PCB Technical - Overview of signal integrity high-speed digital PCB issues

This article introduces a design method of high-speed digital signal PCB board based on signal integrity computer analysis. In this design method, the PCB board-level signal transmission model is first established for all high-speed digital signals, and then the solution space of the design is found through the calculation and analysis of signal integrity, and finally the PCB is completed on the basis of the solution space. Board design and verification.

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 in the design of high-speed digital PCNs. Factors such as the parameters of the components and the PCB board, the layout of the components on the PCB board, and the wiring of high-speed signals will cause signal integrity problems, resulting in unstable system operation or even no operation at all.

How to fully consider the signal integrity factors in the PCB design process and take effective control measures has become a hot topic in the PCB design industry today. The high-speed digital PCB board design method based on signal integrity computer analysis can effectively realize the signal integrity of PCB design.

1. Overview of signal integrity issues

Signal integrity (SI) refers to the ability of a signal to respond with the correct timing and voltage in the circuit. If the signal in the circuit can reach the IC with the required timing, duration and voltage amplitude, the circuit has better signal integrity. Conversely, when the signal cannot respond normally, a signal integrity problem occurs. Broadly speaking, signal integrity problems are mainly manifested in five aspects: delay, reflection, crosstalk, synchronous switching noise (SSN) and electromagnetic compatibility (EMI).

Delay means that the signal is transmitted at a limited speed on the wires of the PCB board, and the signal is sent from the sending end to the receiving end, during which there is a transmission delay. The delay of the signal will affect the timing of the system. In a high-speed digital system, the transmission delay mainly depends on the length of the wire and the dielectric constant of the medium around the wire.

In addition, when the characteristic impedance of the wires on the PCB circuit board (called transmission lines in high-speed digital systems) does not match the load impedance, a part of the energy will be reflected back along the transmission line after the signal reaches the receiving end, causing the signal waveform to be distorted or even appear Signal overshoot and undershoot. If the signal is reflected back and forth on the transmission line, it will produce ringing and ring oscillation.

Since there is mutual capacitance and mutual inductance between any two devices or wires on the PCB, when a device or signal on a wire changes, its change will affect other devices or inductance through mutual capacitance and mutual inductance. Wire, that is, crosstalk. The strength of crosstalk depends on the geometrical size and mutual distance of the devices and wires.

When many digital signals on the PCB board are switched synchronously (such as CPU data bus, address bus, etc.), due to the impedance of the power line and the ground line, synchronous switching noise will be generated, and ground plane bounce will occur on the ground line. Noise (referred to as ground bomb). The strength of SSN and ground bounce also depends on the IO characteristics of the integrated circuit, the impedance of the power supply layer and ground plane layer of the PCB board, and the layout and wiring of high-speed devices on the PCB board.

In addition, like other electronic devices, PCBs also have electromagnetic compatibility problems, which are mainly related to PCB layout and wiring methods.