High Speed Signal in PCB design

A device with high speed signals (PCB design) can transfer the data at a very high rate. The high speed signal lines are called transmission lines.

Which are High speed signals?

• Basically, the frequency greater than 50 MHZ is considered as high speed signals.
• If the length of the trace is greater than lambda/10, is considered as high speed signal.
• Some of the high speed signals are PCIe, USB 3.0, SATA, Ethernet, DDR, HDMI, LVDS, USB 2.0.

SI(Signal Integrity)

SI plays a major role in high speed design. It signifies the signal’s ability to propagate without distortion. Here, below image shows the reason for the poor signal integrity we should be aware of;

Signal Integrity effects

To overcome these issues we should follow some of the layout rules.

Important Layout rules of HIGH SPEED SIGNAL

Layout rules

• Maintain controlled impedance throughout the transmission line to achieve the signal integrity.
• In differential pairs, constant distance should be maintained to avoid the impedance mismatch and skew (inter pair & intra pair).
• Use serpentine traces to achieve trace length.
• Avoid using trace bend at 90 degree. (eg: use 135 degree).
• Don’t place any Via or components between differential pair, it will produce discontinuity in impedance.
• Provide proper return current path. Stitching capacitor is needed when routed over split planes
• Provide the proper reference plane for high speed signals, prefer directly below the signal layer. Use stitching Via when reference plane changes.
• Avoid using long stub and crossing plane.
• Keep the analog and digital signals separate to reduce noise.
• Trace width must be 4 times the trace between adjacent cu, to minimize crosstalk.
• Place AC caps in series to remove dc offset.
• Avoid hot spot by placing Via in a grid.

Thank you,

Sathya Priya N