Impedance Balanced Routing (IBR) allows you to control the impedance of portions of a net. This enables you to match loaded characteristics or loaded differential impedances by adjusting the track width and track spacing of pin pairs. Branched route topologies are used in Impedance Balanced Routing to reduce reflections at junctions.
Impedance Balanced Routing is supported for both single-ended nets and differential pairs. You can route a differential pair using trunking, which follows the track width and spacing rules defined in the assigned topology template. The differential pair trunking is not updated automatically. In order for it to follow an assigned topology, execute Update Widths/Spacings from the assist menu of the Regulate Trunk command. Any errors or warnings are listed in the displayed Update Widths/Spacings Error Report dialog.
Consider the following when executing Impedance Balanced Routing.
- IBR Trunking is possible only when routing a single differential pair with a valid topology.
- Rule stacks are ignored for rule areas.
- Track widths and spacings are not changed
for end routing.
Restrictions when Routing Differential Pairs
For differential pairs that use topology templates to assign track widths and spacings at the pin pair level, the following restrictions apply when composing a trunk. These also apply if you attempt to recompose the trunk after decomposing it.
By default, the Compose command considers the following design rules, in order, when attempting to compose a trunk. The widths and spacings at the pin pair level are not considered at all. For each level, if a value is not set, then the next consecutive level is considered.
- At the differential pair level: the assigned differential pair rule stack.
- At the default differential pair level: differential pair rule stack.
- At the default level: Default track width stack: Typical Track Width, and Default design rule stack: Other Net.
To compose a trunk, you must update the track widths and spacings to one of the above settings. Next, use the Compose command from the Regulate trunk command. Alternatively, a differential pair that has become decomposed could be re-routed.
Switching the Current Topology Pin Pair
When executing Impedance Balanced Routing, you can switch to the next topology pin pair by clicking Next Topology Pin Pair on the assist menu. Alternatively, press the N key. A High Speed license is required. This command is made available when one of the following conditions met.
- There is a topology pin pair before the next Virtual Branch Point (VBP)/pin. This is the VP pin pair.
- There are multiple destinations (VBPs).
Collecting Virtual Branch Points (VBPs)
When executing Impedance Balanced Routing, you can collect the current target VBPs by clicking Collect Next Virtual Branch Point on the assist menu. Alternatively, press C on the keyboard. This command is available only when the current destination is VBP, and either the via pattern is about to be created (the ghost via pattern is shown), or it is just inserted but the next way point has not been created.
- If selected, then the current target VBPs are attached to the vias inside the via pattern, and the via pattern is placed.
- To resume the routing operation, you can select the via pattern.
Checking Spacing and Width Values for IBR
You can check the spacing between the differential pairs, and the width of pin pairs used in Impedance Balanced Routing. This is done using the Differential Pair Spacing Check and Pin Pair Width Check commands, respectively.
