a, when the quarter-wavelength resonator is resonant, the imaginary part of the admittance Y _{dd_in1} seen from the left end of the quarter-wavelength resonator is equal to zero, and for a given voltage, the entire varactor loaded Resonant frequency of the resonator:

;

;

是谐振器的特性导纳；

是第一微带线6的电长度；C是变容二极管在不同电压下的电容值；在图4中显示了图3a中四分之一波长谐振器的谐振频率、变容二极管的电容值C和微带线长度的关系，可以看出随着电容的增大，谐振器的谐振频率下降；随着微带线长度的变短，可调节区间变大；同样对于第二半波长谐振器，谐振频率为： in

is the characteristic admittance of the resonator;

is the electrical length of the first microstrip line 6; C is the capacitance value of the varactor diode at different voltages; in Fig. 4, the resonant frequency of the quarter-wavelength resonator in Fig. The relationship between C and the length of the microstrip line, it can be seen that as the capacitance increases, the resonant frequency of the resonator decreases; as the length of the microstrip line becomes shorter, the adjustable interval becomes larger; the same for the second half-wavelength resonator , the resonant frequency is:

; 

;

时，相同的电压下

;所以两个谐振器频率在电压相同时能够匹配，滤波器能够正常工作。 in is the characteristic admittance of the resonator; is the electrical length of the half-wavelength resonator; C is the capacitance value of the varactor diode at different voltages; in Fig. 5, the resonant frequency of the second half-wavelength resonator in Fig. 3b, the capacitance value C of the varactor diode and micro The relationship between the length of the strip line, it can be seen that the relationship between the resonance frequency and the capacitance value within the length of 10-18mm is similar to the relationship between the quarter-wavelength resonator within the length of 6-8mm, when

, at the same voltage

; So the two resonator frequencies can match when the voltage is the same, and the filter can work normally.

When the first input port and the second input port input a common mode signal, the entire filter forms a magnetic isolation wall at the linear position where the midpoints of the first half-wavelength resonator and the third half-wavelength resonator are located. The first half-wavelength resonator and the third half-wavelength resonator have a current flowing in the middle position, and the capacitor loaded between the first half-wavelength resonator and the third half-wavelength resonator has a current flowing through it. The two quarter-wavelength resonators equivalent to the first half-wavelength resonator and the third half-wavelength resonator need to consider the capacitance loaded in the middle. The actual working equivalent filter structure when the common mode signal is shown in Figure 6. Figure 7 shows an equivalent quarter-wavelength resonator with a resonant frequency of:

;

;

As shown in the figure, Y ₁ is the characteristic admittance of the microstrip line; θ ₁ is the electrical length of the microstrip line; C is the capacitance value of the varactor diode at different voltages; C ₂ is the capacitance value of the loading capacitor.

Since the capacitance values of the second capacitor 13 and the third capacitor 14 loaded are different, the resonant frequencies of the two resonators are different, and the signal cannot pass through; the common mode signal is suppressed. the

随着频率的增大而增大；所以在ABW结构中采用抽头线馈电方式，能更容易控制品质因数

；在级间采用电磁混合耦合的机制，如图2中的虚线框内的耦合区域，同时存在电耦合和磁耦合，能够实现级间耦合系数k随着频率的增大而下降。  It can be seen from FIG. 2 and FIG. 6 that the filter is fed in a tapped line manner. According to the analysis provided in "R. J. Cameron, C. M. Kudsia, and R. R. Mansour, Microwave Filters for Communication Systems: Fundamentals, Design, and Applications , New York: Wiley: John Wiley & Sons, Inc, 2007." For ABW, in Some constant absolute bandwidth,; the actual external quality factor of the filter

It increases with the increase of the frequency; therefore, it is easier to control the quality factor by adopting the tapped line feeding method in the ABW structure

; The electromagnetic hybrid coupling mechanism is adopted between the stages, as shown in the coupling area in the dotted line box in Figure 2, where both electrical coupling and magnetic coupling exist, and the inter-stage coupling coefficient k can be reduced as the frequency increases.

In the following embodiments, the ABW with a constant absolute bandwidth of 60 MHz is fabricated on a dielectric substrate with a relative permittivity of 10.2, a thickness of 0.63 mm, and a dissipation factor of 0.0023. The varactor diode is selected from Toshiba's silicon varactor diode lsv277. the

Implementation Example: A Balanced RF Electrically Adjustable Bandpass Filter with a Constant Absolute Bandwidth of 60MHz

4MHz，基本恒定。图8c显示了在共模工作情况下对共模噪声的抑制，可见在0.2-1.7GHz共模抑制都低于-25dB。 The structure of the balanced RF electronically tuned bandpass filter with a constant absolute bandwidth of 60MHz is shown in Figure 1. The specific parameters are: the length of the first microstrip line 2 is 10.2mm, and the width is 0.8mm; the length of the second microstrip line 3 is 18.7mm; the length of the fifth microstrip line 8 is 24.1mm, and the width is 0.8mm; The length of the strip line 9 is 10.4mm, the length of the eighth microstrip line 16 is 3.3mm, and the width is 0.6mm, the size of the first capacitor 15 is 7pF, and the size of the second capacitor 13 loaded in the middle of the first half-wavelength resonator is 20pF , the size of the third capacitor 14 loaded in the middle of the third half-wavelength resonator is 7pF, and the distance between the second microstrip line 3 and the fifth microstrip line 8 is 0.6mm. Figure 8 shows the results of simulation and actual measurement using the filter designed with the above parameters. The simulation and actual measurement are completed using Agilent's commercial electromagnetic simulation software ADS and E5071C network analyzer respectively. Fig. 8a shows the simulation, calculation and test transmission characteristics of the filter at four special bias voltages in the differential mode operation, the horizontal axis represents the frequency, and the vertical axis represents the transmission characteristic |S _dd21 |. Figure 8b shows the reflection characteristics of the filter, the horizontal axis represents the frequency, and the vertical axis represents the return loss |S _dd11 |. It can be seen from Figure 8a and Figure 8b that the passband frequency of the filter can be adjusted from 549MHz to 775MHz, with a relative adjustment range of 34.1%. For all tuning states, the measured in-band insertion loss is between 3.5-4.2dB and the return loss is below -10dB. 3dB bandwidth of 60

4MHz, basically constant. Figure 8c shows the suppression of common-mode noise in the case of common-mode operation, and it can be seen that the common-mode rejection is lower than -25dB in 0.2-1.7GHz.

The utility model is based on a mirror symmetrical balanced structure, has different equivalent circuits under differential mode and common mode signals, has constant absolute bandwidth, adjustable intermediate frequency, and suppresses common mode noise in a wider frequency band. The bandwidth and passband waveform remain constant over the frequency tuning range. By adjusting the parameters of the design, the bandwidth can be adjusted, that is, this structure can be used to achieve various bandwidth specifications. the

The above descriptions are only preferred examples of the present utility model, and are not intended to limit the present utility model. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present utility model shall be included in the present utility model. within the scope of the new protection. the

Claims (6)

1. the balanced type radio frequency electrically adjusted band-pass filter that has constant absolute bandwidth is characterized in that comprising the microstrip structure on upper strata, the grounded metal of intermediate layer medium substrate and lower floor; The upper strata microstrip structure is attached to intermediate layer dielectric-slab upper surface, and intermediate layer dielectric-slab lower surface is a grounded metal; The upper strata microstrip structure comprises four half-wave resonator, two input feeding networks, two outputs feeding network, two input ports and two output ports; Two input ports are connected with two input feeding networks respectively; Two output ports are connected with two output feeding networks respectively; First half-wave resonator is joined with two input feeding networks respectively with the tap line mode; First half-wave resonator is coupled with second half-wave resonator and the 4th half-wave resonator respectively again; Second half-wave resonator and the 4th half-wave resonator are coupled with the 3rd half-wave resonator respectively again; The 3rd half-wave resonator is joined with two output feeding networks respectively with the tap line mode again, and the centre of first half-wave resonator and the 3rd half-wave resonator all is loaded with the different big or small electric capacity that are used to absorb common-mode signal, and all there is variable capacitance diode at the two ends of above-mentioned all half-wave resonator.

2. the balanced type radio frequency electrically adjusted band-pass filter with constant absolute bandwidth according to claim 1; It is characterized in that said first half-wave resonator is connected and composed by first variable capacitance diode, first microstrip line, second microstrip line, the 3rd microstrip line, the 4th microstrip line and second variable capacitance diode in order; First microstrip line is perpendicular to second microstrip line; The 3rd microstrip line is perpendicular to the 4th microstrip line; The anode of first variable capacitance diode and second variable capacitance diode all passes the intermediate layer medium substrate and links to each other with the lower floor grounded metal, and the 3rd half-wave resonator is identical with the first half-wave resonator structure; Second half-wave resonator is connected and composed by the 3rd variable capacitance diode, the 5th microstrip line, the 6th microstrip line, the 7th microstrip line, the 4th variable capacitance diode in order; The anode of the 3rd variable capacitance diode and the 4th variable capacitance diode all passes the intermediate layer medium substrate and links to each other with the lower floor grounded metal; The 4th half-wave resonator is identical with the second half-wave resonator structure and between first half-wave resonator and the 3rd half-wave resonator, about above-mentioned four half-wave resonator are arranged into together, equal symmetrical structure up and down; Two first input feeding networks of importing in the feeding network are connected and composed by first electric capacity, the 8th microstrip line in order; One end of the 8th microstrip line is connected with first electric capacity, one end, and second microstrip line of first half-wave resonator is connected with the other end of tap line mode with the 8th microstrip line; The structure of the second input feeding network is identical with the first input feeding network; First input end mouth in two input ports is made up of the 9th microstrip line; The 9th microstrip line is connected with the other end of first electric capacity of the first input feeding network; Second input port is identical with the first input end mouth structure; Two input feeding networks are identical with two output feeding network structures, and two input ports are identical with two output port structures; About two input feeding networks, two outputs feeding networks, two input ports, two output ports and above-mentioned four half-wave resonator are arranged into together, equal symmetrical structure up and down.

3. the balanced type radio frequency electrically adjusted band-pass filter with constant relative bandwidth according to claim 2, it is characterized in that first half-wave resonator and the 3rd half-wave resonator in the middle of the other end of the said electric capacity that loads pass the intermediate layer medium substrate and link to each other with the lower floor grounded metal.

4. the balanced type radio frequency electrically adjusted band-pass filter with constant relative bandwidth according to claim 2, second microstrip line that it is characterized in that the 5th microstrip line and first half-wave resonator of second half-wave resonator laterally arranges and constitutes the interstage coupling structure; The 7th microstrip line of second half-wave resonator and the tenth microstrip line of the 3rd half-wave resonator laterally arrange and constitute the interstage coupling structure.

5. the balanced type radio frequency electrically adjusted band-pass filter with constant relative bandwidth according to claim 2, the characteristic impedance that it is characterized in that the transmission line of two input ports and two output ports all is 50 Ω.

6. according to each described radio frequency electrically adjusted filter of balanced type of claim 2～5, it is characterized in that the said first microstrip line length is 10.2mm with constant relative bandwidth; Width is 0.8mm, and the second microstrip line length is 18.7mm, and the 5th microstrip line length is 24.1mm; Width is 0.8mm, and the 6th microstrip line length is 10.4mm, and the 8th microstrip line length is 3.3mm; Width is 0.6mm, and first capacitance size is 7pF, and the said capacitance size that loads in the middle of first half-wave resonator is 20pF; The said capacitance size that loads in the middle of the 3rd half-wave resonator is 7pF, and the distance between second microstrip line and the 5th microstrip line is 0.6mm.

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