Limit Approach Net

This rule checks whether two nets are within a specified distance.

Although system designers tend to use fewer traces to minimize costs, this usually results in tightly packed traces. Tightly spaced traces have more crosstalk and have power handling capability. Depending on the design, tradeoffs may be necessary to accommodate system requirements. The rule of thumb is using 3W-rule to provide a clearance W when the width of each trace is W. However, the clearance W needs to be adjusted for crosstalk and EMI consideration. While the differential pair rule requires two nets to be the same lengths and widths, the Limit Approach Net rule checks just whether two nets are within the specified distance. The differential pair rule reports an error when the separation between two nets is closer than a specified clearance. However, it is passed in Limited Approach Net rule if two nets are closer than the allowable distance. Another function is to check whether the specified net exists between two nets. For example, the ground wall trace should exist between left ear signal and right ear signal in order to prevent sound noise coupled with each other.
  • Item: Input item name.
  • Net1: specify 1st net to check.
  • Net2: specify 2nd net to check.
  • Filter: Enter a filter to choose differential pair net from selected net group. After entering a base net and pair net click Add Filter.
  • Distance: Define allowable distance for the two net types, Net1 and Net2.
  • Layer: Specify PCB layer.
  • Pin Escape: Enter a radius of circular region around pins to be excluded for the rule check.
  • Via Escape: Enter a radius of circular region around vias to be excluded for the rule check.
  • Except Serpentine: DFE do not check serpentine section of net.
  • Net3: Check existence and minimum width of Net3 which exists between Net1 and Net2.
  • Min Width: Define required minimum trace width of Net3.
  • Composite Power Net: DFE uses composited power net instead of single power net.
    • Passive Comp: The DFE makes composite net for which are connected through this passive component.
    • Exclude Net: Nets that should not be merged into composite net.

The difference between Differential Pair and Limit Approach rules:

Differential Pair nets should keep the same trace width and clearance. Traces of Limited Approach rule do not need to keep the same clearance or width, but they should not be apart farther than the given value.