WO2012162972A1 - Balanced radio frequency electrically tunable band-pass filter with constant absolute bandwidth - Google Patents

Abstract

Disclosed in the present invention is a balanced radio frequency electrically tunable band-pass filter with constant absolute bandwidth. The band-pass filter comprises a micro strip structure at an upper layer, a medium substrate at a middle layer and grounding metal at a lower layer. The micro strip structure at the upper layer adopts a balanced circuit and comprises four half-wavelength resonators, two input feed networks, two output feed networks, two input ports and two output ports; each of the four half-wavelength resonators comprises a micro strip and a variable capacitance diode with two connected ends; capacitors are loaded at the middles of the first half-wavelength resonator and the third half-wavelength resonator; the second half-wavelength resonator and the fourth half-wavelength resonator are bent and arranged symmetrically; and the whole filter structure is in mirror symmetry. The balanced radio frequency electrically tunable band-pass filter provided by the invention realizes constant absolute bandwidth during centre frequency tuning and can restrain common mode interference and can be used for a reconstructed radio frequency front end of wireless communication.

Description

 Balanced RF tolerant bandpass filter with constant absolute bandwidth

Technical field

The invention relates to a balanced radio frequency modulation bandpass filter with adjustable center frequency, in particular to a balanced radio frequency modulation band with constant absolute bandwidth applicable in multi-band, wide-band and reconfigurable radio frequency front-end systems. Pass filter.

Background technique

Modern ultra-wideband radar and wireless communications require a high performance reconfigurable RF front end. For example, in cognitive radio systems, in order to fully utilize and integrate various wireless channels and standards, the RF front-end needs to operate at different frequencies, which requires a reconfigurable RF front-end with a center frequency tunable. RF tones bandpass filters are an important part of reconfigurable RF front-ends and are therefore receiving increasing attention. In this respect, there have been some reports that many different tuning devices have been used, such as semiconductor varactors, RF MEMS (RF). MEMS) capacitor tubes and ferroelectric thin film material varactors.

 Regardless of which tuning device is used, the problems faced by the RF tunable bandpass filter include:

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

(2) Interference of ambient noise around the system. The presence of ambient noise causes the performance of the filter to degrade, which affects the overall performance of the RF front end. Therefore, some methods to suppress environmental noise must be taken.

Some methods have been proposed for the problem of constant bandwidth during center frequency tuning. According to 'M. Sanchez-Renedo, R. Gomez-Garcia, JI 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 coupling coefficient can be controlled by inserting a medium between the resonators, so that the bandwidth can be kept constant. According to 'SJ Park, and GM 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 independent electrical coupling and magnetic coupling mechanisms can be used to control the variation of the coupling coefficient to achieve specific bandwidth characteristics. According to 'MA El-Tanani, and GM 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. 'The analysis provided shows that the electromagnetic hybrid coupling mechanism can also satisfy the constant bandwidth. However, the methods proposed above are all single-port circuits, and there is basically no power to suppress the environmental noise.

The balanced structure circuit has a good suppression effect on environmental noise, so the balance circuit is widely used in modern communication systems. Most of the current research focuses on stopband extension, common mode rejection, widening the passband, or using the differential mode response to obtain dual bands. 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 the resistors are loaded in the middle can absorb the common mode signal. However, the balanced filter design described above is not adjustable in frequency. No research report so far has been on balanced RF tonal filters with absolute bandwidth control and common mode rejection.

Summary of the invention

In order to achieve a constant absolute bandwidth and suppress common mode signals such as ambient noise, the present invention provides a balanced RF tones bandpass filter having a constant absolute bandwidth, which is not only absolute at the center frequency tuning The bandwidth is constant and has a good suppression of common mode signals.

The technical solution adopted by the present invention to solve the technical problem thereof is:

A balanced RF tow bandpass filter having a constant absolute bandwidth, comprising an upper microstrip structure, an intermediate layer dielectric substrate and a lower grounded metal; an upper microstrip structure attached to the surface of the intermediate layer dielectric plate, under the intermediate dielectric plate The surface is grounded metal; the upper microstrip structure consists of four half-wavelength resonators, two input feed networks, two output feed networks, two input ports and two output ports, two input ports and two inputs respectively The feed network is connected, 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 by tapping, and the first half-wavelength resonators are respectively respectively The two 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 with the third half-wavelength resonator, and the third half-wavelength resonator is further connected by a tap line Connected to two output feed networks respectively, the first half-wavelength resonator and the third half-wavelength resonator are loaded with different sizes of capacitance for absorbing common mode signals. All the ends of said half-wavelength resonators are varactor diodes.

In the above balanced RF to-be-regulated bandpass filter having a 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, and the anodes of the first varactor diode and the second varactor diode are connected to the underlying grounding metal through the intermediate layer dielectric substrate, and the third half-wavelength resonator is The first half-wavelength resonator has the same structure; the second half-wavelength resonator is composed of a third varactor diode, a fifth microstrip line, a sixth microstrip line, a seventh microstrip line, and a fourth varactor diode. The anodes of the third varactor diode and the fourth varactor diode are both connected to the underlying grounding metal through the intermediate layer dielectric substrate, and the fourth half-wavelength resonator is identical in structure to the second half-wavelength resonator and is located in the first half-wavelength resonator and 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 second half-wavelength resonator Seven microstrip lines and third The tenth microstrip line of the wavelength resonator is arranged in parallel to form an interstage coupling structure; the four half-wavelength resonators are arranged in a left-right, upper-and-up symmetrical structure; the first input feeding network in the two input feeding networks is composed of The first capacitor and the eighth microstrip line are sequentially connected, and the other end of the eighth microstrip line is connected to the second microstrip line of the first half-wavelength resonator by a tap line; the structure of the second input feed network is The first input feed network is the same; the first input port of the two input ports is composed of a ninth microstrip line, and the ninth microstrip line is connected to the first capacitance start end of the first input feed network, and the second input port is connected with The first input port has the same structure. The two input feed networks are identical in structure to the two output feed networks. The two input ports and the two output ports are identical in structure; two input feed networks, two output feed networks, two input ports, and two outputs. The port and the above four half-wavelength resonators are arranged in a structure of left and right, upper and lower symmetry.

In the above balanced RF to band pass filter having a constant absolute bandwidth, the other end of the capacitor is connected to the underlying ground metal through the intermediate layer dielectric substrate.

The balanced RF tones bandpass filter having a constant absolute bandwidth, when the first input port and the second input port input a differential mode signal, the entire filter is resonant at the first half-wavelength resonator and the third half-wavelength resonator An electrical isolation wall is formed at a linear position at the midpoint of the device. Since the coupling between the resonators is mainly electrically coupled, the first half-wavelength resonator and the third half-wavelength resonator have no current at the intermediate position, and are connected to the first half-wavelength resonator and the third half-wavelength resonator. The capacitance of the intermediate loading is negligible, 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 The wavelength resonator is coupled to form a band pass filter structure; when the first input port and the second input port input a common mode signal, the entire filter is at a line where the midpoints of the first half wavelength resonator and the third half wavelength resonator are located A magnetic isolation wall is formed at the location. The first half-wavelength resonator and the third half-wavelength resonator have current flow at an intermediate position, and current flows through the capacitors loaded between the first half-wavelength resonator and the third half-wavelength resonator. The two quarter-wavelength resonators equivalent to the first half-wavelength resonator and the third half-wavelength resonator need to take into account the capacitance of the intermediate loading. Since the capacitances of the intermediate loading on the first half-wavelength resonator and the third half-wavelength resonator are different, the resonant frequencies of the two quarter-wave resonators that are actually equivalent are different, so that the common mode signal cannot pass. , to achieve the effect of inhibition. The all-input and output feeding networks adopt a tapped line feeding mode to obtain a better external quality factor; the electromagnetic hybrid coupling mechanism in the coupling region can satisfy a constant absolute bandwidth.

To further achieve the object of the present invention, the balanced micro-frequency band-pass filter having a constant absolute bandwidth has a first microstrip line length of 10.2 mm, a width of 0.8 mm, and a second microstrip line length of 18.7 mm. The length of the five microstrip line is 24.1 mm, the width is 0.8 mm, the length of the sixth microstrip line is 10.4 mm, the length of the eighth microstrip line is 3.3 mm, the width is 0.6 mm, and the first capacitor size is 7 pF, the first half wavelength The capacitance of the intermediate load of the resonator is 20 pF, the capacitance of the third half-wavelength resonator is 7 pF, and the distance between the second microstrip line and the fifth microstrip line is 0.6 mm.

Compared with the prior art, the present invention adopts a novel balanced structure and a half-wavelength resonator interstage coupling structure, and the electronically adjustable bandpass filter which maintains a constant absolute bandwidth bandwidth at the center frequency tuning and can suppress the common mode interference signal well. . Overall, it has the following advantages and effects:

(1) Due to the balanced structure design, the band-pass filter can work normally for the differential mode signal, and has a better suppression effect on the common mode signal, so it has an immune function for the interference such as environmental noise. The measured common mode rejection levels in the examples exceeded -23 dB.

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

 DRAWINGS

Figure 1 is a schematic diagram of a balanced RF tolerant bandpass filter ABW with a constant absolute pair bandwidth.

Figure 2a is an ABW differential mode equivalent circuit.

Figure 2b is the ABW common mode equivalent circuit.

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

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

Figure 4 is a graph showing the relationship between the frequency of the quarter-wave resonator, the capacitance value, and the length of the microstrip line in Figure 3a.

Figure 5 is a graph showing the relationship between the X-ray frequency, capacitance value and microstrip line length of the half-wavelength resonator of Figure 3b.

Figure 6 is an ABW common mode equivalent circuit.

Figure 7 is an equivalent quarter-wave resonator of the first resonator in the case of ABW common mode.

Figure 8a is a differential mode transmission characteristic of ABW.

Figure 8b is the differential mode return loss curve for ABW.

Figure 8c is a common mode transmission characteristic of ABW.

 detailed description

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

As shown in FIG. 1, a balanced radio frequency modulation band pass filter having a constant absolute bandwidth is characterized by comprising an upper layer microstrip structure, an intermediate layer dielectric substrate and a lower layer grounding metal; and an upper layer microstrip structure attached to the intermediate layer medium. On the upper surface, the lower surface of the intermediate dielectric plate is a 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. Two input ports are respectively connected to two input feed networks, two output ports are respectively connected to two output feed networks, and two input feed networks are respectively connected to the first half-wavelength resonators by tapping lines (ie, Not connected to both ends of the first half-wavelength resonator, but connected to the portion between the two ends), the first half-wavelength resonator is respectively connected to the second half-wavelength resonator and the fourth half-wavelength resonator, respectively The two half-wavelength resonators and the fourth half-wavelength resonators are respectively coupled to the third half-wavelength resonator, and the third half-wavelength resonators are respectively connected to the two output feed networks by tapping lines, Both the half-wavelength resonator and the third half-wavelength resonator are loaded with different sized capacitors for absorbing common mode signals; all of the above half-wavelength resonators have varactor diodes at both ends.

The first half-wavelength resonator is composed 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. Sequentially connected, the anodes of the first varactor diode 1 and the second varactor diode 6 are connected to the underlying grounding metal through the intermediate layer dielectric substrate, and the third half-wavelength resonator is identical in structure to the first half-wavelength resonator; The two half-wavelength resonators are sequentially connected by 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, and the third varactor diode 7 and the anode of the fourth varactor diode 11 are both connected to the underlying grounding metal through the intermediate layer dielectric substrate, and the fourth half-wavelength resonator is identical in structure to the second half-wavelength resonator and is located in the first half-wavelength resonator and the third half 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 half-wavelength resonator is seventh The microstrip line 10 and the tenth microstrip line 12 of the third half-wavelength resonator are arranged in parallel Inter-coupling structure; the above four half-wavelength resonators are arranged in a left-right, up-and-down symmetrical structure; the first input feeding network 17 in the two input feeding networks is compliant by the first capacitor 15 and the eighth microstrip line 16 The secondary connection is formed, and the other end of the eighth microstrip line 16 is connected to the second microstrip line 3 of the first half-wavelength resonator by a tap line; the structure of the second input feed network 18 and the first input feed network 17 The same; the first input port IN of the two input ports is constituted by the ninth microstrip line 21, and the ninth microstrip line 21 is connected to the beginning of the first capacitor 15 of the first input feed network 17, and the second input port IN' The same structure as the first input port IN, The two input feed networks are identical in structure to the two output feed networks. The two input ports and the two output ports are identical in structure; two input feed networks, two output feed networks, two input ports, and two outputs. The port and the above four half-wavelength resonators are arranged in a structure of left and right, upper and lower symmetry. A second capacitor 13 and a third capacitor 14 of different sizes for absorbing the common mode signal are loaded in the middle of the first half wavelength resonator and the third half wavelength resonator, and the second capacitor 13 and the third capacitor 14 are both One end is connected to the underlying ground metal through the intermediate layer dielectric substrate.

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

Adjust the parameters of the filter to balance the filter across the structure. When the first input port IN and the second input port IN' input a differential mode signal, the entire filter forms an electrical isolation wall at a linear position where the midpoints of the first half-wavelength resonator and the third half-wavelength resonator are located. Since the coupling between the resonators is mainly electrically coupled, the first half-wavelength resonator and the third half-wavelength resonator have no current at the intermediate position, and are connected to the first half-wavelength resonator and the third half-wavelength resonator. The intermediately loaded second capacitor 13 and third capacitor 14 are negligible, so that under differential mode excitation, the first half-wavelength resonator and the third half-wavelength resonator are equivalent to two quarter-wavelength resonators. At the same time, the second half-wavelength resonator is coupled to form a band-pass filter structure; the equivalent structure of the filter is shown in FIG. Figures 3a and 3b show equivalent quarter-wave resonators and second half-wavelength resonators in the case of differential mode. According to "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." The analysis provided shows that in Figure 3a, when the quarter-wave resonator resonates, the imaginary part of the admittance Ydd_in1 seen from the left end of the quarter-wave resonator is equal to zero, for a given one. Voltage, the resonant frequency of the entire resonator loaded with the varactor:

f _dd1 = (Y ₁ tan ^-1 θ ₁ ) /2πC;

 Where QUOTE is the characteristic admittance of the resonator; QUOTE Is the electrical length of the first microstrip line 6; C is the capacitance value of the varactor diode at different voltages; Figure 4 shows the resonant frequency of the quarter-wave resonator in Figure 3a, the capacitance value of the varactor diode The relationship between the length of C and 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; also for the second half-wavelength resonator , the resonant frequency is:

f _dd2 = Y ₂ (1 + tan θ ₂ ² ) ^1⁄2 /2πCtan θ ₂ ;

Where 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; Figure 5 shows the resonant frequency of the second half-wavelength resonator in Figure 3b, the capacitance value of the varactor diode C and micro With the relationship of the length of the line, it can be seen that the relationship between the resonant frequency and the capacitance value in the range of 10-18 mm in length is similar to that of the quarter-wavelength resonator in the length of 6-8 mm. 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 a 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 current flow at an intermediate position, and current flows through the capacitors loaded between the first half-wavelength resonator and the third half-wavelength resonator. The two quarter-wavelength resonators equivalent to the first half-wavelength resonator and the third half-wavelength resonator need to take into account the capacitance of the intermediate loading. The actual working equivalent filter structure for the common mode signal is shown in Figure 6. Figure 7 shows an equivalent quarter-wavelength resonator with a resonant frequency of:

f _cc2 = Y ₁ [2C + C ₂ + (4C ² + 4CC ₂ + C ₂ ² + 8CC ₂ tan θ ₁ ² ) ^1⁄2 ] / 4πCC ₂ tan θ ₁ ;

As shown in the figure, Y1 is the characteristic admittance of the microstrip line; θ1 is the electrical length of the microstrip line; C is the capacitance value of the varactor diode at different voltages; C2 is the capacitance value of the loading capacitor.

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

As can be seen in Figures 2 and 6, the filter is fed by a tapped line. According to "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, The analysis provided by Inc, 2007.” shows that for ABW, at a certain absolute bandwidth, the actual external quality factor of the filter As the frequency increases, so the tap line feed mode is used in the ABW structure, which makes it easier to control the quality factor. The mechanism of electromagnetic hybrid coupling is used between the stages, as shown in the dotted region in the dotted line frame of Fig. 2, and there are both electrical and magnetic couplings, which can realize that the inter-stage coupling coefficient k decreases with increasing frequency.

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

Implementation Example: Balanced RF Electrical Bandpass Filter with 60MHz Constant Absolute Bandwidth

A balanced RF tolerant bandpass filter with a constant absolute bandwidth of 60 MHz is shown in Figure 1. The specific parameters are: the first microstrip line 2 has a length of 10.2 mm, a width of 0.8 mm, the second microstrip line 3 has a length of 18.7 mm, and the fifth microstrip line 8 has a length of 24.1 mm and a width of 0.8 mm. The strip 9 has a length of 10.4 mm, the eighth microstrip line 16 has a length of 3.3 mm, a width of 0.6 mm, the first capacitor 15 has a size of 7 pF, and the second capacitor 13 loaded in the middle of the first half-wavelength resonator has a size of 20 pF. The third capacitor 14 loaded in the middle of the third half-wavelength resonator is 7 pF in size, and the distance between the second microstrip line 3 and the fifth microstrip line 8 is 0.6 mm. Figure 8 shows the results of simulation and actual measurements using the filters designed with the above parameters. The simulation and actual measurements were performed using Agilent's commercial electromagnetic simulation software ADS and E5071C network analyzer, respectively. Figure 8a shows the transmission characteristics of the simulation, calculation and test at four special bias voltages for the differential mode operation of the filter. The horizontal axis represents the frequency and the vertical axis represents the transmission characteristic |Sdd21|. Figure 8b shows the reflection characteristics of the filter, with the horizontal axis representing the frequency and the vertical axis representing the return loss |Sdd11|. As can be seen from Figures 8a and 8b, the passband frequency of the filter can be adjusted from 549 MHz to 775 MHz with a relative adjustment range of 34.1%. For all tuning states, the measured in-band insertion loss is between 3.5 and 4.2 dB and the return loss is below -10 dB. 3dB bandwidth is 60 4MHz, basically constant. Figure 8c shows the suppression of common mode noise in the case of common mode operation, showing that common mode rejection is less than -25 dB at 0.2-1.7 GHz.

The invention is based on a mirror-symmetric balanced structure, has different equivalent circuits under differential mode and common mode signals, has a constant absolute bandwidth, and has an adjustable intermediate frequency to suppress common mode noise in a wide frequency band. The bandwidth and passband waveforms remain constant over the frequency tuning range. The bandwidth can be adjusted by adjusting the parameters of the design, ie this structure can be used to implement various bandwidth specifications.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., which are within the spirit and scope of the present invention, should be included in the scope of the present invention. within.

Claims (1)

1. 1. A balanced radio frequency modulation bandpass filter having a constant absolute bandwidth, comprising: an upper layer microstrip structure, an intermediate layer dielectric substrate and a lower layer grounded metal; and an upper microstrip structure attached to the intermediate layer dielectric plate Surface, the lower surface of the intermediate layer dielectric plate 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, two The input ports are respectively connected to two input feed networks, and the two output ports are respectively connected to two output feed networks, and the first half-wavelength resonators are respectively connected to the two input feed networks by tapping lines, the first half The wavelength resonator is coupled to the second half-wavelength resonator and the fourth half-wavelength resonator, respectively, and 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 resonance The device is connected to two output feeding networks by tapping, and the first half-wavelength resonator and the third half-wavelength resonator are loaded with a common mode signal for absorbing common mode signals. Capacitors of different sizes, all of the above half-wavelength resonators have varactor diodes at both ends.

   2. The balanced radio frequency modulation bandpass filter having a constant absolute bandwidth according to claim 1, wherein said first half-wavelength resonator comprises a first varactor diode, a first microstrip line, and a second The microstrip line, the third microstrip line, the fourth microstrip line and the second varactor diode are sequentially connected, the first microstrip line is perpendicular to the second microstrip line, and the third microstrip line is perpendicular to the fourth microstrip line a line, the anode of the first varactor diode and the second varactor diode are connected to the underlying grounding metal through the intermediate layer dielectric substrate, the third half-wavelength resonator is identical in structure to the first half-wavelength resonator; the second half-wavelength resonator The third varactor diode, the fifth microstrip line, the sixth microstrip line, the seventh microstrip line, and the fourth varactor diode are sequentially connected, and the anodes of the third varactor diode and the fourth varactor diode are both worn. The intermediate layer dielectric substrate is connected to the lower grounding metal, and the fourth half-wavelength resonator is identical in structure to the second half-wavelength resonator and is located between the first half-wavelength resonator and the third half-wavelength resonator, and the four half-wavelength resonances Arranged together into left and right, upper a symmetric structure; the first input feed network in the two input feed networks is formed by sequentially connecting the first capacitor and the eighth microstrip line, and one end of the eighth microstrip line is connected to one end of the first capacitor, first The second microstrip line of the half-wavelength resonator is connected in a tapped manner to the other end of the eighth microstrip line; the structure of the second input feed network is the same as that of the first input feed network; the first of the two input ports The input port is composed of a ninth microstrip line, and the ninth microstrip line is connected to the other end of the first capacitor of the first input feed network, and the second input port has the same structure as the first input port. The two input feed networks are identical in structure to the two output feed networks. The two input ports and the two output ports are identical in structure; two input feed networks, two output feed networks, two input ports, and two outputs. The port and the above four half-wavelength resonators are arranged in a structure of left and right, upper and lower symmetry.

   3. A balanced radio frequency modulation bandpass filter having a constant relative bandwidth according to claim 2, wherein the other end of said capacitor loaded between the first half-wavelength resonator and the third half-wavelength resonator The dielectric substrate is connected to the underlying ground metal through the intermediate layer.

   4. A balanced radio frequency modulation bandpass filter having a constant relative bandwidth according to claim 2, wherein the fifth microstrip line of the second half-wavelength resonator and the second micro-wavelength of the first half-wavelength resonator The strip lines are arranged in parallel to form an interstage coupling structure; the seventh microstrip line of the second half-wavelength resonator and the tenth microstrip line of the third half-wavelength resonator are arranged in parallel to form an interstage coupling structure.

   5. A balanced radio frequency modulation bandpass filter having a constant relative bandwidth according to claim 2, wherein the characteristic impedance of the transmission lines of the two input ports and the two output ports is 50 Ω.

   The balanced radio frequency econc filter with constant relative bandwidth according to any one of claims 2 to 5, wherein the first microstrip line has a length of 10.2 mm and a width of 0.8 mm, and a second The length of the microstrip line is 18.7 mm, the length of the fifth microstrip line is 24.1 mm, the width is 0.8 mm, the length of the sixth microstrip line is 10.4 mm, the length of the eighth microstrip line is 3.3 mm, and the width is 0.6 mm. The capacitance is 7 pF, the capacitance of the first half-wavelength resonator is 20 pF, and the capacitance of the third half-wave resonator is 7 pF, and between the second microstrip line and the fifth microstrip line. The distance is 0.6mm.

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