Reducing crosstalk between microstrip lines using CSR structure

Aiming at the problem of crosstalk between microstrip lines, a method of reducing crosstalk by using cross‐shape resonators (CSR) structures is proposed. On the premise of not changing the spacing of microstrip lines, this method adds CSR structures between the coupled microstrip lines to increase the capacitive coupling and thus to suppress the far‐end crosstalk. Based on the analysis of the equivalent circuit of CSR structure, the parameters simulation and verification are carried out by ADS and HFSS software. Through HFSS simulation and physical test of the designed CSR structure, the results show that: the CSR structure can significantly reduce the far‐end crosstalk by about 15 dB in the frequency of 0–10 GHz, and the maximum can reach 43 dB. Compared with 3W crosstalk reduction method and rectangular‐shape resonators crosstalk reduction method, the crosstalk reduction effect is improved.


INTRODUCTION
With the continuous advancement of science and technology, people have higher requirements on the miniaturization and high performance of electronic products, resulting in the continuous compression of the wiring spacing of the interconnection transmission lines in electronic products, and the continuous increase of signal frequency, which in turn causes crosstalk that seriously interferes with signal transmission.As one of the four major signal integrity problems, crosstalk will cause signal distortion during transmission.Severe signal distortion will directly affect circuit functions, resulting in product performance degradation or even damage. 1,2Therefore, studying the crosstalk problem between transmission lines in high-speed interconnection is an important direction to ensure the normal function of high-speed circuits, improve product performance, and promote the further development of integrated circuits.When industrial design continuously increases the operating frequency of integrated circuits to improve performance, and continuously compresses the wiring spacing of transmission lines to pursue product miniaturization, reducing crosstalk becomes an important prerequisite for ensuring correct signal transmission.Crosstalk can be divided into near-end crosstalk and far-end crosstalk.Because far-end crosstalk will seriously affect the correct judgment of the signal at the receiving end, and the damage to the circuit is more serious.Therefore, far-end crosstalk is the main factor that limits signal integrity in high-speed interconnections. 35][6] On the basis of the crosstalk formation principle, a method of using guard trace to reduce crosstalk is formed.
8][9] Although the structure of the guard trace is simple, but the suppression effect of crosstalk is not good and may cause resonance problems.Some researchers have also designed a crosstalk reduction method using a mushroom-shaped dielectric structure, 10 but the use of 3D printing technology increases industrial manufacturing costs, and the effect of reducing far-end crosstalk is not ideal.Based on previous research, crosstalk can be suppressed to a certain extent when the microstrip line spacing is three times the line width.Therefore, the 3W rule crosstalk reduction is generated. 11Although this method can reduce crosstalk, it wastes limited wiring space and not suitable for circuit integration.On the basis of the crosstalk reduction method using guard trace, the rectangular-shaped resonator (RSR) structure can be arranged between microstrip lines to further reduce crosstalk. 12However, this structure has a certain crosstalk suppression effect only when the line spacing is 3W.In particular, in ensuring the effective use of wiring space, when the RSR structure is used for microstrip lines with small spacing, the effect is not ideal, and the effect of reducing crosstalk must be improved. 13nspired by the RSR structure, this article designs a cross-shape resonator (CSR) structure with better crosstalk reduction effect, which can further reduce the far-end crosstalk under the premise of ensuring the distance between the microstrip lines.The CSR structure as a resonator is a metal structure located between microstrip lines, and its specific physical structure allows it to affect the transmission effect of signals on microstrip lines.This method is not only simple in structure and easy to implement, but it also has a good suppression effect on the far-end crosstalk within 0-10 GHz.In this article, the proposed CSR structure is initially simulated and verified from the perspective of equivalent circuit, and then the CSR crosstalk reduction method is modeled and simulated using the three-dimensional electromagnetic simulation software high-frequency structure simulator (HFSS).Furthermore, a real object is made and tested using a vector network analyzer.The effectiveness of this method is verified from the two dimensions of simulation and actual measurement.

THE PRINCIPLE ANALYSIS OF CROSSTALK BETWEEN COUPLED MICROSTRIP LINES
The development and application of integrated electronic design has forced the distance between transmission lines to be greatly reduced.Therefore, when a high-speed signal is transmitted on the transmission line, the electromagnetic field generated by the transmission line can easily radiate to the adjacent transmission lines, thereby damaging the transmission quality of the signal.This harmful interference caused by electromagnetic field coupling is called crosstalk.
In high-speed interconnection, the impact of crosstalk on high-speed systems will become more significant with the reduction of transmission line spacing, and excessive crosstalk will cause wrong signal decisions, thereby affecting the normal function of system circuits.Taking two coupled microstrip lines as the object of analysis, the electromagnetic field coupling phenomenon between microstrip lines can be equivalent to mutual capacitance and mutual inductance, as shown in Figure 1.For two coupled microstrip lines, one of the transmission lines to which high-speed signals are applied is known as the attack line, and the other one that interfered with the attack line is known as the victim line.The side of the victim line close to the sending end of the attack line is known as the near end, and the side close to the receiving end of the attack line is known as the far end.Therefore, the crosstalk generated at both ends of the victim line is defined as the near-end crosstalk and far-end crosstalk respectively.The far-end crosstalk is more important because it will seriously affect the correct decision of the signal at the receiver. 14I G U R E 1 Coupled microstrip lines and equivalent circuit model.
Crosstalk is due to the coupling effect caused by mutual capacitance C m and mutual inductance C m between the attack line and victim line.In Figure 1, when a high-speed signal is input on the attack line, the far-end crosstalk on the victim line is measured as follows: 14 where Z 0 is the characteristic impedance of the microstrip line, l is the length of the coupling microstrip line, V a (t) is the input signal on the attack line, and C m and L m are the mutual capacitance and mutual inductance of the unit length between the attack line and victim line, respectively.As shown in Formula (1), the far-end crosstalk amplitude is proportional to the difference between C m and L m .Therefore, reducing the far-end crosstalk between microstrip lines can be achieved by increasing the capacitive coupling without changing the inductive coupling.

CSR STRUCTURE AND CROSSTALK REDUCTION METHOD
Based on the analysis of the mechanism behind far-end crosstalk between microstrip lines, it has been determined that reducing far-end crosstalk can be achieved by increasing the capacitive coupling between microstrip lines.To achieve this, a special structure can be added between microstrip lines to increase mutual capacitance.Accordingly, this article proposes a method to reduce crosstalk between microstrip lines by utilizing the capacitive-coupled metal resonator structure known as the CSR.The CSR structure is analyzed, which is located between two parallel-coupled microstrip lines and serves as a metal resonator.Its thickness matches the thickness of the microstrip line, and it increases mutual capacitance between microstrip lines without increasing mutual inductance.The model for arranging CSR structures at equal intervals between coupled microstrip lines is shown in Figure 2. As shown in Figure 2, when the CSR structure is excited by the electromagnetic field generated from the attack line, the current will flow from one side of the gap to the other in the form of strong displacement current.Thus, the gap between them is equivalent to the distributed capacitance.Considering that the CSR structure is placed perpendicular to the attack line, that is, perpendicular to the direction of the magnetic field generated by the attack line, it will not increase the mutual inductance between the microstrip lines.Therefore, the CSR structure arranged between the coupled microstrip lines can be equivalent to the series connection of two capacitors. 15The equivalent circuit is shown in Figure 3.For better analysis, the coupling microstrip lines with a corresponding length of the CSR structure is selected as the analysis object.Figure 3A shows the equivalent circuit model of the coupled microstrip lines with this length.Figure 3B shows the equivalent circuit model of the coupled microstrip lines after adding the CSR structure.L 01 represents the self-inductance of the microstrip line; L m1 is the mutual inductance between the two microstrip lines; C m1 indicates the mutual capacitance between the microstrip lines; C g1 represents the equivalent capacitance between the microstrip line and the reference ground, and C csr1 and C csr2 denote the distributed capacitance between the CSR structure and two microstrip lines.
Based on the equivalent circuit model, the total mutual capacitance after adding the CSR structure between the coupled microstrip lines C mm is measured as follows: The total mutual capacitance between coupled microstrip lines increases after adding a CSR structure.Based on the calculation formula of the far-end crosstalk, this crosstalk decreases.In confirming the effectiveness of the CSR structure in reducing crosstalk, HFSS software was used to extract parasitic parameters and simulate crosstalk.The simulation model of the crosstalk reduction method using the CSR structure is established through HFSS.The model consists of  three parts, namely, reference ground plane, dielectric layer, and wiring layer.The specific structure is shown in Figure 4.
The specific parameters of microstrip line are as follows: the line width of microstrip w = 3 mm, the height m = 0.06 mm, the distance between lines s = 2 mm, the thickness of the dielectric substrate h = 1.6 mm, the relative dielectric constant is 4.4, and the characteristic impedance of microstrip line is 50 Ω.
In achieving the best crosstalk reduction effect of the CSR structure, the abovementioned model is simulated and optimized for many times, and the specific structure of CSR is determined after repeated optimization, as shown in Figure 5.The specific parameters are as follows: a = 0.2 mm, b = 0.6 mm, c = 0.7 mm, d = 0.6 mm, e = 0.6 mm, g = 0.3 mm.The distance between the CSR structure and microstrip line is determined by the distance s and b, c, and d of the CSR structure.
To confirm the accuracy of the equivalent circuit model parameters mentioned above, the advanced design system (ADS) was employed to simulate the equivalent circuit and compare it against the HFSS simulation results, as presented in Figure 6.Overall, the ADS simulation outcomes align well with those of HFSS.Thus, the aforementioned equivalent circuit is suitable for modeling and analyzing CSR structures, as well as for exploring methods to mitigate crosstalk through CSR structures.Additionally, parasitic parameters for coupled microstrip lines featuring CSR structure (Figure 6) are derived and presented in Table 1.The extraction yields have found that incorporating the CSR structure results in increased mutual capacitance between the coupled microstrip lines, while the mutual inductance remains nearly constant.

TA B L E 1
Mutual capacitance and mutual inductance between coupled microstrip lines.

Method L m (nH) C m (pF)
Without using crosstalk reduction method 0.03052 0.11306 Crosstalk reduction method using a CSR structure 0.03052 0.12960

ANALYSIS OF CSR STRUCTURE DISTRIBUTION
To analyze the influence of CSR structure on the coupling phenomenon between microstrip lines, propose the CSR structure that exhibits the highest potential for reducing crosstalk, the position layout of CSR structure is simulated and analyzed.In practical applications, microstrip lines may have different lengths, making it difficult to place the CSR structure at the far end of the microstrip line.Therefore, we simulated and analyzed three different distributions of the CSR structure shown in Figure 7. Figure 7A represents a scenario where as many CSR structures as possible are placed, and the CSR structure is truncated when the far end position is insufficient.Figure 7B shows the scenario where the integrity of each CSR structure is maintained, and the CSR structure is placed with two equally distant ends.Finally, Figure 7C shows the scenario where the far end of the microstrip line is not sufficient to place a complete CSR structure.The simulation results of the three cases are shown in Figure 8.The analysis reveals that, while the suppression effect of Case 2 on far-end crosstalk is not as significant as that of Case 1, the suppression effect of Cases 3 and 1 on far-end crosstalk is not significantly different.Therefore, for practical use of the CSR structure to reduce crosstalk, it is most effective to place it at the beginning of the transmission line to suppress far-end crosstalk.When the microstrip line ends insufficiently, it can be directly truncated, which does not significantly affect far-end crosstalk suppression, as in Case 1.

EXPERIMENT AND RESULT ANALYSIS
After verifying the rationality of the method at the theoretical through the equivalent circuit, this section verifies the effectiveness of the crosstalk reduction method proposed in this article.First, use HFSS software to establish a general coupled microstrip line model with x = 20 mm and y = 36 mm, as shown in Figure 9A.Place the CSR structure in Figure 5 evenly between the microstrip lines.The number of CSR structures is determined by the length of the microstrip line.
For CSR structures that are not completely placed, they can be directly truncated, as shown in Figure 9B.In order to compare the crosstalk reduction effect with other methods, the simulation models corresponding to the 3W crosstalk reduction method and the RSR structure crosstalk reduction method are established.The four simulation models are  shown in Figure 9. Except that the line spacing in Figure 9C should meet the 3W principle, the specific parameters of other microstrip lines are consistent with those shown in Figure 4.
The S parameter simulation is carried out for the four abovementioned models.Meanwhile, the sample of the method proposed in this article is made in accordance with the model parameters, as shown in Figure 10, and the vector network analyzer is used for measurement.The simulation and measurement results are shown in Figures 11-13.Figure 11 shows the comparison between the simulation and measurement results of the return loss (S 11 ) of the four methods in the frequency range of 0-10 GHz, Figure 12 shows the comparison between the simulation and measurement results of the far-end crosstalk (S 41 ), and Table 2 shows the maximum remote crosstalk reduction for three crosstalk reduction methods.By comparing the simulation and test results of S 11 , it can be found that the CSR structure ensures low reflection of microstrip lines on signals.Compared with the general coupled microstrip lines, the three crosstalk reduction methods can suppress the far-end crosstalk.By contrast, the crosstalk reduction method using CSR structure proposed in this   Figure 13 illustrates the simulation and measurement findings for insertion loss (S 21 ) in the four models described above.Upon comparison, all three crosstalk reduction methods offered improvements to insertion loss.However, the RSR structure limited the effect on reducing crosstalk, with reduced benefits especially at higher frequencies.The application of the CSR structure for crosstalk reduction has a positive impact on signal quality transmitted by

2
Model of the crosstalk reduction method using the CSR structure.

3 5
Equivalent circuit of the coupled microstrip lines in two cases.(A) Coupled microstrip lines equivalent circuit model (B) Coupled microstrip lines equivalent circuit model with a CSR structure.F I G U R E 4Model of the crosstalk reduction method using the CSR structure.CSR structure and parameters.

6
ADS simulation and HFSS simulation results.

F I G U R E 8
The influence of three different distributions on the coupling between microstrip lines.

9
Four simulation analysis models (A) General coupled microstrip line (B) method using the CSR structure (C) method using the 3W rule (D) method using the RSR structure.F I G U R E 10Sample object: (A) S 21 test object and (B) S 41 test object.Coupled microstrip lines 3W rule Coupled microstrip lines + RSR structure Coupled microstrip lines + CSR structure The measurement result of the sample F I G U R E 11 S 11 simulation and test results.
Coupled microstrip lines 3W rule Coupled microstrip lines + RSR structure Coupled microstrip lines + CSR structure The measurement result of the sample F I G U R E 12 S 41 simulation and test results.

TA B L E 2
Coupled microstrip lines 3W rule Coupled microstrip lines + RSR structure Coupled microstrip lines + CSR structure The measurement result of the sample F I G U R E 13 S 21 simulation and test results.Reduction effect of far-end crosstalk.Different methods Far-end crosstalk reduction effect 3W crosstalk reduction method Maximum of approximately 11 dB RSR crosstalk reduction method Maximum of approximately 18 dB CSR crosstalk reduction method proposed in this article Maximum of approximately 43 dB article has more advantages in suppressing far-end crosstalk, and it can reduce the far-end crosstalk by approximately 15 dB, with a maximum reduction of 43 dB.The test results are basically consistent with the simulation results, and the observed differences are primarily due to process accuracy, dielectric constant and and other factors.