TWI528624B - Balanced tri - band band - pass filter - Google Patents

Description

Balanced three-band pass filter

The present invention relates to a filter, and more particularly to a balanced three-band pass filter.

In recent years, with the rapid development of wireless communication technology, a single electronic product, not only to meet the application of a variety of communication systems, but also to be lighter and thinner, but this also makes the density of components in electronic products higher and higher, noise and The problem of interference is also more serious.

Referring to FIG. 1, a band pass filter 9 is disclosed in the specification of the U.S. Patent No. 5,825,263, which includes a first filtering unit 91 and a second filtering unit 92.

The first filtering unit 91 includes a first input end 911 and a first output end 912. The second filtering unit 92 includes a second input end 921 and a second output end 922. When the first input terminal 911 and the second input terminal 921 receive a pair of differential mode signals (two signals with a phase difference of 180 degrees), the first output terminal 912 and the second output terminal 922 also output a pair. The differential mode filtering signal (the two phases are also 180 degrees apart) related to the pair of differential mode input signals; and the first input terminal 911 and the second input terminal 921 receive a pair of common mode signals (two in phase The first output terminal 912 and the second output terminal 922 also output a pair related to the pair The common mode filter signal of the analog input signal (both phases are in phase).

The pair of differential mode filtering signals and the pair of common mode filtering signals can pass through a passband, so there is no common mode signal rejection effect, so in the actual application, a balance unbalance converter (Balun) is also needed. 8. The balun 8 can convert the two differential mode filtered signals having phase differences into an in-phase summing output, and convert the two common mode filtered signals in phase to have a phase difference of 180 degrees. Eliminate, so the effect of common mode signal rejection is achieved.

However, the band-pass filter 9 has the disadvantage that the balun 8 must be additionally combined to achieve the common mode signal rejection effect, thereby increasing the extra path transmission loss, volume and cost, so that it does not conform to the current electronic products. Demand.

Accordingly, it is an object of the present invention to provide a balanced three-band pass filter that achieves common mode signal rejection without the need for an additional balun.

Therefore, the balanced three-band pass filter of the present invention comprises a substrate, a ground plane, a first resonant unit to a fourth resonant unit, and two through holes.

The substrate has a first surface and a second surface that are spaced apart from each other.

The ground plane is disposed on the first surface of the substrate.

The first to sixth resonance units are disposed on the base a second surface of the board and spaced apart from each other, and the second resonating unit and the third resonating unit are interposed between the first resonating unit and the fourth resonating unit, the first resonating unit and the second resonating unit portion Parallelly spaced to be coupled to each other, the third resonating unit and the fourth resonating unit are spaced apart in parallel to be coupled to each other, the second resonating unit and the third resonating unit are partially spaced apart to be coupled to each other, and the first resonating unit is coupled to the Each of the fourth resonating units includes two resonators on opposite sides of a plane of symmetry, respectively.

Moreover, the first resonating unit is provided with two input ports for receiving signals, and the fourth resonating unit is provided with two output ports for outputting signals.

The through holes are disposed along the plane of symmetry, and the second resonant unit is electrically connected to the ground plane via one of the through holes, and the third resonant unit is electrically connected to the ground plane via another through hole.

When the balanced three-band pass filter is operated in a differential mode, the plane of symmetry is equivalent to a perfect electric wall, and the resonators of the first resonating unit and the fourth resonating unit are electrically connected to each other through the perfect electric wall. The ground plane, each resonator resonates at an Nth resonant frequency, the Nth resonant frequency corresponds to an Nth passband, wherein the parameter N is an odd number increasing from 1 and the pair of differential mode signals received by the inputs 埠The passbands are coupled to the output ports.

In the common mode operation, the first resonating unit and the fourth resonating unit allow the presence of the common mode signal because the symmetry plane is not connected to the ground plane. In contrast, the through holes are connected to the second resonating unit. And the first The common mode signal of the three resonant units, thereby destroying the common mode signals that may exist in the second and third resonant units (or, in other words, the common holes of the second resonant unit and the third resonant unit The analog signal is shorted to the ground plane). When the input signals are used to receive a pair of common mode signals, the common mode signal excited by the first resonant unit is electrically connected to the through hole of the second resonant unit when coupled to the second resonant unit. When the remaining common mode signal is coupled to the third resonant unit, it is further destroyed by the other through hole, and thus coupled to the fourth resonant unit, so that the common mode signal arriving at the output port is negligible. That is, the pair of common mode signals received by the inputs are substantially not coupled to the output ports.

9‧‧‧Bandpass filter

91‧‧‧First filter element

911‧‧‧ first input

912‧‧‧ first output

92‧‧‧Second filter element

921‧‧‧ second input

922‧‧‧second output

8‧‧‧Balanced unbalance converter

1‧‧‧Substrate

11‧‧‧ first surface

12‧‧‧ second surface

XPOS‧‧‧symmetric plane

YPOS‧‧‧ mirror plane

2‧‧‧ ground plane

3‧‧‧First Resonance Unit

31‧‧‧Resonator

311‧‧‧First line segment

312‧‧‧Second line

313‧‧‧ third line segment

3131‧‧‧ openings

4‧‧‧Second Resonance Unit

41‧‧‧Resonator

411‧‧‧First line segment

412‧‧‧second line

413‧‧‧ third line segment

5‧‧‧3rd Resonance Unit

51‧‧‧Resonator

511‧‧‧First line

512‧‧‧second line

513‧‧‧ third line segment

6‧‧‧4th Resonance Unit

61‧‧‧Resonator

611‧‧‧First line

612‧‧‧second line

613‧‧‧ third line segment

6131‧‧‧ openings

34‧‧‧First coupling gap

56‧‧‧Second coupling gap

45‧‧‧ Third coupling gap

32‧‧‧ Input埠

62‧‧‧ Output埠

7‧‧‧through holes

D1‧‧‧Linear distance

G12‧‧‧Coupling spacing

G22‧‧‧ coupling spacing

Other features and effects of the present invention will be apparent from the following description of the drawings, wherein: Figure 1 is a schematic diagram of a conventional bandpass filter; Figure 2 is a balanced three-bandpass filter of the present invention. FIG. 3 is another schematic diagram of the preferred embodiment; FIG. 4 is a diagram showing the relationship between an external quality factor and the signal feeding position change of the preferred embodiment; FIG. A relationship diagram of a differential mode coupling coefficient of a preferred embodiment as a function of a coupling width of a first coupling gap; FIG. 6 is a coupling width of another differential mode coupling coefficient of the preferred embodiment with a third coupling gap a diagram of change; Figure 7 is an S-parameter diagram of the preferred embodiment for differential mode operation; and Figure 8 is an S-parameter diagram of the preferred embodiment for common mode operation.

Referring to FIG. 2 and FIG. 3, a preferred embodiment of the balanced three-band pass filter of the present invention comprises a substrate 1, a ground plane 2, a first resonant unit to a fourth resonant unit 3, 4, 5, 6 and Two through holes 7.

The substrate 1 has a first surface 11 and a second surface 12 stacked at intervals.

The ground plane 2 is disposed on the first surface 11 of the substrate 1 .

The first resonating unit to the fourth resonating unit 3, 4, 5, and 6 are disposed on the second surface 12 of the substrate 1 and spaced apart from each other, and the second resonating unit 4 and the third resonating unit 5 are interposed Between the first resonating unit 3 and the fourth resonating unit 6, the first resonating unit 3 and the second resonating unit 4 are partially spaced apart to be coupled to each other, and the third resonating unit 5 and the fourth resonating unit 6 are partially coupled The parallel spacing is coupled to each other, and the second resonating unit 4 and the third resonating unit 5 are partially spaced apart to be coupled to each other.

Each of the first to fourth resonating units 3, 4, 5, 6 includes two resonators 31, 41, 51, 61 respectively located on opposite sides of a plane of symmetry XPOS, and the first resonance The two resonators 31 of the unit 3 are mirror-symmetrical to each other in the plane of symmetry XPOS, and the two resonators 41 of the second resonating unit 4 are mirror-respected to each other in the plane of symmetry XPOS, the two resonances of the third resonating unit 5 The mirrors 51 are opposite to each other in the plane of symmetry XPOS, and the two resonators 61 of the fourth resonating unit 6 are mirrored to each other. Relatively referred to as the symmetry plane XPOS.

Moreover, the first resonating unit 3 and the fourth resonating unit 6 are mirror-opposed to each other in a mirror plane YPOS perpendicular to the plane of symmetry XPOS, and the second resonating unit 4 and the third resonating unit 5 are also mirror-referenced to each other. The mirror plane YPOS. The eight resonators 31, 41, 51, 61 have substantially the same electrical length.

Each of the first to seventh resonating units 3, 4, 5, and 6 is a three-stage stepped-impedance resonators (TSSIRs). And each of the resonators 31, 41, 51, 61 has a first line segment 311, 411, 511, 611, a second line segment 312, 412, 512, 612 and a third line segment 313 of different line widths, 413, 513, 613.

The two first line segments 311, 411, 511, 611 of each of the first to fourth resonance units 3, 4, 5, 6 are mutually mirror-referenced to the plane of symmetry XPOS, the two The second line segments 312, 412, 512, 612 are mirror-referenced to the plane of symmetry XPOS, and the two third line segments 313, 413, 513, 613 are mirror-referenced to each other in the plane of symmetry XPOS.

Each of the first line segments 311, 411, 511, 611 has a first end located at the plane of symmetry XPOS and a second end opposite the first end. Each of the second line segments 312, 412, 512, 612 has a second connection of the first line segment 311, 411, 511, 611 corresponding to the electrical connection a first end of the end and a second end opposite the first end. Each of the third line segments 313, 413, 513, 613 has a first end of the second end of the second line segment 312, 412, 512, 612 corresponding to the electrical connection, and a free end opposite the first end.

Each of the first line segments 311, 411, 511, 611 extends from the plane of symmetry XPOS toward a direction away from the plane of symmetry XPOS, and each of the second line segments 312, 412, 512, 612 is electrically connected to the first line segment. The second ends of 311, 411, 511, and 611 extend away from the plane of symmetry XPOS, and the first resonating unit 3 and the second resonating unit 4 collectively define a direction extending in a direction perpendicular to the plane of symmetry XPOS a first coupling gap 34, the third resonating unit 5 and the fourth resonating unit 6 also jointly define a second coupling gap 56 extending in a direction perpendicular to the plane of symmetry XPOS, the second resonating unit 4 and the third The two third line segments 413, 513 of the resonating unit 5 on the opposite sides of the opposite sides of the symmetry plane XPOS respectively extend from the second ends of the second line segments 412, 512 electrically connected to each other, and then approaching The symmetry plane XPOS direction extends, and the two third line segments 413, 513 collectively define a third coupling gap 45 extending in a direction perpendicular to the symmetry plane XPOS.

Each of the third line segments 313, 613 of the first resonating unit 3 and the fourth resonating unit 6 is substantially U-shaped with an opening 3131, 6131 facing a direction parallel to the plane of symmetry XPOS, and the first resonating unit 3 a U-shaped opening formed by each of the third line segments 313 and a U-shaped opening formed by each of the third line segments 613 of the fourth resonating unit 6 The directions of the 6131 are opposite to each other, and the third line segments 313 of the first resonating unit 3 are partially adjacently coupled to the second line segments 412 of the second resonating unit 4, and the third resonating units 6 are the third lines. Segment 613 is also partially coupled adjacent to the second line segment 512 of the third resonating unit 5, respectively.

Moreover, the first resonating unit 3 is provided with two input ports 32 for receiving signals, and the fourth resonating unit 6 is provided with two output ports 62 for outputting signals.

The through holes 7 are disposed along the symmetry plane XPOS, and the second resonating unit 4 is electrically connected to the ground plane 2 via one of the through holes 7, and the third resonating unit 5 is electrically connected via another through hole 7. Go to the ground plane 2.

In the differential mode operation, the symmetric plane XPOS is equivalent to a perfect electric wall, and the four resonators 31 and 61 of the first resonating unit 3 and the fourth resonating unit 6 pass through the perfect electric wall, etc. The power is electrically connected to the ground plane 2, and each of the resonators 31, 41, 51, 61 resonates at an Nth resonant frequency, and the Nth resonant frequency corresponds to an Nth passband, wherein the parameter N is an odd number increasing from 1 For example, N=1, 3, 5, 7, ..., a pair of differential mode signals received by the input ports 32 are coupled to the output ports 62 through the pass bands.

Moreover, the straight line distance D1 of each input 埠32 to the symmetry plane XPOS is inversely proportional to an external value factor, and the larger the external value factor, the smaller the bandwidth of each pass band. In the present specification, the resonant frequency of the fundamental mode of each of the resonators 31, 41, 51, 61 The rate is the first resonant frequency, the first high-order mode resonant frequency is the third resonant frequency, and the second high-order mode resonant frequency is the The fifth resonant frequency, and so on.

In the view of the standing wave, when the input ports 32 are used to receive the pair of differential mode signals (hereinafter referred to as differential mode operation), any of the eight resonators 31, 41, 51, 61 One end of the symmetry plane XPOS is electrically connected to the ground plane 2, and the other end is an open end free end, so the eight resonators 31, 41, 51, 61 can be regarded as eight quarter-wave resonance The resonator, in other words, an electrical length of the free end of each of the resonators 31, 41, 51, 61 to the plane of symmetry XPOS is an odd multiple of a quarter wavelength, l = [(1/4 ×λ]×N, where the parameter l is the electrical length and the parameter λ represents the electrical length of one wavelength.

Since N is a positive odd number that is incremented from 1, the balanced three-band pass filter does produce three passbands during differential mode operation.

And a coupling coefficient between the first resonating unit 3 and the second resonating unit 4, a coupling coefficient between the third resonating unit 5 and the fourth resonating unit 6, and the second resonating unit 4 and the third The coupling coefficients between the resonant units 5 are all positively related to the bandwidth of each pass band.

When the preferred embodiment is in the common mode operation, a common mode signal excited by the first resonating unit 3 is electrically connected to the through hole 7 of the second resonating unit 4 when coupled to the second resonating unit 4. When the residual common mode signal is coupled to the third resonating unit 5, the other through hole 7 is Further destroying, thus being coupled to the fourth resonating unit 6, so that the common mode signal arriving at the output 埠62 is already negligible, in other words, the through holes 7 the second resonating unit 4 and the third resonating The common mode signal of unit 5 is shorted to the ground plane 2 such that the pair of common mode signals received by the input ports 32 are substantially uncoupled (less than one percent in the preferred embodiment) to the outputs埠62.

Explaining in more detail, during the common mode operation, an electrical length of each of the resonators 31, 61 of the first and fourth resonating units 3, 6 is an integer multiple of one-half wavelength, l' = [( 1/2)×λ]×k, where the parameter l′ is the electrical length, the parameter λ represents the electrical length of one wavelength, the parameters k=1, 2, 3, ...; differently, the second And each of the resonators 41, 51 of the third resonating units 4, 5 is physically connected to the ground plane 2 via the corresponding through hole 7, so that the electrical length is broken by an integral multiple of one-half of the wavelength. The common mode resonant signal, therefore, the signals of the second resonant frequency, the fourth resonant frequency, and the sixth resonant frequency ( f ₂ , f ₄ , f ₆ ...) corresponding to the resonators 31, 61 during common mode operation The coupling to the resonators 41, 51 may be destroyed or may not exist. In other words, the common mode signal from the first resonating unit 3 is hardly coupled to the second resonating unit 4, and is coupled to the third resonating unit. The common mode signal of 5 is weaker after being re-damaged due to the double common mode rejection of the second resonating unit 4 and the third resonating unit 5. Such input port 32 receives a common mode signal is not substantially coupled to the resonance unit 6 and the fourth port 62 such an output.

The characteristic impedance of the first line segment to the third line segment 311~313, 411~413, 511~513, 611~613 of each resonator 31, 41, 51, 61 and the first to fifth resonances The relationship between the frequencies f ₁ to f ₅ satisfies the following conditions:

Where Z ₁ is the characteristic impedance of the first line segments 311, 411, 511, 611, Z ₂ is the characteristic impedance of the second line segments 312, 412, 512, 612, and Z ₃ is the third line segment Characteristic impedance of 313, 413, 513, 613.

For the convenience of design, in the preferred embodiment, the electrical lengths of the twelve line segments 311, 411, 511, 611, 312, 412, 512, 612, 313, 413, 513, 613 are substantially equal, but only By changing the characteristic impedances Z ₁ , Z ₂ , Z ₃ , the resonant frequency ratios f ₂ / f ₁ , f ₃ / f ₁ , f ₄ / f ₁ , f ₅ / f shown in the above four equations can be adjusted. ₁ .

In the preferred embodiment, the first resonant frequency f ₁ , the third resonant frequency f _{3 ,} and the fifth resonant frequency f ₅ are designed at 1.57 GHz, 3.5 GHz, and 5.25 GHz, respectively, to respectively cover the GPS L1 system ( 1.557~1.593GHz), WiMAX (3.4~3.7GHz) and WLAN (5.15~5.35GHz) three communication bands. The substrate 1 is a microwave substrate of the model Duroid 6010 having a plate thickness of 0.635 mm, a dielectric constant of 10.2, and a loss tangent of 0.0023.

First, we determine the impedance values Z ₁ , Z ₂ and Z ₃ according to the required resonant frequency ratios f ₃ / f ₁ and f ₅ / f ₁ .

Since N is a positive odd number, the even second and fourth resonant frequencies f ₂ and f ₄ represented by the above formulas of f ₂ / f ₁ and f ₄ / f ₁ do not exist, so we only need to consider f ₃ / f ₁ and f ₅ / f ₁ of the two formulas, so the selected parameters K ₁ and K ₂ are 1.89, 1.67, respectively; the electrical length θ ₀₁ corresponding to the first resonant frequency (1.57 GHz) is 40.3 degrees, wherein

In this design, in order to take into account the accuracy limit of the implementation, we take Z ₁ = 21.6 Ω, Z ₂ = 36 Ω, and Z ₃ = 68 Ω, and the line widths of the first line segments 311, 411, 511, and 611 are each 2.31 mm, the line widths of the second line segments 312, 412, 512, 612 are each 1.04 mm, and the line widths of the third line segments 313, 413, 513, 613 are each 0.23 mm.

Next, we can further design the coupling pitch and the coupling length between the first to third coupling gaps 34, 56, 45 (the coupling pitch is defined as the width parallel to the XPOS direction of the symmetry plane, and the coupling length is defined as perpendicular to the The length of the symmetry plane XPOS direction), and the linear distance D1 of each input 埠32 and each output 埠62 to the symmetry plane XPOS.

First, the ratios (Δ) of the first passband, the third passband, and the fifth passband corresponding to the first resonant frequency, the third resonant frequency, and the fifth resonant frequency are set to be 8%, 10%, respectively. 6% (Same reason, for the sake of safety, this value is slightly higher than the three frequencies of GPS, WiMAX, and WLAN required by the above-mentioned actual specifications by 2.3%, 8.57%, and 3.8%), and the fourth-order Butterworth function is selected. For the response of the filter, the values of the components of the low-pass filter prototype can be found in the Pozar Microwave Engineering book. The required coupling factor and the external quality factor of the linear distance D1 can be calculated as follows.

The external quality factor Q _dd =g0×g1/Δ required for the first and fourth resonating units 3 and 6 to operate in the first passband and the third passband can be calculated as Q _dd of 9.57, 7.65 and 12.76, respectively. The coupling coefficient M _34,56 =Δ/√(g0×g1) between the first and second resonating units 3, 4 or the third and fourth resonating units 5, 6 can be calculated as 0.067, 0.084, respectively. 0.05, and the coupling coefficient M ₄₅ = Δ / √ (g2 × g3) between the second and third resonance units 4, 5 is 0.043, 0.054, and 0.032. After knowing the coupling coefficients M ₃₄ , ₅₆ , M ₄₅ , the required coupling pitch and coupling length can be determined, and the calculated linear distance D1 can be determined by the calculated external quality factor.

Referring to Fig. 4, we first simulate the external quality factor Q _dd and the linear distance D1 when the differential mode is operated by the electromagnetic simulation software (HFSS), and then select the value of the feeding position (the linear distance D1) by referring to FIG. On the other hand, in our case the fixed coupling length (FIG. 2), the simulation software to differential mode coupling coefficient M _34,56, M ₄₅ each of the first to third coupling gap coupled 34,56,45 The relationship of the coupling pitch.

Referring to FIG. 5, the calculated coupling coefficient M _{34, 56 is} calculated to determine the coupling pitch G12 between the first resonating unit 3 and the second resonating unit 4, and the third resonating unit 5 and the fourth resonating The coupling pitch between the units 6 is equal to the coupling pitch G12 = 1 mm.

Referring to FIG. 6, the calculated coupling coefficient M _{45 is} determined to determine the coupling distance G22=0.4 mm between the second resonating unit 4 and the third resonating unit 5.

Referring to FIG. 7, in the differential mode operation, the measured values of the first resonant frequency, the third resonant frequency, and the fifth resonant frequency are 1.57 GHz, 3.55 GHz, and 5.245 GHz, respectively; the first resonant frequency The analog values of the third resonant frequency and the fifth resonant frequency are 1.55 GHz, 3.56 GHz, and 5.245 GHz, respectively; the measured results of the first resonant frequency, the third resonant frequency, and the 3 dB bandwidth corresponding to the fifth resonant frequency are respectively 1.53~1.61GHz, 3.39~3.71GHz and 5.13~5.36GHz; the simulation results of the 3dB bandwidth corresponding to the first resonant frequency, the third resonant frequency and the fifth resonant frequency are 1.49~1.61GHz, 3.38~3.73GHz and respectively 5.11~5.38GHz; the minimum value of the intervention loss is 2.56dB, 1.8dB and 2.67dB; the minimum simulation of the insertion loss is 2.4dB, 1.3dB and 2.45dB. The imbalance amplitudes (amplitude imbalance, || S ₂₁ |-| S _{2 ' 1} ||) measured by the first passband, the third passband and the fifth passband are respectively between 0.09 and 0.23 dB, 0.52 and 0.89. dB and between 0.04 and 1.89dB, and the unbalanced phase (∠ S ₂₁ -∠ S _2'1 ) measured by the first passband, the third passband and the fifth passband are respectively _179.08 ° Between 181.51°, 172.09° and 187.89° and 171.76° and 188.34°.

Referring to FIG. 8, the three passbands of the first resonant frequency, the third resonant frequency, and the fifth resonant frequency are measured and simulated in the common mode operation, and the interference is obtained in the range of 1 to 7 GHz. The loss is greater than 21.5dB, and it does have the effect of common mode signal rejection.

Referring to the attached one, which is a photograph of the implementation of the preferred embodiment, the total area of the circuit board (ie, the substrate 1 of FIG. 2) is 45×32.5 mm ² , and the above measurement results are actually made through the preferred embodiment. Do the measurement.

In summary, the preferred embodiment receives the two boundary conditions corresponding to the differential mode signal and the pair of common mode signals respectively, so that the first resonating unit 3 and the fourth resonating unit 6 are electrically connected correspondingly. And electrically connected to the ground plane 2, and the second resonating unit 4 is made by using the through holes 7. And the third resonant unit 5 is constantly electrically connected to the ground plane 2 at each end of the symmetric plane XPOS, thereby generating the effect of differential mode bandpass and common mode signal rejection, and the object of the present invention can be achieved.

The above is only the preferred embodiment of the present invention, and the scope of the present invention is not limited thereto, that is, the simple equivalent changes and modifications made by the patent application scope and patent specification content of the present invention, All remain within the scope of the invention patent.

1‧‧‧Substrate

XPOS‧‧‧symmetric plane

YPOS‧‧‧ mirror plane

3‧‧‧First Resonance Unit

31‧‧‧Resonator

311‧‧‧First line segment

312‧‧‧Second line

313‧‧‧ third line segment

3131‧‧‧ openings

4‧‧‧Second Resonance Unit

41‧‧‧Resonator

411‧‧‧First line segment

412‧‧‧second line

413‧‧‧ third line segment

5‧‧‧3rd Resonance Unit

51‧‧‧Resonator

511‧‧‧First line

512‧‧‧second line

513‧‧‧ third line segment

6‧‧‧4th Resonance Unit

61‧‧‧Resonator

611‧‧‧First line

612‧‧‧second line

613‧‧‧ third line segment

6131‧‧‧ openings

34‧‧‧First coupling gap

56‧‧‧Second coupling gap

45‧‧‧ Third coupling gap

32‧‧‧ Input埠

62‧‧‧ Output埠

7‧‧‧through holes

D1‧‧‧Linear distance

Claims (10)

A balanced three-band pass filter comprising: a substrate having a first surface and a second surface spaced apart; a ground plane disposed on the first surface of the substrate; a first resonant unit to a The fourth resonating unit is disposed on the second surface of the substrate and spaced apart from each other, and the second resonating unit and the third resonating unit are interposed between the first resonating unit and the fourth resonating unit, the first The resonant unit and the second resonant unit are partially spaced apart to be coupled to each other, the third resonant unit and the fourth resonant unit are spaced apart in parallel to be coupled to each other, and the second resonant unit and the third resonant unit are partially spaced apart to be coupled to each other And each of the first to fourth resonating units includes two resonators respectively located on opposite sides of a symmetric plane, and the first resonating unit is provided with two input ports for receiving signals, The fourth resonating unit is provided with two output ports for outputting signals; and two through holes are disposed along the plane of symmetry, and the second resonating unit is electrically connected via one of the through holes The grounding surface, the third resonant unit is electrically connected to the ground plane via another through hole, and the balanced three-band pass filter is equivalent to a perfect electric wall when operating in a differential mode, the first Resonant unit and the first The resonators of the four resonance units are electrically connected to the ground plane through the perfect electric wall, and each resonator resonates at an Nth resonance frequency, and the Nth resonance frequency corresponds to an Nth pass band, wherein the parameter N For an odd number increasing from 1, the pair of differential mode signals received by the input ports are coupled to the output ports through the pass bands, and the balanced three-band pass filters are operated in a common mode, the through holes A common mode signal of the second resonating unit and the third resonating unit is shorted to the ground plane. The balanced three-band pass filter of claim 1, wherein the resonators have substantially the same electrical length. The balanced three-band pass filter of claim 1, wherein each of the first to fourth resonating units is a three-step stepped impedance resonator, the three-step stepped impedance resonator having a first line segment, a second line segment and a third line segment having different line widths, the first line segment having a first end located at the plane of symmetry, and a second end opposite to the first end, The second line segment has a first end electrically connected to the second end of the first line segment, and a second end opposite to the first end, the third line segment having a second end electrically connected to the second line segment One end, and one opposite the free end of the first end. The balanced three-band pass filter of claim 3, wherein the two first line segments of each of the first to fourth resonance units are mutually mirror-symmetrical to the plane of symmetry, The two The second line segments are mirror-referenced to the plane of symmetry, and the two third line segments are mirror-referenced to the plane of symmetry. The balanced three-band pass filter of claim 3, wherein, when N=1, 3, and 5, the characteristic impedance of the first line segment to the third line segment of each resonator and the first resonant frequency f ₁ , the relationship between the third resonance frequency f ₃ and the fifth resonance frequency f ₅ satisfies the following conditions: Wherein Z ₁ is a characteristic impedance of the first line segment, Z ₂ is a characteristic impedance of the second line segment, and Z ₃ is a characteristic impedance of the third line segment, and the first line segment, the second line segment, and The electrical lengths of the third line segments are substantially equal. The balanced three-band pass filter of claim 5, wherein a linear distance of each input to the plane of symmetry is inversely proportional to an external value factor of the balanced three-band pass filter, and the external value The larger the factor, the smaller the bandwidth of each passband. The balanced three-band pass filter of claim 5, wherein a coupling between the first resonating unit and the second resonating unit The number, the coupling coefficient between the third resonating unit and the fourth resonating unit, and a coupling coefficient between the second resonating unit and the third resonating unit are positively related to the bandwidth of each pass band. The balanced three-band pass filter of claim 3, wherein each first line segment extends from the symmetry plane away from the plane of symmetry, and each second line segment is electrically connected to the first line from the phase a second end of the segment extends away from the plane of symmetry, and the first resonating unit and the second resonating unit collectively define a first coupling gap extending in a direction perpendicular to the plane of symmetry, the third resonating unit And the fourth resonating unit also collectively defines a second coupling gap extending in a direction perpendicular to the plane of symmetry, the second resonating unit and the third resonating unit being located on the same side of the opposite sides of the plane of symmetry The two third line segments respectively extend from the second ends of the second line segments electrically connected to each other, and then extend toward the plane of symmetry, and the two third line segments collectively define a direction perpendicular to the plane of symmetry An extended third coupling gap. The balanced three-band pass filter of claim 8, wherein each third line segment of the first resonating unit and the fourth resonating unit is substantially U-shaped with an opening oriented in a direction parallel to the plane of symmetry And a U-shaped opening direction formed by each third line segment of the first resonating unit and each third line segment of the fourth resonating unit The formed U-shaped opening directions are opposite to each other, and the third line segments of the first resonating unit are partially adjacently coupled to the second line segments of the second resonating unit, and the fourth resonating unit is the third line The segments are also partially adjacently coupled to the second line segments of the third resonant unit, respectively. The balanced three-band pass filter of claim 1, wherein the first resonating unit and the fourth resonating unit are mutually mirror-symmetrical to a mirror plane perpendicular to the plane of symmetry, the second resonating unit and the second The three resonant units are also mirrored relative to each other in the mirror plane.