Return Path

One of the ways to make high speed signals stable is by making a return path shorter. If there is enough ground structure above and/or below the signal propagation path, the return path usually follows the signal path in high speed design. If ground patterns are cut by other structures or voids, return paths must go around such voids so the return path lengths become longer than the desired length. This item calculates the shortest signal return path along ground path. The shortest signal return path may become several times longer than signal path. This item checks for the ratio between the shortest return path length and signal length on board.

• Item: Input item name.
• Net: Select a target Net from the Net Class list.
• Reference: Select a Ground Net from the Net Class list to search for the Return Path.
• Pass/Fail Criteria: Define the value (Ratio or Distance) to detect the fail of Return Path and Signal.
• Check ratio: Check ratio between net length and return current path.
  • Check distance: Check maximum allowable detour distance of return path.

    
    
    Figure 1.

• Consideration: Addition factors for Return Path search.
• Select Start Component in case of Branch net: When searching Return Path of multiple pin (more than 2 pins) net, a start component must be defined. Return paths are searched from the start component to all other components or pins.
• Except Component Group: Certain types of components or pins can be excluded from searching return path. (For example, Test Points)
• Flow current along via location: To search for the shortest return path one multiple ground layers connected by via. When it is not checked, return path is searched on a superimposed ground layer.
  • Not checked: The return current always goes through capacitance between two planes regardless of via existence.

    
    
    Figure 2.

  • Check: The return current can go through only if there is stitching via.

    
    
    Figure 3.

• Ignore return path for near area to pin: Complete the return path search to the ground plane edge.
  • Will not check pin area: Enter a radius of circular region around pins to be excluded for the rule check.
  • Return path to pin: Select the way to complete the return path search.
  • Consider Return Path from GND Pin to GND Pin: Complete the return path search to the ground pins that are closest to the signal pins. (Fig. below, path ‘B’)
  • Force make return path to pin: Force return path to connect to directly to GND pins as shown in Fig. below. (Fig. below, path ‘A’)

    
    
    Figure 4. Return path connection compensation

The signal propagates between the signal trace and the reference plane and it acts like a loop antenna. The electrical field strength of loop antenna is:

Figure 5.

$$E=\mathrm{1.31.6}\times {10}^{-16}\cdot \left(f^2\cdot I\cdot A\right)\cdot \left(\frac{1}{r}\right)\cdot \sin \theta$$