Uncoupled Spacing

Summary of the Uncoupled Spacing PRC, checking differential pair spacing to ensure effective electromagnetic coupling, consistent impedance, and minimal crosstalk.

Description

Uncoupled Spacing calculates the maximum trace length in which the differential pair is uncoupled. The nominal spacing is determined based on desired differential impedance and the specific stackup. A differential pair trace segment is considered “uncoupled” if it’s projected distance from the other trace in the other net is outside of a tolerance of 10% when compared to the nominal spacing.

Passing Criteria: This check passes if the trace length of uncoupled spacing between the two trace paths in the nets of interest is below a specified length tolerance

Reporting Units: CENTIMETERS (cm)

Examples

Passing Message:

Uncoupled spacing of 0.06cm within acceptable range (0cm to 1cm)

Failing Message:

Uncoupled spacing of 6.57cm outside acceptable range (0cm to 1cm)

Physics Justification

The distance between differential pairs is important because it affects how well they work together and resist interference. A few reasons why this is important include:

• Electromagnetic Coupling

  Differential pairs create small electromagnetic fields as signals travel. If the traces are too far apart, they won’t couple well, reducing their ability to cancel out external noise. Conversely, if they’re too close, they might interfere with each other or with other signals.

• Crosstalk

  If differential pairs are too close to other traces, their signals can interfere with neighboring traces (or vice versa), causing crosstalk, which is unwanted noise from other signals.

• Impedance Control

  The spacing affects the differential impedance, which is the resistance the signals encounter. Maintaining a specific distance ensures the impedance remains consistent, helping the signal to stay clear and fast.

Helpful Definitions

• Crosstalk

  Crosstalk is unwanted interference caused by signals in one trace inducing noise into a nearby trace. It happens because the electromagnetic fields from one trace can "leak" into another, disrupting the signal.

• Differential Impedance

  Differential impedance is the combined resistance a differential pair encounters as signals travel together. It's determined by the trace width, spacing between the traces, and the surrounding materials. Proper impedance ensures the signal integrity without reflections or losses.