CN101916892A - Tunable Band-Stop Filter with Constant Absolute Bandwidth Based on Modular Structure - Google Patents

Abstract

The invention discloses an adjustable band-stop filter with constant absolute bandwidth based on a modular structure, which comprises an upper-layer microstrip structure, a middle-layer dielectric substrate and a lower-layer grounded metal plate. The filter can be composed of one modular unit or multiple modular units, and each modular unit includes two resonators and the main transmission line; both resonators are half-wavelength resonators with the same structure, about the central longitudinal axis of the microstrip structure Symmetrically arranged; the main transmission line includes a coupling part and a non-coupling part; its coupling part is composed of the seventh microstrip line, the eighth microstrip line and the ninth microstrip line, which are connected in turn to form an n-shaped structure, located in the n-shaped resonator coupling part The inner side of the structure; the varactor diodes of the two resonators are set with the same bias voltage; the invention can be used in various reconfigurable radio frequency front-end circuits, and has the characteristic of constant absolute bandwidth during frequency tuning. By cascading two or more modular units, an adjustable band-stop filter with higher frequency selectivity and constant absolute bandwidth can be obtained.

Description

Tunable Band-Stop Filter with Constant Absolute Bandwidth Based on Modular Structure

technical field

The invention relates to a band-stop filter with adjustable center frequency, in particular to an adjustable band-stop filter based on a modular structure, whose absolute bandwidth is constant when the center frequency is tuned, and which can be applied to radio frequency front-end circuits.

Background technique

The development of modern ultra-wideband radar and wireless communication requires high-performance reconfigurable RF subsystems. For example, the development of mobile communication requires miniaturized, multi-standard, multi-mode, and multi-band transceivers. In order to meet this requirement, the RF front-end needs to be based on Working mode to adjust its working frequency and bandwidth. This type of RF front-end requires a variety of RF tunable filters to select useful signals and suppress interference signals, among which RF electrically tunable band-stop filters are an important category of RF tunable filters. In the RF front-end circuits of multi-band broadband transceivers and software radios, band-stop filters with adjustable center frequencies can be used to suppress strong interference signals that appear near the frequency of weaker useful signals.

At present, there are many design methods of tunable band-stop filters, among which there are several typical methods. The first method is to tune the frequency of the stopband by adjusting the resonant frequency of the resonator coupled to the main transmission line, as in I.C.Hunter and J.D.Rhodes, “Electronically tunable microwave bandstop filters,” IEEE Trans.Microw.Theory Tech., vol. MTT-30, no.9, pp.1361-1367, Sep.1982. The second method is to use the DGS (defective ground) structure integrated with varactor diodes to design tunable bandstop filters, such as A.M.E.Safwat, F.Podevin, P.Ferrari and A.Vi lcot, "Tunable bandstop defective ground structure resonator using reconfigurable dumbbell-shaped coplanar waveguide,” IEEE Trans. Microw. Theory Tech., vol.54, no.9, pp.3559-3564, Sep.2006. The third method is to adjust the impedance of the resonator to match the source/load impedance in the stopband frequency range, so that the signal energy is absorbed in the lossy oscillator, such as D.R. Jachowski, "Cascadable lossy passive biquad bandstop filter ,” in IEEE MTT-S Int Microwave Symp.Dig., pp.513-516, 2005.

No matter which filter design method is adopted, the design of the electronically tuned band-stop filter faces two problems: one is that the absolute bandwidth of the stop band will change when the center frequency of the stop band is tuned; the other is the design of high-order electronically tuned filters. The problem is that when designing a high-order electronically tuned filter, the design parameters of each stage must be adjusted instead of directly cascading two low-order band-stop filters, resulting in complex design.

Contents of the invention

The purpose of the present invention is to solve the problems existing in the prior art, to provide an adjustable band-stop filter based on a modular structure and having a constant absolute bandwidth when the center frequency is tuned; an adjustable band-stop filter based on a modular structure with a constant absolute bandwidth The filter has the characteristics of constant absolute bandwidth; at the same time, it has the characteristics of modular design, and directly cascades two or more adjustable band-stop filter module units to obtain a band-stop filter with higher frequency selectivity; based on modular The adjustable band-stop filter with a constant absolute bandwidth of the structure can solve the problem that the absolute bandwidth of the stop band changes when the center frequency is tuned and the problem that high-order filter designs cannot be directly cascaded.

For realizing the object of the present invention, the technical scheme adopted in the present invention is as follows:

其中c为光速，ε_r为介质基板的相对介电常数，

f_min和f_max分别为谐振器的谐振频率f可调范围的最小值与最大值；谐振器的电长度L+ΔL为谐振频率f对应的波长λ的二分之一；其中，L为实际微带线长度，ΔL为变容二极管等效微带线长度；实际微带线长度L为第一微带线、第二微带线、第三微带线、第四微带线、第五微带线和第六微带线的长度之和；谐振器与主传输线之间的耦合方式是一种电耦合与磁耦合混合的耦合方式，耦合区间的长度等于第三微带线，第四微带线和第五微带线的长度总和；在最高谐振频率f_max和最低谐振频率f_min上谐振器总的等效微带线的中点落在耦合区间内；该耦合区间内电磁耦合中磁耦合占主导地位，耦合强度随着频率的增加而减小。An adjustable band-stop filter with constant absolute bandwidth based on a modular structure, including the upper microstrip structure, the dielectric substrate of the middle layer and the grounded metal plate of the lower layer; the upper microstrip structure is attached to the upper surface of the middle dielectric board, and The lower surface of the dielectric plate is grounded metal; the upper microstrip structure includes two resonators and the main transmission line; the two resonators are half-wavelength resonators with the same structure, and are symmetrically arranged about the central longitudinal axis of the microstrip structure; each resonator The resonator is composed of a microstrip line and a varactor diode; the varactor diodes of the two resonators set the same bias voltage; the microstrip line of the resonator is divided into a coupling part and an uncoupling part; the microstrip line coupling part of the resonator is composed of The third microstrip line, the fourth microstrip line and the fifth microstrip line are sequentially connected to form an n-shaped structure; the uncoupled part of the microstrip line of the resonator includes the first microstrip line, the second microstrip line and the sixth microstrip line line; one end of the first microstrip line is open, and the other end is connected to the second microstrip line; the other end of the second microstrip line is connected to the third microstrip line; one end of the sixth microstrip line is connected to the fifth microstrip line , the other end is connected to the varactor diode; the other end of the varactor diode is connected to the ground metal of the lower layer through the metallized via hole passing through the intermediate layer dielectric substrate; the main transmission line includes a coupling part and a non-coupling part; The seven microstrip lines, the eighth microstrip line and the ninth microstrip line are sequentially connected to form an n-shaped structure, which is located inside the n-shaped structure of the resonator coupling part; a width of 0.1 is provided between the resonator coupling part and the main transmission line coupling part. The electromagnetic coupling spacing of mm-0.8mm; the uncoupled part of the main transmission line includes the port microstrip line and the connecting microstrip line; the connecting microstrip line is a serpentine line; the port microstrip line is symmetrically arranged on both sides of the connecting microstrip line, the seventh Microstrip line, the eighth microstrip line, and the ninth microstrip line; one end of the port microstrip line is connected to the seventh microstrip line; the connecting microstrip line is connected to the ninth microstrip line; the length of the connecting microstrip line

Where c is the speed of light, ε _r is the relative permittivity of the dielectric substrate,

f _min and f _max are the minimum and maximum values of the adjustable range of the resonant frequency f of the resonator respectively; the electrical length L+ΔL of the resonator is one-half of the wavelength λ corresponding to the resonant frequency f; among them, L is the actual The length of the microstrip line, ΔL is the equivalent microstrip line length of the varactor diode; the actual microstrip line length L is the first microstrip line, the second microstrip line, the third microstrip line, the fourth microstrip line, the fifth The sum of the lengths of the microstrip line and the sixth microstrip line; the coupling mode between the resonator and the main transmission line is a coupling mode in which electrical coupling and magnetic coupling are mixed, and the length of the coupling interval is equal to the third microstrip line, the fourth The sum of the lengths of the microstrip line and the fifth microstrip line; the midpoint of the total equivalent microstrip line of the resonator on the highest resonant frequency f _max and the lowest resonant frequency f _min falls within the coupling interval; the electromagnetic coupling in this coupling interval The magnetic coupling is dominant in the middle, and the coupling strength decreases with the increase of frequency.

In order to further realize the object of the present invention, the adjustable resonant frequency range of the adjustable band-stop filter is 1.73-2.2GHz, wherein the length of the first microstrip line and the third microstrip line is 5.6mm, and the length of the second microstrip line The length of the line is 1.88mm, the length of the fourth microstrip line is 3.75mm, the length of the fifth microstrip line is 9.8mm, the distance between the seventh microstrip line and the ninth microstrip line is 2mm, the sixth microstrip line The length of the stripline is 1.7mm, the coupling spacing between the resonator and the main transmission line is 0.12mm, the width of the sixth microstrip line is 0.7mm, the width of the connecting microstrip line is 0.3mm, the first microstrip line and the second The width of the second microstrip line is 0.7mm, the width of the third microstrip line, the fourth microstrip line and the fifth microstrip line is 0.4mm, the seventh microstrip line, the eighth microstrip line and the ninth microstrip line The width of the port microstrip line is 0.8mm, the width of the port microstrip line is 1.9mm, and the characteristic impedance of the port microstrip line is 50Ω. The length L ₁₂ of the connecting microstrip line is 25.4 mm.

Two identical adjustable band-stop filter module units are connected through a 50Ω transmission line to obtain a two-stage adjustable band-stop filter, and the length of the transmission line is greater than 1.5 times the thickness of the dielectric substrate.

Compared with the prior art, the present invention has the following advantages:

(1) The absolute bandwidth is constant. For common adjustable band-stop filters, when the center frequency of the stop band is tuned, its absolute bandwidth will change. The center frequency of the band-stop filter in the embodiment of the present invention keeps constant when tuning, which is better than ordinary band-stop filters. adjustable band-stop filter.

(2) Modular structure. Multiple modular units can be easily cascaded to form high-order filters to improve frequency selectivity. When designing a high-order electronically tunable filter with a common band-stop filter structure, it is necessary to adjust the design parameters of each stage instead of directly cascading two common low-order band-stop filters, resulting in a complex design. The module unit in the present invention is a modular symmetrical structure, and the impedance seen from the two ports is the same. When the impedance of the two ports is adjusted to 50Ω, the two module units can be directly cascaded to realize a high-order filter , thereby improving its frequency selectivity.

Description of drawings

Fig. 1 is the structural diagram of a module unit of the constant adjustable band stop filter based on the absolute bandwidth of modular structure;

Fig. 2 (a) is the equivalent schematic diagram of the electromagnetic coupling structure of the adjustable band-stop filter;

Fig. 2 (b) is the equivalent circuit diagram of the resonator of the adjustable band-stop filter under different bias voltages;

Fig. 3 is a schematic structural diagram of an adjustable band-stop filter using a modular unit;

Fig. 4 is a transmission characteristic curve diagram of an adjustable band-stop filter adopting a modular unit;

Fig. 5 is a structural schematic diagram of an adjustable band-stop filter using two modular units;

Fig. 6 is a curve diagram of transmission characteristics of an adjustable band-stop filter using two modular units.

specific implementation plan

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

其中c为光速，ε_r为介质基板的相对介电常数，

f_min和f_max分别为谐振器的谐振频率f可调范围的最小值与最大值。As shown in Figure 1, the tunable band-stop filter with constant absolute bandwidth based on the modular structure includes the upper microstrip structure, the middle dielectric substrate and the lower ground metal plate; the upper microstrip structure is attached to the middle dielectric plate The upper surface and the lower surface of the intermediate dielectric plate are grounded metal; the upper microstrip structure includes two resonators and the main transmission line; both resonators are half-wavelength resonators with the same structure, symmetrical about the central longitudinal axis of the microstrip structure setting; each resonator is made of a microstrip line and a varactor diode 7; the microstrip line of the resonator is divided into a coupling part and an uncoupling part; the microstrip line coupling part of the resonator is composed of a third microstrip line 3, a fourth microstrip line The microstrip line 4 and the fifth microstrip line 5 are sequentially connected to form an n-shaped structure; the uncoupled part of the microstrip line of the resonator includes the first microstrip line 1, the second microstrip line 2 and the sixth microstrip line 6; One end of a microstrip line 1 is open, and the other end is connected to the second microstrip line 2; the other end of the second microstrip line 2 is connected to the third microstrip line 3; one end of the sixth microstrip line 6 is connected to the fifth microstrip line The other end is connected to the varactor diode tube 7; the other end of the varactor diode 7 is connected to the lower ground metal plate through the metallized via hole of the dielectric substrate. The main transmission line is arranged symmetrically with respect to the central longitudinal axis of the microstrip structure, including a coupling part and an uncoupling part; the coupling part is sequentially connected by the seventh microstrip line 9, the eighth microstrip line 10 and the ninth microstrip line 11 to form an n-shaped structure , located inside the n-shaped structure of the coupling part of the resonator. The uncoupled part of the main transmission line includes a port microstrip line 8 and a connection microstrip line 12; the connection microstrip line 12 is a serpentine line; the port microstrip line 8 and the seventh microstrip line 9 are arranged symmetrically on both sides of the connection microstrip line 12 , the eighth microstrip line 10, the ninth microstrip line 11; one end of the port microstrip line 8 is connected to the seventh microstrip line 9; the connecting microstrip line 12 is connected to the ninth microstrip line 11; the port microstrip line 8 is connected to the ninth microstrip line The second microstrip line 2 is parallel, and the distance between each other is greater than 1.5 times the thickness of the dielectric substrate to prevent electromagnetic coupling; the characteristic impedance of the port microstrip line 8 is 50Ω; the width between the resonator coupling part and the main transmission line coupling part is 0.1 The electromagnetic coupling spacing of mm-0.8mm realizes electromagnetic coupling; the electromagnetic coupling spacing is determined by the strength of the coupling. The connection of the main transmission line to the microstrip line 12 plays the role of impedance transformation, and the length of the connection to the microstrip line 12

Where c is the speed of light, ε _r is the relative permittivity of the dielectric substrate,

f _min and f _max are the minimum value and maximum value of the adjustable range of the resonant frequency f of the resonator, respectively.

调整谐振器的变容二极管7的偏置电压，则变容二极管7的等效电容会发生改变，其等效微带线长度也会随之改变，从而谐振频率发生变化；如图2(b)所示，当变容二极管的等效电容C_v1＞C_v2时，对应的等效微带线长度ΔL₁＞ΔL₂，对应的谐振频率f₁＜f₂。因此通过调整变容二极管的偏压，可以调整阻带滤波器的中心频率。选定变容二极管7和确定滤波器工作的谐振频率调谐范围f_min、f_max之后，可以确定变容二极管的等效微带线长度的变化范围，然后根据等效微带线的总长度为半波长的特性就可以确定实际微带线的长度L。实际微带线长度L为图1中第一微带线1、第二微带线2、第三微带线3、第四微带线4、第五微带线5和第六微带线6的长度之和。The resonator is composed of a microstrip line and a varactor diode. One end of the microstrip line is connected to a varactor diode, and the other end is open circuit; the first microstrip line 1 of the resonator, the second microstrip line 2, and the third microstrip line 3, The length of the fourth microstrip line 4, the fifth microstrip line 5 and the sixth microstrip line 6 and the total length of the microstrip line equivalent to the varactor diode 7 are half the wavelength at the filter resonance frequency. The resonant frequency of the resonator is mainly adjusted by the bias voltage of the varactor diode. When ignoring the parasitic effect, the varactor diode can be equivalent to a microstrip line with an open circuit. As shown in Figure 2(a), the slash area represents the real microstrip line with a length of L; the grid area represents the microstrip line equivalent to the varactor diode with a length of ΔL; the electrical length of the resonator is L+ΔL It is one-half of the wavelength λ corresponding to the resonance frequency f; the resonance frequency f is inversely proportional to the electrical length, that is

Adjust the bias voltage of the varactor diode 7 of the resonator, then the equivalent capacitance of the varactor diode 7 will change, and its equivalent microstrip line length will also change accordingly, so that the resonant frequency will change; as shown in Figure 2(b ), when the equivalent capacitance C _v1 >C _v2 of the varactor diode, the corresponding equivalent microstrip line length ΔL ₁ >ΔL ₂ , and the corresponding resonant frequency f ₁ <f ₂ . Therefore, by adjusting the bias voltage of the varactor diode, the center frequency of the stop-band filter can be adjusted. After selecting the varactor diode 7 and determining the resonant frequency tuning range f _min and f _max of the filter operation, the variation range of the equivalent microstrip line length of the varactor diode can be determined, and then according to the total length of the equivalent microstrip line as The characteristic of the half wavelength can determine the length L of the actual microstrip line. The actual microstrip line length L is the first microstrip line 1, the second microstrip line 2, the third microstrip line 3, the fourth microstrip line 4, the fifth microstrip line 5 and the sixth microstrip line in Fig. 1 The sum of the lengths of 6.

其中C_v为变容二极管电容，Δw为滤波器阻带的绝对带宽，Z₀为应用滤波器的射频电路的特征阻抗。由于J₀₁与谐振器和主传输线之间的耦合系数|K|成正比，可得

当变容二极管的电容C_v减小时，谐振频率f增大。由上述关系可得，阻带中心频率调谐时阻带的绝对带宽Δw恒定不变的理论要求为：耦合系数|K|要随着阻带中心频率的增大而减小。该理论要求可以通过下列方式实现：耦合区间如图2(a)中的虚线部分所示，谐振器耦合部分微带线的长度须使变容二极管7在最低偏置电压下和最高偏置电压下的谐振器总等效微带线的中点都在谐振器耦合部分微带线上，即在最高谐振频率f_max和最低谐振频率f_min上谐振器总的等效微带线的中点落在耦合区间内，使得电磁耦合中磁耦合占主导地位。在此基础上，调整耦合区间的大小即调整图2(a)中d₁和d₂的位置。在此耦合区间，磁耦合强度随频率的增大而减小，电耦合强度随阻带中心频率的增大而增大，而总的耦合强度为磁耦合强度减去电耦合强度，因此总的耦合系数就会随着频率的增大而减小，从而可以满足阻带中心频率调谐时绝对带宽保持恒定的理论要求。耦合区间的长度d₂-d₁等于第三微带线3，第四微带线4和第五微带线5的长度总和；第一微带线1与第二微带线2长度之和为d₁；第六微带线长度为L-d₂。耦合区间中的微带线之间的耦合间距决定了总的耦合强度，耦合间距越小，则总的耦合强度越强。The coupling method between the resonator and the main transmission line of the adjustable band-stop filter with constant absolute bandwidth based on the modular structure is a hybrid electromagnetic coupling method. As shown in Figure 1, the coupling structure consists of the third microstrip line 3, the fourth microstrip line 4, the fifth microstrip line 5, the seventh microstrip line 9, the eighth microstrip line 10, and the ninth microstrip line 11 composition. The coupling structure between the resonator and the main transmission line can be represented by the admittance converter J ₀₁ , the resonator can be equivalent to the parallel connection of the inductor and the capacitor, and the capacitance of the varactor diode is controlled by the bias voltage. According to JSHong and MJLancaster, Microwave Filter for RF/Microwave Application, New York: John Wiley, 2001. The classic filter design theory introduced in the book and the circuit structure of the filter module unit, the required

Where C _v is the capacitance of the varactor diode, Δw is the absolute bandwidth of the filter stop band, and Z ₀ is the characteristic impedance of the RF circuit to which the filter is applied. Since J ₀₁ is proportional to the coupling coefficient |K| between the resonator and the main transmission line, it can be obtained

When the capacitance C _v of the varactor diode decreases, the resonant frequency f increases. It can be obtained from the above relationship that the theoretical requirement that the absolute bandwidth Δw of the stop band is constant when the center frequency of the stop band is tuned is: the coupling coefficient |K| should decrease with the increase of the center frequency of the stop band. This theoretical requirement can be realized in the following way: the coupling interval is shown as the dotted line part in Fig. The midpoint of the total equivalent microstrip line of the resonator below is on the microstrip line of the resonator coupling part, that is, the midpoint of the total equivalent microstrip line of the resonator on the highest resonant frequency f _max and the lowest resonant frequency f _min Falling in the coupling interval, the magnetic coupling dominates the electromagnetic coupling. On this basis, adjusting the size of the coupling interval means adjusting the positions of _d1 and _d2 in Figure 2(a). In this coupling interval, the magnetic coupling strength decreases with the increase of the frequency, and the electrical coupling strength increases with the increase of the center frequency of the stop band, and the total coupling strength is the magnetic coupling strength minus the electric coupling strength, so the total The coupling coefficient will decrease as the frequency increases, thus meeting the theoretical requirement that the absolute bandwidth remains constant when the center frequency of the stop band is tuned. The length d ₂ -d ₁ of the coupling section is equal to the sum of the lengths of the third microstrip line 3, the fourth microstrip line 4 and the fifth microstrip line 5; the sum of the lengths of the first microstrip line 1 and the second microstrip line 2 is d ₁ ; the length of the sixth microstrip line is Ld ₂ . The coupling spacing between the microstrip lines in the coupling section determines the overall coupling strength, and the smaller the coupling spacing, the stronger the overall coupling strength.

The two resonators and the main transmission line are arranged symmetrically about the central longitudinal axis of the microstrip structure. When viewed from the left and right ends, they have the same impedance characteristics and form a modular unit; by cascading two or more modular units, the frequency can be obtained Higher selectivity tunable band-stop filter with constant absolute bandwidth.

Example

The structure of a modular unit of an adjustable band-stop filter with constant absolute bandwidth based on a modular structure is shown in Figure 1, and the relevant dimensions are shown in Figure 3 below. The thickness of the dielectric substrate is 0.76mm, the relative permittivity is 2.94, and the loss tangent is 0.0012. The connecting microstrip line 12 adopts a serpentine zigzag structure, which can reduce the size of the circuit. The varactor diode 7 is Toshiba's 1sv277, and one end of the varactor diode is grounded through a metallized via hole. The total length L ₁₂ of the connecting microstrip line 12 is 25.4 mm, which is a quarter wavelength of the center frequency of the tunable resonant frequency range. As shown in Figure 3, the size parameters of the microstrip lines of the filter are as follows: the length L 1 of the first microstrip line 1 and the third microstrip line 3 = 5.6mm; the length L ₂ of the second microstrip line ₂ = 1.88mm, The length of the fourth microstrip line 4 is L ₃ =3.75mm, the length of the fifth microstrip line 5 is L ₄ =9.8mm, the distance between the seventh microstrip line 9 and the ninth microstrip line 11 is L ₅ =2mm, the sixth The length of the microstrip line 6 is L ₆ =1.7mm, the coupling distance g ₁ between the resonator and the main transmission line is 0.12mm, the width of the sixth microstrip line 6 is W ₁ =0.7mm, and the width of the connecting microstrip line 12 is W ₂ =0.3 mm, the width W ₄ of the first microstrip line 1 and the second microstrip line 2 =0.7mm, the width W 3 of the third microstrip line 3 , the fourth microstrip line 4 and the fifth microstrip line ₅ =0.4mm, The width of the seventh microstrip line 9 , the eighth microstrip line 10 and the ninth microstrip line 11 is W ₅ =0.8 mm, and the width of the port microstrip line 8 is W ₆ =1.9 mm. The respective length and width of these microstrip lines are selected to obtain the desired input/output impedance characteristics, in-band transmission characteristics, and out-of-band attenuation characteristics. Fig. 4 is the simulation and actual test result of the adjustable band-stop filter adopting a modular unit designed according to the above-mentioned parameters; the horizontal axis in the transmission characteristic graph represents the frequency, and the vertical axis represents the transmission characteristic |S ₂₁ |; dotted line is the simulation result, and the solid line is the test result. The test results are consistent with the simulation results, and the simulation and testing are completed using Agilent's commercial electromagnetic simulation software ADS and E5071C network analyzer respectively. It can be seen from the test results that the center frequency of the stop band can be adjusted within the range of 1.73-2.2GHz; the transmission characteristic curve in Figure 4 is that the center frequency of the stop band is 1.73GHz, 1.8GHz, 1.9GHz, 2.0GHz, 2.1GHz , 2.2GHz, and taking the -20dB suppression level commonly used by band-stop filters as a standard, the bandwidths at -20dB are 48MHz, 48MHz, 49MHz, 52MHz, 53MHz, and 53MHz respectively; it can be seen that the bandwidth at -20dB is 50±3MHz, which means that the absolute bandwidth remains almost unchanged during frequency tuning. The test results show that this embodiment achieves the goal of constant absolute bandwidth to be achieved by the present invention.

FIG. 5 is a schematic structural diagram of an adjustable band-stop filter using two modular units. The purpose of using two modular units is to improve the suppression level of the stop band. The left and right ports of the filter module unit have the same impedance characteristics. When the impedance of the two ports is adjusted to 50Ω, the two module units can be directly cascaded to improve frequency selectivity and realize a high-order filter. Two identical filter module units in Fig. 1 are cascaded through a 50Ω transmission line 13 to obtain an adjustable band-stop filter with higher frequency selectivity; in order to avoid mutual coupling between the two filter module units, the length of the transmission line 13 should be Greater than 1.5 times the thickness of the dielectric substrate. Figure 6 is the simulation and actual test results of the adjustable band-stop filter using two modular units. The horizontal axis in the transmission characteristic graph represents the frequency, and the vertical axis represents the transmission characteristic |S ₂₁ |; the dotted line is the simulation result, and the solid line For the test results; the simulation and test were completed using Agilent's commercial electromagnetic simulation software ADS and E5071C network analyzer respectively. The test results show that the center frequency of the stop band can be tuned in the range of 1.73-2.2GHz; Measured at GHz, the bandwidths at -40dB are 58MHz, 62MHz, 61MHz, 62MHz, 58MHz, and 57MHz respectively; it can be seen that the bandwidth at -40dB is 60±3MHz, which means that with the tuning of the center frequency of the stopband, the stopband The absolute bandwidth of the band remains constant, achieving the goal of constant bandwidth.

The simulation and test results of the embodiment show that when the center frequency of the stop band is tuned, the absolute bandwidth in the embodiment remains basically unchanged, and the goal of constant absolute bandwidth is achieved. The invention has a modular feature, and can realize an adjustable band rejection filter with higher frequency selectivity through simple cascading.

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

Claims (3)

1. based on the constant tunable band-stop filter of the absolute bandwidth of modular construction, comprise the microstrip structure on upper strata, the medium substrate in intermediate layer and the grounding plate of 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; It is characterized in that: the microstrip structure on upper strata comprises two resonators and main transmission line; Two resonators all are half-wave resonator, and structure is identical, are symmetrical arranged about the center longitudinal axis of microstrip structure; Each resonator is made of microstrip line and variable capacitance diode; The variable capacitance diode of two resonators is provided with identical bias voltage; The microstrip line of resonator is divided into coupling unit and non-coupling unit; The microstrip line coupling unit of resonator is by the 3rd microstrip line, and the 4th microstrip line and the 5th microstrip line are in turn connected into n shape structure; The non-coupling unit of the microstrip line of resonator comprises first microstrip line, second microstrip line and the 6th microstrip line; The one end open circuit of first microstrip line, the other end links to each other with second microstrip line; The other end of second microstrip line is connected with the 3rd microstrip line; The 6th microstrip line one end is connected with the 5th microstrip line, and the other end links to each other with variable capacitance diode; The metallization via hole of the other end of variable capacitance diode through passing the intermediate layer medium substrate links to each other with the lower floor grounded metal; Described main transmission line comprises coupling unit and non-coupling unit; Its coupling unit is by the 7th microstrip line, and the 8th microstrip line and the 9th microstrip line connect and compose n shape structure successively, is positioned at the inboard of resonator coupling unit n shape structure; Being provided with width between resonator coupling unit and the main transmission line coupling unit is the electromagnetic coupled spacing of 0.1mm-0.8mm; The non-coupling unit of main transmission line comprises the port microstrip line and is connected microstrip line; The connection microstrip line is a serpentine; Connect the microstrip line bilateral symmetry port microstrip line, the 7th microstrip line, the 8th microstrip line, the 9th microstrip line are set; Port microstrip line one end is connected with the 7th microstrip line; Connecting microstrip line is connected with the 9th microstrip line; The length of described connection microstrip line

Wherein c is the light velocity, ε _rBe the relative dielectric constant of medium substrate,

f _MinAnd f _MaxBe respectively the minimum value and the maximum of the resonance frequency f adjustable extent of resonator; The electrical length L+ Δ L of resonator be resonance frequency f correspondence wavelength X 1/2nd; Wherein, L is actual microstrip line length, and Δ L is a variable capacitance diode equivalence microstrip line length; Actual microstrip line length L is the length sum of first microstrip line, second microstrip line, the 3rd microstrip line, the 4th microstrip line, the 5th microstrip line and the 6th microstrip line; Length between the coupled zone equals the 3rd microstrip line, the length summation of the 4th microstrip line and the 5th microstrip line.

2. according to claim 1 based on the constant tunable band-stop filter of the absolute bandwidth of modular construction, it is characterized in that, the turnable resonator frequency range of described tunable band-stop filter is 1.73-2.2GHz, the length of first microstrip line and the 3rd microstrip line is 5.6mm, the length of second microstrip line is 1.88mm, the length of the 4th microstrip line is 3.75mm, the length of the 5th microstrip line is 9.8mm, spacing between the 7th microstrip line and the 9th microstrip line is 2mm, the length of the 6th microstrip line is 1.7mm, coupling spacing between resonator and the main transmission line is 0.12mm, the width of the 6th microstrip line is 0.7mm, the width that connects microstrip line is 0.3mm, the width of first microstrip line and second microstrip line is 0.7mm, the 3rd microstrip line, the width of the 4th microstrip line and the 5th microstrip line is 0.4mm, the 7th microstrip line, the width of the 8th microstrip line and the 9th microstrip line is 0.8mm, the width of port microstrip line is 1.9mm, connects the length L of microstrip line ₁₂Be 25.4mm.

3. according to claim 1 based on the constant tunable band-stop filter of the absolute bandwidth of modular construction, it is characterized in that, two identical tunable band-stop filter modular units connect the tunable band-stop filter that obtains two-stage by 50 Ω transmission lines, and the length of transmission line is greater than 1.5 times of dielectric substrate thickness.

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