Absorptive common mode suppression filter

Abstract

The present application belongs to the technical field of microwave filter manufacturing, and particularly relates to an absorption type common mode rejection filter. The absorption type common mode rejection filter comprises a plurality of cascaded single-stage filters; the structure of the single-stage filter is in an M shape, and comprises two microstrip lines and a high-resistance filter network; the single-stage filter comprises an upper layer structure and a lower layer structure, and the upper and lower layer structures are connected into a whole by a via; two ends of each microstrip line are connected to the lower layer by the via, and one end of each microstrip line is grounded, and the other end is connected to one end of a defective ground structure; the defective ground structure connects the two microstrip lines in the lower layer; the middle part of the defective ground structure is truncated, and a resistor is arranged at the truncated position; the high-resistance filter network comprises a capacitor and an inductor, the inductor is formed by the via with metal on the inner side, and the capacitor is formed by the ground capacitance of the defective ground structure. The present application has the characteristic that the common mode noise can be completely eliminated from the system.

Description

Technical Field

[0001] This invention belongs to the field of microwave filter manufacturing technology, specifically relating to an absorption common-mode rejection filter. Background Technology

[0002] With the increasing popularity of high-speed digital applications, differential signals have attracted much attention due to their immunity to external interference. Differential lines are increasingly used in high-speed digital systems. However, due to factors such as corners, asymmetrical signal rise / fall times, and amplitude imbalances, common-mode noise inevitably appears in differential lines. Furthermore, common-mode signals are highly susceptible to radiation, which can occur at switching ports or parasitic antennas, leading to signal integrity (SI), power integrity (PI), electromagnetic compatibility (EMC), and electromagnetic interference (EMI) issues.

[0003] To address the aforementioned issues, ferrite chokes are used to suppress common-mode noise. While providing high impedance to common-mode signals and reflecting them, this method is only applicable to the megahertz (MHz) frequency range, has insufficient suppression levels, and is quite bulky. To suppress common-mode noise in the gigahertz (GHz) frequency range, common-mode filters (CMFs) utilizing reflected waves have been proposed, hence the name Reflective Common-mode Filter (R-CMF). However, reflected waves may enter amplifiers, antennas, and other components, potentially causing severe electromagnetic compatibility issues, reducing system lifespan, and posing potential hazards. Therefore, a suitable destination for the reflected energy is needed to avoid electromagnetic compatibility problems. To address this issue, Arbsorpive Common-mode Filters (A-CMFs) have recently been proposed. In Arbsorpive Common-mode Filters, common-mode noise is not reflected but dissipated by being converted into heat.

[0004] Typical common-mode rejection filters (CMRR) achieve total reflection or absorption at a specific frequency by designing common-mode circuitry, while ensuring the differential-mode (DM) circuitry maintains full-pass characteristics. Due to varying application scenarios, some CRR filters can completely suppress common-mode noise when only a certain frequency band of differential-mode signal is needed. However, current CRR filter designs either employ multi-layer board structures, leading to increased costs, or large defective ground structures (DGS), requiring pre-allocated area when integrating with other components.

[0005] Therefore, it is essential to design an absorptive common-mode rejection filter in a more economical and convenient way in modern communication systems. Summary of the Invention

[0006] The present invention aims to overcome the problem that existing technologies are unable to suppress common-mode noise signals generated during differential line signal transmission, and provides an absorptive common-mode suppression filter that can completely eliminate common-mode noise from the system.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] An absorptive common-mode rejection filter comprises several cascaded single-order filters; the single-order filters have an M-shaped structure, including two microstrip lines and a high-impedance filter network.

[0009] The single-order filter includes an upper structure and a lower structure, which are connected as a whole by vias. Each microstrip line in the upper layer is connected to the lower layer through vias at both ends, and one end of each microstrip line is grounded, while the other end is connected to one end of the defective ground structure. The defective ground structure connects two microstrip lines in the lower layer. The defective ground structure is truncated in the middle, and a resistor is provided at the truncated position.

[0010] The high-impedance filter network includes capacitors and inductors; the inductor in the high-impedance filter network is formed by vias with metal on the inside; the capacitor in the high-impedance filter network is composed of ground capacitance formed by defective ground structures.

[0011] Preferably, both microstrip lines are rectangular.

[0012] Ideally, both microstrip lines are located exactly in the middle of the two difference lines.

[0013] Preferably, the defective ground structure is a square area after defective ground technology has been used.

[0014] Preferably, all the vias are cylindrical.

[0015] Preferably, the via is tangent to the microstrip line and the defective ground structure on the connection surface.

[0016] Preferably, the portion of the defective structure that is cut off in the middle is square; the resistance welding is performed at the cut-off location.

[0017] Preferably, the absorptive common-mode rejection filter occupies only the space between the two differential lines.

[0018] Preferably, each single-order filter connects the two microstrip lines and the defective ground structure into a single unit via four vias.

[0019] Compared with the prior art, the beneficial effects of this invention are: (1) The absorptive common-mode rejection filter of this invention has broadband and ultra-wideband common-mode noise absorption bands, and does not occupy additional area. It can be achieved simply by adding a structure in the middle of the differential traces; (2) This invention solves the problems of narrow absorption bandwidth and the need to reserve area for defective grounding technology in the previous technology, and has the advantages of low cost and simple manufacturing; This invention plays a very positive role in promoting the industrialization process of absorptive common-mode rejection filters. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a single-order structure of the absorptive common-mode suppression filter of the present invention;

[0021] Figure 2 This is a top view of the upper layer of the single-order structure of the absorptive common-mode suppression filter of the present invention;

[0022] Figure 3 This is a bottom view of the lower layer of the single-order structure of the absorptive common-mode suppression filter of the present invention;

[0023] Figure 4 This is an equivalent circuit diagram of the absorptive common-mode rejection filter of the present invention;

[0024] Figure 5 This is a simulation result diagram of a single-order structure of the absorptive common-mode suppression filter of the present invention;

[0025] Figure 6 This is an experimental result diagram of a third-order structure of the absorption common-mode suppression filter of the present invention. Detailed Implementation

[0026] To more clearly illustrate the embodiments of the present invention, specific implementation methods will be described below with reference to the accompanying drawings. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings and other implementation methods can be obtained based on these drawings without any creative effort.

[0027] Example:

[0028] This invention provides an absorptive common-mode rejection filter, comprising multiple cascaded single-order filters; each single-order filter has an M-shaped structure, including two microstrip lines and a high-impedance filter network, as shown in the specific structure below. Figure 1 As shown;

[0029] The single-order filter includes an upper structure and a lower structure, which are connected as a whole by vias. Each microstrip line in the upper layer is connected to the lower layer through vias at both ends, and one end of each microstrip line is grounded, while the other end is connected to one end of the defective ground structure. The defective ground structure connects two microstrip lines in the lower layer. The defective ground structure is truncated in the middle, and a resistor is provided at the truncated position.

[0030] The high-impedance filter network includes capacitors and inductors; the inductor in the high-impedance filter network is formed by vias with metal on the inside; the capacitor in the high-impedance filter network consists of ground capacitance formed by a defective ground structure.

[0031] Considering the narrow bandwidth of single-order filters, in order to extend the bandwidth, this embodiment cascades three basic single-order filters and selects the resistance value according to different absorption rate requirements to achieve common-mode noise absorption.

[0032] This invention adopts the following theory:

[0033] Theory 1, Odd-Even Mode Circuit Analysis Method;

[0034] Theory 2, Coupled Line Theory.

[0035] Theory 1 is as follows:

[0036] A four-port network can be decomposed into two two-port networks, a differential-mode equivalent circuit, and a common-mode equivalent circuit, depending on the type of input signal. Analyzing these two two-port networks yields the responses to different input signals. To simplify the analysis of each two-port network, it can be further converted into two one-port networks: a differential-mode network and a common-mode network. These two one-port networks together contain all the information of the corresponding two-port network.

[0037] For common-mode and differential-mode networks, the S-parameters are represented by the terminal impedance Z0 and their respective Z-parameters.ine / Z ine The input impedances are represented as follows:

[0038]

[0039]

[0040] For a two-port network, it is easy to know that:

[0041]

[0042]

[0043] Substitute it into the above equation:

[0044]

[0045]

[0046] For an absorption filter, the common-mode two-port network should have no reflection or output of the input energy, so that the energy is dissipated within the network. This achieves the absorption function, which can be expressed in terms of S-parameters as follows:

[0047] S 11 =0

[0048] S 21 =0

[0049] That is

[0050] Z ine Z ino -Z0 2 =0

[0051] Z ine -Z ino =0

[0052] That is, it should satisfy

[0053] Z ine =Z ino =Z0

[0054] Theory 2 is as follows:

[0055] For a coupled line segment, the port current-voltage relationship is as follows:

[0056]

[0057] in:

[0058]

[0059]

[0060]

[0061]

[0062] For Z 0e With Z 0o This can be determined by determining the coupling coefficient C:

[0063]

[0064]

[0065] For the designed filter, we now obtain:

[0066] V1=-Z1I1

[0067] V4 = -Z4I4

[0068] Substituting into equation (*), we can see

[0069]

[0070]

[0071] in

[0072] c = 1 - abZ 41 Z 14

[0073] d=abZ 41 Z 12 -bZ 12

[0074] e = abZ 41 Z 13 -bZ 13

[0075] f = abZ 41 Z 12 -bZ 42

[0076] g = abZ 41 Z 13 -bZ 43

[0077]

[0078]

[0079] Substituting equations (a) and (b) into equation (*), and taking port 2 as input 1′ and port 3 as output 2′, we can obtain the impedance matrix of a two-port network:

[0080]

[0081] For any two-port network, its [Z] matrix is ​​often related to the port voltages and currents, and can be written in matrix form as follows:

[0082]

[0083] Its input impedance can be determined by the impedance matrix [Z] and the load impedance Z. L Let's say we have an input at port 1' and a load connected to port 2'.

[0084] V′2=-Z L I'2

[0085] Substitute (**) into the expression,

[0086]

[0087] Substitute I2 into equation (**),

[0088]

[0089] For the input impedance Z of the two circuits ine With Z ino This can be easily derived using the preceding coupled-line theory.

[0090] For the designed filter, we can know

[0091] Z 1e =jωL1

[0092]

[0093]

[0094] Z 1o =jωL1

[0095]

[0096] Z Lo =jωL m

[0097] In practical applications, a 50Ω load is commonly used. The input impedances of the ee circuit and the oo circuit are respectively...

[0098]

[0099]

[0100] Substituting, we can obtain

[0101]

[0102]

[0103] Substituting this into the above formula yields the S-parameters. Plotting the corresponding calculation results reveals an absorption resonant point at 5 GHz, exhibiting high impedance characteristics. This completes the filter design required by this invention.

[0104] In summary, the integrated design process of the absorption common-mode rejection filter of the present invention is very simple. By determining the values ​​of the coupling line length l, capacitor C1 and inductor L1, the band-stop resonant frequency can be determined, and by determining the value of resistor R, better absorption can be achieved.

[0105] Furthermore, both microstrip lines are rectangular and located precisely in the middle of the two differential lines. The microstrip line width (coupling gap) should be appropriate, neither too narrow nor too wide.

[0106] Furthermore, the defective ground structure is a square area after the defective ground technology has been used.

[0107] Furthermore, all the vias are cylindrical.

[0108] Furthermore, the vias are tangent to the microstrip lines and defect structures at the connection surface.

[0109] Furthermore, the truncated portion of the defective structure is square; the resistance weld is applied at the truncated location. The welding resistance should minimize the overlap between the welded portion and the original metal portion.

[0110] Furthermore, the absorptive common-mode rejection filter only occupies the space between the two differential lines.

[0111] Furthermore, each single-order filter connects two microstrip lines and the defective ground structure into a single unit via four vias.

[0112] In addition, the substrate material of the present invention is FR4 with a thickness of 1.8mm, and the whole is a double-layer board structure.

[0113] Figure 2 This diagram shows a top-level view of the single-order absorption common-mode rejection filter proposed in this invention. It can be seen that the signal lines and the central microstrip structure exhibit a coupling line effect in the common-mode / differential-mode half-circuit, which can be analyzed using the coupling line theory described earlier. The location of the resonant point is determined by the length of the central microstrip line; a longer line results in a lower resonant frequency, and vice versa.

[0114] Figure 3 The bottom view of the proposed single-order absorption common-mode rejection filter is shown. It can be seen that the structure truncates the square region in the middle to leave space for welding resistors.

[0115] Figure 4 The equivalent circuit diagram of the proposed single-order absorption common-mode rejection filter is shown. Figures (a) and (b) are the common-mode half-circuit and differential-mode half-circuit, respectively, and Figures (c) and (d) are the common-mode half-circuit and differential-mode half-circuit of the common-mode half-circuit, respectively. Using the equivalent circuit diagram, and combining the odd-even mode circuit analysis method and coupled-line theory described above, the required absorption common-mode rejection filter can be designed.

[0116] Figure 5 Simulation results of a single-order absorption common-mode rejection filter are shown. It can be seen that common-mode noise neither passes through nor reflects back near 5 GHz, indicating that it has been absorbed by the circuit and dissipated as heat. Furthermore, the simulated S-parameters exhibit high impedance, which can be utilized by cascading the filter to take advantage of the absorption resonant point and the high impedance phenomenon.

[0117] Figure 6 The experimental results for a third-order absorptive common-mode rejection filter are shown. From... Figure 4 It can be seen that although it can absorb common-mode noise, its bandwidth is too narrow, making it difficult to apply in practice. Therefore, it is cascaded in three layers to increase its bandwidth. When cascading, it is important to note that the following should be changed... Figure 2 The lengths of the microstrip lines in the middle of the signal lines are then cascaded. Looking from the signal input port, the length of each microstrip line decreases progressively, and the resistance values ​​(R1) in the last two units should be the same. The resistance value (R2) in the first unit differs from the other two units and should be smaller. Let R1 = 1000Ω and R2 = 240Ω. Experimental results show that, based on a 95% absorption rate, its absorption bandwidth reaches 0.81GHz. If based on an 80% absorption rate, simply changing the resistance value of R1 to 300Ω achieves an absorption bandwidth >5.14GHz, greatly increasing its potential in practical applications.

[0118] The absorptive common-mode rejection filter of this invention has broadband and ultra-wideband common-mode noise absorption bands, and does not occupy additional area; it can be implemented simply by adding a structure in the middle of the differential traces. This invention solves the problems of narrow absorption bandwidth and the need to reserve area for defective grounding technology in previous technologies, and has the advantages of low cost and simple manufacturing. This invention plays a very positive role in promoting the industrialization of absorptive common-mode rejection filters.

[0119] The above description is merely a detailed explanation of preferred embodiments and principles of the present invention. For those skilled in the art, there may be changes in specific implementation methods based on the ideas provided by the present invention, and these changes should also be considered within the scope of protection of the present invention.

Claims

1. An absorption-type common-mode rejection filter, characterized in that, It includes several cascaded single-order filters; the single-order filter has an M-shaped structure, including two microstrip lines and a high-impedance filter network; The single-order filter includes an upper structure and a lower structure, which are connected as a whole by vias. The upper structure includes two microstrip lines, each connected to the lower structure at both ends through vias. One end of each microstrip line is grounded through the via, and the other end is connected to one end of the defective ground structure through the via. The lower structure includes the defective ground structure, which connects the two microstrip lines of the upper layer. The defective ground structure is truncated in the middle, and a resistor is provided at the truncated position. The high-impedance filter network includes a capacitor and an inductor; the inductor in the high-impedance filter network is formed by the via with metal on the inside; the capacitor in the high-impedance filter network is composed of the ground capacitance formed by the defect ground structure. Both microstrip lines are located exactly in the middle of the two difference lines.

2. The absorptive common-mode rejection filter according to claim 1, characterized in that, Both microstrip lines are rectangular.

3. The absorptive common-mode rejection filter according to claim 1, characterized in that, The defective ground structure is a square area after defective ground technology has been used.

4. The absorptive common-mode rejection filter according to claim 1, characterized in that, All the vias are cylindrical.

5. The absorptive common-mode rejection filter according to claim 4, characterized in that, The via is tangent to the microstrip line and the defective ground structure on the connection surface.

6. The absorptive common-mode rejection filter according to claim 1, characterized in that, The defective structure has a square-shaped section cut off in the middle; the resistance welding is performed at the cut-off location.

7. The absorptive common-mode rejection filter according to claim 1, characterized in that, The absorptive common-mode rejection filter occupies only the space between the two differential lines.

8. The absorptive common-mode rejection filter according to claim 1, characterized in that, Each single-order filter connects two microstrip lines and the defective ground structure into a single unit via four vias.

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