PCB Blog - 10 important knowledge sharing related to high-speed PCB board design

In the study of high-speed PCB board design, there are many knowledge points that need to be understood and mastered, such as common signal integrity, reflection, crosstalk, power supply noise, filtering, etc. This article will share 10 important knowledge related to high-speed PCB board design with you, hoping to help you in your study.

1. Signal integrity
Signal integrity refers to the quality of the signal on the transmission path. The transmission path can be an ordinary metal wire, an optical device, or other media.
In short distance, low bit rate situations, a simple conductor can faithfully transmit the signal. On the other hand, if a long-distance, high-bit-rate signal passes through several different conductors, various effects can reduce the reliability of the signal, so that the system or device does not work properly. With the increase in the output switching speed of integrated circuits and the increase in PCB density, signal integrity has become one of the issues that must be concerned in the design of high-speed digital PCB boards. Factors such as the parameters of components and PCB boards, the layout of components on the PCB board, and the wiring of high-speed signals can cause signal integrity problems, resulting in unstable system operation, or even no work at all. Signal integrity issues to consider mainly include ringing, crosstalk, ground bounce, skew, signal loss, and noise in the power supply.

2. Reflection
A reflection is an echo on the transmission line. A portion of the signal power (voltage and current) is transmitted to the line and reaches the load, but a portion is reflected. If the source and load have the same impedance, reflections will not occur. The impedance mismatch between the source and the load will cause reflections on the line, and the load will reflect a portion of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative, and if the load impedance is greater than the source impedance, the reflected voltage is positive. Variations in trace geometry, incorrect wire termination, transmission through connectors, and discontinuities in power planes can all cause such reflections.

3. Crosstalk
Crosstalk is the coupling between two signal lines, and the mutual inductance and mutual capacitance between the signal lines cause noise on the line. Capacitive coupling induces coupling current, while inductive coupling induces coupling voltage. The parameters of the PCB board, the signal line spacing, the electrical characteristics of the driving end and the receiving end, and the line termination method have a certain influence on the crosstalk.

4. Characteristic impedance
Let’s clarify a few concepts first. We often see impedance, characteristic impedance, and instantaneous impedance. Strictly speaking, they are different, but they are still the same. They are still the basic definition of impedance: the input at the beginning of the transmission line. Impedance is referred to as impedance for short; the instantaneous impedance that the signal encounters at any time is called instantaneous impedance; if the transmission line has a constant instantaneous impedance, it is called the characteristic impedance of the transmission line. Characteristic impedance describes the transient impedance that a signal experiences as it propagates along a transmission line, which is a major factor affecting signal integrity in transmission line circuits. Unless otherwise specified, the characteristic impedance is generally referred to as the transmission line impedance.
PS: For high-speed PCB board design, our goal is to keep the impedance as stable as possible during the transmission process, and this must keep the characteristic impedance of the transmission line stable.

5. Power Integrity
Power integrity (PI) for short is to confirm whether the voltage and current of the power supply and the destination meet the requirements. Power integrity is very important in today's electronic products. There are several levels of power integrity: the chip level, the chip package level, the board level, and the system level. Among them, the power integrity at the board level must meet the following three requirements: make the voltage ripple of the chip pins smaller than the specification (for example, the error between the voltage and 1V is less than +/-50 mV), and control the ground bounce (Also known as Synchronous Switching Noise SSN, Synchronous Switching Output SSO) Reduces Electromagnetic Interference (EMI) and maintains Electromagnetic Compatibility (EMC): Power Distribution Networks (PDNs) are conductors on a circuit board and are therefore prone to transmit and receive noise 's antenna.

6. Power supply noise
Power supply noise is a kind of electromagnetic interference, and the frequency spectrum of its conducted noise is roughly 10kHz~30MHz, and can reach 150MHz. Power supply noise, especially transient noise interference, has fast rising speed, short duration, high voltage amplitude and strong randomness, which is easy to cause serious interference to microcomputers and digital circuits. In high-frequency circuits, the noise of the power supply has a particularly significant impact on high-frequency signals. Therefore, firstly, the power supply is required to be low-noise. Here, clean ground is as important as clean power.

7. Filtering
Wave filtering is the operation of filtering out specific band frequencies in the signal, and it is an important measure to suppress and prevent interference. The filtering is divided into classic filtering and modern filtering.

8. Parallel bus
A bus is a shared physical path for two or more devices to communicate, a collection of signal lines, a common connection between multiple components, and is used to transmit information between components. According to different working modes, the bus can be divided into two types: one is a parallel bus, the other is a serial bus. Parallel bus: can transmit multiple bits of data at the same time, like a wide road that allows multiple cars to drive side by side, and it also has two-way and one-way points.

9. Serial bus
Only one piece of data can be transmitted at the same time, like a narrow road that only allows one car to travel. The data must be transmitted one by one, which looks like a long data string, so it is called "serial".

10. Topology
It refers to the form in which each site in the network is connected to each other, and the topology in the PCB board design refers to the connection relationship between chips. Topologies commonly used in PCB board include point-to-point, daisy-chain, remote cluster, star, etc.