In the study of high-speed PCB design, there are many knowledge points that everyone needs to understand and master, such as common signal integrity, reflection, crosstalk, power supply noise, filtering, etc.
1. Signal integrity
Signal integrity (SI) refers to the quality of signals on the transmission path. The transmission path can be ordinary metal wires, optical devices, or other media.
Over short distances and low bit rates, a simple conductor can faithfully transmit the signal.
If long-distance, high-bit-rate signals pass through several different conductors, various effects can reduce the reliability of the signal, so that the system or equipment cannot work properly.
As the output switching speed of integrated circuits increases and the density of PCB boards increases, signal integrity has become one of the issues that must be concerned about in high-speed digital PCB design.
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 all cause signal integrity problems, causing the system to work unstable or even not work at all.
Signal integrity issues that need to be considered include ringing, crosstalk, ground bounce, skew, signal loss and noise in the power supply.
2. Reflection
Reflection is an echo on a transmission line. Part of the signal power (voltage and current) travels down the line and reaches the load, but part of it 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 part of the voltage back to the source.
If the load impedance is less than the source impedance, the reflected voltage is negative; conversely, if the load impedance is greater than the source impedance, the reflected voltage is positive.
Variations in cabling geometry, incorrect wire termination, transmission through connectors, and power plane discontinuities can all cause such reflections.
3. Crosstalk
Crosstalk is the coupling between two signal lines. 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. PCB board layer parameters, signal line spacing, power characteristics of the driving end and receiving end, and line termination pipes all have a certain impact on crosstalk.
4. Characteristic impedance
Let’s first clarify a few concepts. We often see impedance, characteristic impedance, and transient impedance. Strictly speaking, they are different, but they remain the same. They are still the basic definitions of impedance:
4.1 The input impedance at the beginning of the transmission line is referred to as impedance;
4.2 The real-time impedance that a signal encounters at any time is called transient impedance;
4.3 If the transmission line has a constant transient impedance, it is called the characteristic impedance of the transmission line.
4.4 Characteristic impedance describes the transient impedance encountered when a signal propagates along a transmission line. This is a major factor affecting signal integrity in transmission line circuits.
4.5 Unless otherwise specified, characteristic impedance is generally used to collectively refer to transmission line impedance.
PS: For high-speed PCB design, our goal is to keep the impedance of the signal as stable as possible during the transmission process, and this must maintain the stability of the characteristic impedance of the transmission line.
5. Power integrity
Power integrity, referred to as PI, is to confirm whether the voltage and current at the source and destination of the power supply meet the requirements.
Power integrity is very important in today's electronic products. There are several levels of power integrity: chip level, chip package level, circuit board level, and system level.
Among them, power integrity at the circuit board level must meet the following three requirements:
Make the voltage ripple at the chip pin smaller than the specification (for example, the error between the voltage and 1V is less than +/-50 mV)
Controlling ground bounce (also known as synchronous switching noise SSN, synchronous switching output SSO)
Reduce electromagnetic interference (EMI) and maintain electromagnetic compatibility (EMC): The power distribution network (PDN) is a conductor on the circuit board, which is also an antenna that easily emits and receives noise.
6. Power supply noise
Power supply noise is a type of electromagnetic interference, and the spectrum of its conducted noise is roughly 10kHz~30MHz, up to 150MHz.
Power supply noise, especially transient noise interference, has fast rising speed, short duration, high voltage amplitude and strong randomness, which can cause serious interference to microcomputers and digital circuits.
In high-frequency circuits, the noise contained in the power supply has a particularly obvious impact on high-frequency signals. For this reason, the power supply is first required to be low-noise. Here, a clean ground and clean power supply are equally important.
7. Filtering
Wave filtering is the operation of filtering out specific frequency bands in signals. It is an important measure to suppress and prevent interference. Filtering is divided into classic filtering and modern filtering.
8. Parallel bus
A bus is a common physical path for communication between two or more devices. It is a collection of signal lines and a common connection between multiple components. It is used to transmit information between various components.
Depending on the working mode, the bus can be divided into two types: one is the parallel bus and the other is the serial bus.
Parallel bus: It can transmit multiple bits of data at the same time, just like a spacious road that allows multiple cars to drive side by side, and it is also bidirectional and unidirectional.
9. Serial bus
Serial bus: Only one piece of data can be transmitted at the same time, just like a narrow road that only allows one car to walk. The data must be transmitted one after another, and it looks like a long data string, so it is called "serial".
10. Topology
Topology refers to the way in which various sites in the network are connected to each other. Topology in PCB design refers to the connection relationship between chips.
Commonly used topologies include point-to-point, daisy chain, remote cluster, star, etc.
The above shares with you 10 important matters related to high-speed PCB design. I hope it will be helpful to your learning.