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Optimize PCB routing to minimize crosstalk

Optimize PCB routing to minimize crosstalk


Today, a variety of portable computing devices are designed with dense printed circuit board (PCB) designs and use multiple high-speed digital communication protocols such as PCIe, USB and SATA, which support data throughput rates up to Gb and have The difference between the hundreds of millivolts. Designers must carefully plan PCB high-speed serial signal traces to minimize line-to-line crosstalk and prevent channel transmission from damaging data.

 


An aggressive signal and a victim signal can cause crosstalk when it is coupled to an energy or magnetic field. The electric field is coupled by mutual capacitance between the signals, and the magnetic field is coupled by mutual inductance.

Equation (1) and (2) are the induced voltage and current calculation formulas of the intrusion signal to the victim signal respectively. Equations (3) and (4) are the mutual capacitance and mutual inductance calculation formulas between the intrusion signal and the victim signal respectively.

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Chinese and English

The induced voltage of the victim signal

Mutual inductance between victim and aggressor: mutual inductance between the victim signal and the intrusion signal

Transient edge rate of current due to aggressor: Transient current edge rate affected by intrusion signal

 


The induced current of a victim signal

Mutual capacitance between victim and aggressor: mutual capacitance between intrusion signal and intrusion signal

Dielectric permittivity: dielectric constant

Overlapped conductive area between victim and aggressor: the overlapping conductive area between the victim signal and the intrusion signal

Distance between victim and aggressor: the distance between the victim signal and the intrusion signal

Transient edge rate of voltage due to aggressor: Transient voltage edge rate affected by intrusion signal

As shown in equations (1), (2), (3) and (4), the inductance and capacitive coupling between the victim signal and the intrusion signal decreases as the distance increases. However, due to the need to meet the compact design requirements of portable computing devices, PCB size is limited, to increase the gap between the line is very difficult.

Microstrip Line Crossover Wiring and Stripe Line Transfers and Crossover Wiring Methods can mitigate crosstalk or coupling problems.

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Figure 1: transmitted pair: received pair:

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Figure 2 passive pair: transmitted pair:

Cross mode is applied when far-end crosstalk (FEXT) is much larger than near-end crosstalk (NEXT). On the contrary, when the near-end crosstalk is far greater than the far-end crosstalk for non-cross-wiring. The near-end crosstalk represents the crosstalk caused by the victim network adjacent to the intrusion signal transmitter. The far-end crosstalk represents the crosstalk caused by the victim's adjacent intrusion signal receiver. By analyzing the S-parameter and transient response of the two closely coupled signals of the intrusion signal and the victim signal, we can compare the far-end crosstalk and near-end crosstalk between the microstrip line and the stripline.

II. Simulation

Figure 3 and Figure 4 are ADS S parameters and transient analysis simulation model. In Figure 3, the 100Ω differential impedance and the 3-inch-long victim signal and the single-mode S-parameter of the incoming network signal line are mathematically transformed into the differential mode. Port 1 and Port 2 represent the input and output ports of the incoming signal pair, and Port 3 and Port 4 respectively represent the input and output ports of the victim network pair. The gap between the intrusion signal and the victim signal is set to 8 mil (1 times the wiring width).

Figure 4, the middle of the transmission line that the victim network signal pairs, both ends of the transmission line termination resistance. In the victim network signal on the top and bottom of the transmission line were injected with 30ps edge rate of the square wave, as an intrusion signal.

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Figure 3: Connected model: coupled pairs:

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Figure 4: Transient analysis Coupled pairs:

The differential S parameter Sdd31 represents the near-end crosstalk, and Sdd41 represents the far-end crosstalk. Sdd31 is defined as the gain ratio of the induced voltage of port 3 (victim network signal input) relative to port 1 (intrusion network signal input), and Sdd41 is defined as port 4 (victim network signal output). The induced voltage is proportional to port 1 (Intrusion network signal input) The gain ratio of the incident voltage.

Figure 5 and Figure 6 are the simulated S-parameters of the coupled microstrip lines and stripline pairs. Figure 6 shows that Sdd41 is below Sdd41, indicating that the Sdd41 or far crosstalk gain is better than Sdd31 or near-end crosstalk for wiring using microstrip lines; Figure 6 shows that the Sdd31 gain using the stripline is higher than Sdd41.

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Figure 5: Simulation of microstrip lines Sdd31 and Sdd41 (FEXT: far-end crosstalk; NEXT: near-end crosstalk)

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Figure 6: Simulation strip lines Sdd31 and Sdd41 (FEXT: far-end crosstalk; NEXT: near-end crosstalk)

Figures 7 and 8 show the simulation of the far-end crosstalk and near-end crosstalk time-domain transient responses of coupled microstrip lines and stripline pairs, respectively. As shown in Figure 7, when the intrusion line signal transient rise or fall, the peak of the far-end induced voltage (0.3V) of the victim line of the microstrip line is much larger than the near peak (0.05V); Figure 8 stripline simulation Shows that the peak value of the far-end induced voltage of the victim signal line is comparable to that of the proximal end (0.05V). The false triggering or inductive peak of the victim signal increases the receiver IC (noise) margin margin and increases the bit error rate (BER).

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Figure 7: Microstrip line far-end crosstalk and near-end crosstalk time-domain response simulation (Waveform: waveform; Aggressor: intrusion signal)

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Figure 8: Far-band crosstalk and near-end crosstalk time-domain response simulation (Waveform: waveform; Aggressor: intrusion signal)

In order to minimize the crosstalk between tightly coupled pairs, microstrip lines are used to send and receive crossover wiring and stripline applications to send and receive non-intersecting cabling is a better choice.

III. Prototype PCB measurement

In order to verify the correlation between the simulation results and the actual measurements, we need to make the prototype PCB. Figures 9 and 10 show the S-parameter measurements of the coupled microstrip lines and striplines. As shown in Figure 9, the near-end crosstalk is lower than far-end crosstalk; in Figure 10, the far-end crosstalk is below the near-end crosstalk.

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Figure 9: S-parameter measurement of microstrip lines

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Figure 10: S-parameter measurement of stripline

Figure 11 and Figure 12 show the results of the far-end crosstalk and near-end crosstalk time-domain transient response measurements of coupled microstrip lines and stripline pairs. In Figure 11, the peak of the far-end induced voltage (0.3V) of the victim line is much larger than the near-peak (0.1V) when the signal of the intrusion line rises or falls. In Figure 12, the peak of the far-end induced voltage of the victim line is Proximal peak equivalent (0.1V).

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Figure 11: Microstrip line far-end crosstalk and near-end crosstalk time-domain response measurements (nsec: nanosecond)

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Figure 12: Far-band crosstalk and near-end crosstalk time-domain response measurements (nsec: nanoseconds)

IV. Summary

This article describes how to optimize signal routing to significantly reduce crosstalk. S parameter and time domain transient response analysis results show that: using microstrip line transceiver crossover wiring and stripline non-cross-wiring scheme can minimize crosstalk. To achieve very high data rates, the PCB design must optimize the signal wiring to ensure superior signal quality.
 

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