How to Define Differential Pair Rules in OrCAD X

Defining Electrical Rules for Differential Pairs

Step 1: Open the provided design in OrCAD X Presto.

Step 2: If the Constraints panel is not visible, select View > Panels > Constraints from the menu.

Step 3: The Constraints panel opens on the right side of the PCB canvas. Select Advanced under Electrical to define advanced electrical rules including:

• Wiring

• Differential Pairs

• Propagation Delay

• Return Path

• Impedance

Step 4: Select Differential Pair to define a differential pair rule set.

Creating a Differential Pair CSet

Step 5: Select the plus sign for Create Electrical CSet to create a new constraint set.

Step 6: The Create Electrical CSet window opens. Enter CAM_DP for the name.

Step 7: Click OK. The constraint set is automatically selected from the Rule Set dropdown.

Step 8: Select Ignore from the Gather Control dropdown towards the bottom of the subpanel. This option determines whether to include or ignore decoupling for pin connections in the uncoupled length.

Step 9: Set the Maximum Uncoupled Length to 6. This will set the maximum uncoupled length or the maximum length at which the routed differential pair traces can be spaced farther than their assigned primary gap, to 6mm.

Step 10: Set the Static Phase Tolerance to 0.508. This defines the maximum distance for the differential pair to be out of phase to 0.508mm.

Assigning Electrical Rules for Differential Pairs

Step 11: The differential pair constraint set must be assigned to the appropriate nets before it can be checked. In the Selection Filter subpanel, disable All Objects and enable Nets to allow the selection of nets.

Step 12: Click and drag to select all CAM1 pads on connector J11 on the left side of the board. Hold CTRL and click the GND plane to deselect it. The Constraints panel is populated with common constraints for these nets.

Step 13: Select CAM_DP from the Rule Set dropdown. The constraint set is assigned to all CAM1 nets.

Step 14: Click anywhere to deselect the nets.

Defining Physical Rules for Differential Pairs

Step 15: Physical rules such as minimum line width and primary gap can be assigned for differential pairs as well, and constraints can also be defined and assigned in the Constraint Manager. Select Electrical Analysis > Constraint Manager from the menu to activate the Constraint Manager.

Step 16: The Constraint Manager window opens, showing a directory of constraint domains and worksheets. Select the Physical domain.

Step 17: Select the Physical Constraint Set > All Layers worksheet.

Step 18: Right-click the Default constraint set and select Create > Physical CSet.

Step 19: Enter CAM_DP for the name and click OK to create the constraint set.

Step 20: Assign the following constraint values:

• Line Width: 0.243

• Primary Gap: 0.254

• Neck Length: 6

• Neck Width: 0.127

• Neck Gap: 0.15