Design of T-shaped non-uniform microstrip lines for reducing crosstalk in high-speed PCBs

High-speed and high-density PCB circuit layout design faces serious signal integrity problems. Crosstalk is one of the key problems affecting signal quality. Periodic non-uniform differential microstrip lines have the possibility of reducing crosstalk. This paper proposes a T-shaped non-uniform microstrip line, which adds the T-shaped bump on the inner side of the parallel microstrip lines. The microstrip line adopts the minimum batch production process linewidth to maintain high density, which is suitable for occasions where the line spacing is three times the linewidth. The principle of crosstalk mitigation for T-shaped microstrip lines is explained, and the evolution process of T-shaped microstrip lines is given, compared with parallel microstrip lines and tabbed lines with the same density. The simulation resulting in the time domain and frequency domain shows that the transmission performance of the T-shaped microstrip line is good, and crosstalk can be effectively suppressed in a high-frequency band, even better in some specific frequencies due to the crosstalk fluctuation with frequency.


Introduction
A printed circuit board (PCB) supports electronic components and the carrier for electrical interconnection.With the development of the fifth generation communication technology and the progress of integrated circuit technology, the working frequency of electronic equipment has gradually increased from MHz to GHz, and PCB design has to face high-frequency and high-speed issues [1].On the one hand, the clock frequency of the system continues to increase and the working voltage continues to decrease [2]; on the other hand, a large number of components are integrated into the circuit board, and the layout density of the PCB is increasing, the edge of the signal is becoming steeper and steeper, and the electromagnetic interference (EMI) between transmission lines will become more and more serious [3].
The transmission signal is very vulnerable to interference, resulting in distortion and integrity problems.Crosstalk has become one of the main limiting factors to improving the data transmission rate [4].In a highly compact PCB, when the parallel microstrip lines are getting closer and closer, farend crosstalk (FEXT) will seriously reduce the received signal [5].Currently, many non-uniform transmission lines (NUTL) exist in the actual high-speed circuit system [6].In fact, due to the possibility of reducing crosstalk, the introduction of periodic non-uniform differential microstrip lines has attracted more and more attention in recent years.To analyze the crosstalk mitigation mechanism of non-uniform transmission lines and fully utilize this characteristic, researchers have explored various methods.paraffin material to the signal line with labels to suppress FEXT [10] and increased the cost of batch production.Ma et al. proposed a kind of guard trace based on a subwavelength periodic structure to reduce the FEXT of parallel microstrip lines [11] and limit the high-density layout.In 2015, Intel Corporation proposed a non-uniform transmission line that can reduce crosstalk and meet the requirements of high-speed and high-density integration.As shown in Figure 1, interdigital trapezoidal lugs are introduced between two closely coupled microstrip lines.The non-uniform transmission line can realize crosstalk mitigation and impedance management to improve the performance of dual data rate (DDR) channels [12].This paper focuses on the layout design of transmission lines in high-speed and high-density PCBs.By reference to Intel's commercial tabbed non-uniform transmission line design, the pattern of microstrip line for remote crosstalk suppression is studied, and a new line type, a T-shaped microstrip line, is proposed.The principle for reducing remote crosstalk is expounded from the perspective of increasing mutual capacitance.The parallel, tabbed, and T-shaped microstrip lines with the same density are compared regarding scattering parameters, remote crosstalk peak voltage, and eye diagram simulation.

Principle of crosstalk reduction
Crosstalk is a kind of interference noise generated by the mutual coupling of electromagnetic fields between microstrip lines.It is one of the basic problems of signal integrity.Taking two parallel microstrip lines as an example, crosstalk is illustrated in Figure 2. The microstrip line that sends the crosstalk signal is the attack line, and the microstrip line affected by the crosstalk signal is the victim line.Once the noise current is transmitted from the attack line to the victim line, it will transmit along the victim line and cause near-end and far-end crosstalk.
Step excitation The FEXT is the ratio of the far-end crosstalk peak voltage Vf to the signal voltage Va on the attack line.In addition to the intrinsic parameters of the coupled transmission line, FEXT is proportional to the coupling length and rise time.The relationship is as follows:

Capacitive coupling current
where CL is the capacitance per unit length (in pF/mm), CmL is the mutual capacitance per unit length (in pF/mm), LL is the inductance per unit length (in nH/mm), LmL is the mutual inductance per unit length (in nH/mm), l is the length of the coupling region between microstrip lines, tr is the signal rise time, and v is the signal propagation speed on the line.Since the dielectric constant of the surrounding air is less than that of the dielectric material below the microstrip line, the relationship between the capacitance ratio and the inductance ratio of the microstrip line is as follows: According to Equation ( 1), minimizing the difference between CmL/CL and LmL/LL can minimize the far-end crosstalk, which is the reason why specially designed non-uniform transmission lines may reduce crosstalk by increasing mutual capacitance between microstrip lines.First, only a rectangular bulge is added inside the parallel microstrip line, as shown in Figure 5.The basic parameters are l=30 mm, n=28, D=1.016 mm, a=1w~4w, h=1w.The results of S 41 , which characterizes the far-end crosstalk in the scattering parameters of HFSS simulation, are shown in Figure 6.Even if the value of a is increased, the S 41 curve is almost coincident; that is, the crosstalk has not improved or even slightly deteriorated.The reason is that the self-capacity of such microstrip lines increases slightly.Yet, the mutual capacitance between microstrip lines has not changed due to no relative area between the bumps.

Evolution and simulation of T-shaped microstrip lines
Then increase the bulge height, the resulting microstrip line called stub-alternated NUTL 13 [13] is shown in Figure 7.According to [13], when h=2w, the far-end crosstalk suppression effect is the best.This line type is the case of the T-shaped microstrip line proposed in this paper when b=a.The HFSS simulation results are shown in Figure 8.The S 41 is reduced with an average reduction of 5 dB in the 3-16 GHz band.We further modify the above structure, stretching half of the height h transversely to form the Tshaped microstrip line with short side a and long side b, as shown in Figure 4.The parameter scanning results of S 41 , which varies with b at a=0.1 mm and varies with a at b=0.4 mm, are shown in Figure 9 and Figure 10, respectively.Through the simulation results, it can be seen that after the T-shaped structure is formed by stretching, the far-end crosstalk decreases significantly, and S 41 fluctuates with frequency obviously, which is different from the uniform microstrip lines.It can be concluded that Tshaped short side a cannot increase the relative area between T-shaped bulges, but the increase of a slightly increases the self-capacity of a single microstrip line.The larger the T-shaped long side b, the greater the mutual capacitance between microstrip lines.According to Equation ( 1), the purpose of suppressing far-end crosstalk can be achieved.The characteristics of FEXT fluctuation with frequency make it possible to design T-shaped microstrip lines for extremely low FEXT at a specific frequency.Finally, the structure parameters of a T-shaped microstrip line with a length of 30 mm are determined as a=0.1 mm, b=0. 3

Comparison of different types of microstrip lines
The T-shaped microstrip line, the uniform microstrip line, and the tabbed line with the same PCB parameters are compared.The three types of microstrip lines are set as the same line length of l =30 mm, line width of w=0.1 mm, and line spacing of 3w.The three microstrip line structures are shown in Figure 11.The long side of the tab is 0.12 mm, the short side is 0.11 mm, the height is 0. Three microstrip line models are established in HFSS for frequency domain simulation.The results of S 41 are compared in Figure 12.The S 41 of parallel microstrip lines is the largest and monotonically increases with frequency.When the frequency is higher than 1.36 GHz, the S 41 of the T-shaped microstrip line is much smaller than that of the parallel microstrip line and the tabbed line.At the frequency of 3.2 GHz, the S 41 of the parallel microstrip line is -23.6 dB, the S 41 of the tabbed line is -31.8576dB, and the S 41 of the T-shaped microstrip line is -53.69 dB, which is 56% lower than that of the parallel line and 40.7% lower than that of the tabbed line, indicating that the T-shaped microstrip line has an excellent effect on suppressing the far end crosstalk at the same PCB density.The HFSS models of three Microstrip are imported into ADS for time-domain simulation.
Step excitation is added, the voltage is set to 1 V, and the signal rise time is 0.1075 ns.The far-end crosstalk voltages of three kinds of microstrip lines are as shown in Figure 13.The far-end crosstalk peak voltage V f of the parallel microstrip line is 17 mV, that of the tabbed line is 7.5 mV, and that of the T-shaped microstrip line is 3.4 mV, which is 80% lower than that of the parallel microstrip line and 54.7% lower than that of the tabbed It also shows that the T-shaped microstrip line can effectively suppress crosstalk when PCB density is the same.

Conclusion
According to the principle of increasing mutual capacitance and suppressing crosstalk, a T-shaped microstrip line structure is proposed by simulating the gradual evolution of the structural parameters.The microstrip line adopts the minimum process linewidth to maintain high density, which is suitable for occasions where the line spacing is three times the linewidth.Due to the periodic special T-shaped structure (b>a), the far-end crosstalk fluctuates with frequency, and the far-end crosstalk suppression effect is better in some specific high-frequency bands.T-shaped, tabbed, and parallel uniform microstrip lines in the same density are compared by simulation in the time domain and frequency domain.S 41 of the T-shaped microstrip line is -53.69 dB, which is 56% lower than that of the parallel microstrip line and 40.7% lower than that of the tabbed line at 3.2 GHz; The far-end crosstalk voltage of the T-shaped microstrip line is 3.4 mV, which is 80% lower than that of the parallel microstrip line and 54.7% lower than that of the tabbed line; The eye diagrams of the three lines are similar and excellent.In general, the overall effect of a T-shaped microstrip line is good, providing a wiring type for reducing crosstalk and improving wiring density.

Figure 9 .
Figure 9. S 41 dependence on b at a=0.1 mm.

Figure 10 .
Figure 10.S 41 dependence on a at b=0.4 mm.

Figure 11 .
Figure 11.Schematic diagram of three transmission line structures.

Figure 12
Figure 12. S 41 comparison of three structures of transmission lines.

Table 1 .
Compared with the parallel microstrip line, the eye height of the T-shaped microstrip line is increased by 2 mV, and the eye width is reduced by 0.8 ps.Compared with the tabbed line, the eye height is reduced by 3 mV, and the eye width is unchanged, indicating that the T-shaped microstrip line has suitable eye pattern parameters and signal transmission performance.

Table 1 .
Eye diagram parameter comparison.