1 Overview
In high-speed digital circuit PCB design, when the wiring length is greater than one-twentieth of the wavelength or the signal delay exceeds one-sixth of the rising edge of the signal, the PCB wiring can be regarded as a transmission line. There are two types of transmission lines: microstrip and stripline. Transmission line characteristics relevant to EMC design include: characteristic impedance, propagation delay, intrinsic capacitance, and intrinsic inductance. Reflection and crosstalk will affect signal quality, and from the perspective of EMC, they are also the main sources of EMI.
2. Transmission line model
This part is introduced in detail in the SI tutorial, here is a brief introduction.
https://mxioum.blog.csdn.net/article/details/123969250
3. Types of transmission lines
3.1 Microstrip (microstrip)
Definition: Surface routing adjacent to a reference plane.
For our commonly used FR-4 dielectric material, er=4.5. Transmission delay TPD=142.2 (ps/inch)
Stripline
Definition: PCB routing between two reference planes
For our commonly used FR-4 dielectric material, er=4.5. Transmission delay TPD=180.3 (ps/inch)
3.3 Embedded Microstrip Line
K is related to the embedding thickness, and decreases as the embedding thickness increases. When it is less than 15mil, K65, and when it is 20mils, K=60.
4. Reflection of the transmission line
Any inhomogeneity in the transmission process (such as impedance changes, right-angled corners) will cause signal reflection. The result of the reflection is a standing wave for analog signals (sine waves), and ringing for digital signals as rising and falling edges. and overshoot. On the one hand, this overshoot forms strong electromagnetic interference, and on the other hand, it causes damage or even failure to the protection diode of the input circuit of the subsequent stage.
Generally speaking, measures should be taken if the overshoot exceeds 0.7V. In the figure below, the signal source impedance, load impedance is what causes the signal to reflect back and forth.
The following is a comparison of matching circuits and no matching circuits:
Overshoot, undershoot, and ringing are caused on the rising and falling edges of the signal due to reflections. These overshoots and ringing not only affect the integrity of the signal, but are also the main source of EMI emissions.
5. Crosstalk
The circuit shown in the figure below, due to the parasitic capacitance Csv between adjacent PCB wiring, high-frequency signals will cause mutual interference through Csv, when there is a pulse signal passing through one path, there will be a gap between the rising and falling edges of the pulse on the other path. Interfering pulses appear, which is crosstalk between PCB traces. On the one hand, crosstalk affects signal quality, and crosstalk pulses are also the main emission source of EMI. The figure below illustrates the factors related to crosstalk between PCB traces.
That is, the factors that affect the crosstalk between transmission lines are: coupling length L, input and output impedances of the source end and load end, dielectric constant, width W of the transmission line, thickness T, and height H of the reference plane (another angle: distributed capacitance CSV, parasitic Capacitance Cti, coupled inductance L)
The following table lists the crosstalk under different conditions:
