Top Ten "Tricks" for High-frequency PCB Wiring(III)

[eighth trick] avoid loops formed by traces

Do not form a loop as much as possible for all types of high-frequency signal traces. If it is unavoidable, make the loop area as small as possible.

[Ninth move] must ensure good signal impedance matching

During the transmission of the signal, when the impedance does not match, the signal will reflect in the transmission channel, and the reflection will overshoot the composite signal, causing the signal to fluctuate around the logic threshold.

The fundamental way to eliminate the reflection is to make the impedance of the transmitted signal match well. Since the difference between the load impedance and the characteristic impedance of the transmission line is larger, the reflection is also larger. Therefore, the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible. At the same time, it should be noted that the transmission line on the PCB should not be abrupt or corner, try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflection between the segments of the transmission line. This requires the following wiring rules to be observed when performing high-speed PCB routing:

USB Wiring Rules: USB signal differential routing is required. The line width is 10 mils, the line spacing is 6 mils, and the ground and signal lines are 6 mils apart.

HDMI cabling rules: HDMI signal differential routing is required, linewidth is 10mil, line spacing is 6mil, and the spacing between each pair of HDMI differential signal pairs exceeds 20mil.

The LVDS routing rules require LVDS signal differential traces with a linewidth of 7 mils and a line pitch of 6 mils. The purpose is to control the HDMI differential signal pair impedance to 100+-15% ohm DDR routing rules. The DDR1 routing requires that the signal should not pass through the hole as much as possible. The signal line is equal in width and the line is equidistant from the line. The line must meet the 2W principle to reduce crosstalk between signals. For high-speed devices with DDR2 and above, high-frequency data is required. The lines are equal in length to ensure impedance matching of the signals.

[Tenth move] to maintain the integrity of signal transmission

Maintain the integrity of signal transmission and prevent "ground bounce" caused by ground segmentation.