Advances in semiconductor technology have led to an increasing scale of chip integration, and the clock frequency of the chip is getting higher and higher, resulting in shorter signal rise/fall times. When the clock frequency exceeds 50 MHz, the signal trace of the PCB must be considered by the transmission line.
1 Signal Integrity OverviewSignal integrity is the ability of a signal to respond in the circuit with the correct timing and voltage. Conversely, if the signal does not respond at the correct timing and voltage level in the circuit, it means that signal integrity issues have arisen. Reflection and crosstalk are the more common factors leading to signal integrity problems [1].
Reflection is the echo on the transmission line. If the impedance on the transmission line is not continuous, it will cause reflection of the signal. The size of the reflected signal component is mainly determined by the reflection coefficient [2]. The reflection coefficient is calculated as equation (1):
Among them, Z0 is the characteristic impedance of the transmission line, and Zt is the impedance that causes discontinuity.
The characteristic impedance Z0 of the transmission line is defined as the ratio of voltage to current at any point on the transmission line. In PCB design, the transmission line mainly considers both microstrip lines and strip lines. Therefore, when calculating the characteristic impedance, it should be approximated according to the corresponding transmission line type [3]. The characteristic impedance calculation formula of the microstrip line is as shown in equation (2):
Where W (mm) is the conductor width, T (mm) is the conductor thickness, H (mm) is the dielectric thickness, and εr is the dielectric constant of the board material.
Crosstalk refers to the undesired noise voltage interference signal generated on adjacent signal lines due to the mutual coupling of electromagnetic fields when the signals propagate on the transmission line, that is, the energy coupling between different transmission lines. As shown in Figure 1.
Crosstalk is the result of the combined action of mutual capacitance Cm and mutual inductance Lm. It is generally defined that the crosstalk of the interfered transmission line near the end of the driver is near-end crosstalk (also called backward crosstalk), and the crosstalk of the interfered transmission line away from the end of the driver is far-end crosstalk (also called forward crosstalk).
Mutual inductance, that is, inductive coupling, is electromagnetic interference caused by a magnetic field generated by a current change on a driven transmission line causing an induced voltage on a transmission line that is not driven. The magnitude of the mutual inductance Lm can be calculated by equation (4):
2.1 Reflex measures
Reflections on the transmission line can have a serious negative impact on the performance of digital systems. Therefore, effective measures must be taken to suppress the reflection. According to the reason of reflection, there are essentially three ways to reduce the influence of reflection: (1) reduce the system frequency; (2) shorten the PCB trace; (3) terminate one end of the transmission line and match the characteristic impedance of the transmission line. Impedance to eliminate reflections. In contrast, only the third method is more reasonable.
There are two main strategies for impedance matching: (1) matching at the load end, that is, parallel termination matching; (2) matching at the source end, that is, serial termination matching. From a system design perspective, strategy (1) should be preferred because it eliminates reflections before signal energy returns to the source, eliminating one reflection, reducing noise, electromagnetic interference (EMI), and radio frequency interference (RFI). Strategy (2) is relatively simple to implement and widely used in practice [7].
2.2 Crosstalk resolution measures
Crosstalk is the result of a combination of factors. It is impossible to completely eliminate crosstalk in the PCB design, and only effective measures can be taken to minimize it, as long as the crosstalk is suppressed within the allowable range of noise. By analyzing the causes of crosstalk, the following measures can be taken to suppress crosstalk during PCB design: in the case of sufficient space, the distance between the wires can be increased as much as possible; and the parallel length of the wires between adjacent networks should be minimized; Wiring between the two layers should be done with vertical wiring to reduce crosstalk between adjacent layers; ground lines can be inserted between the two lines, or critical signal lines can be shielded with grounding wires [8, 9].
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