What is PCB signal integrity

Signal integrity refers to the quality of the signal on the signal line, that is, the ability of the signal to respond in the circuit with the correct timing and voltage. If the signal in the circuit can reach the receiver in the required timing, duration, and voltage amplitude, the circuit can be determined to have good signal integrity. Conversely, when the signal does not respond properly, there is a signal integrity problem.

With the increasing use of high-speed devices and the design of high-speed digital systems, the system data rate, clock rate and circuit density are constantly increasing. In this design, the system has fast slope transients and high operating frequencies, and cables, interconnects, printed boards (PCB), and silicon wafers will behave very differently than in low speed designs, presenting signal integrity issues.

Signal integrity problems can cause or directly cause such as signal distortion, timing error, incorrect data, address, control line and system error, and even make the system crash, which has become a very noteworthy problem in high-speed product design. This paper first introduces the problem of PCB signal integrity, then expounds the steps of PCB signal integrity, and finally introduces how to ensure the integrity of PCB design signal.

PCB signal integrity problem

The signal integrity problems of PCB mainly include signal reflection, crosstalk, signal delay and timing error.

1. Reflection

When the signal is transmitted on the transmission line, when the characteristic impedance of the transmission line on the high-speed PCB does not match the impedance of the signal source or the impedance of the load, the signal will be reflected, causing the signal waveform to overflush, underflush and the resulting ringing phenomenon. Overs hoot is the first peak (or valley) of a signal jump, which is an extra voltage effect over the power level or below the reference ground level;

Unders hoot is the next valley (or peak) at which the signal jumps. Too large overshoot voltage often causes long-term impact of the device damage, downshoot will reduce the noise tolerance, ringing increases the time required for signal stability, thus affecting the system timing.

2. Crosstalk

In PCB, crosstalk refers to the unexpected noise interference caused by electromagnetic energy on adjacent transmission lines through mutual capacitance and mutual inductance coupling when the signal is propagated on the transmission line. It is generated by the interaction of electromagnetic fields caused by different structures in the same area.

Mutual capacitance causes coupling current, which is called capacitive crosstalk. The mutual inductance induces coupling voltage, which is called inductive crosstalk. On printed circuit board, crosstalk is related to the length of the wires, the distance between the signal wires, and the condition of the reference ground plane.

3. Signal delay and timing errors

The signal is transmitted at a limited speed on the wire of the PCB circuit board. The signal is sent from the driver end to the receiver end with a transmission delay. Excessive signal delay or signal delay mismatch may result in timing errors and logic device confusion.

High-speed digital system design analysis of signal integrity analysis can not only effectively improve product performance, but also shorten product development cycle and reduce development cost. With the development of digital system towards high speed and high density, it is very urgent and necessary to master this design tool.

With the continuous improvement of signal integrity analysis model and computational analysis algorithm, the digital system design method using signal integrity for computer design and analysis will be widely and comprehensively applied.