Issue link: https://resources.pcb.cadence.com/i/1533023
High speed and high frequency transmission lines typically require a guard trace or grounded copper flood that is pulled back a specified distance from the transmission lines. At the same time, the walled garden is closing the gap to the ordinary signals beyond. Safety is first which makes performance secondary. High voltage will require sufficient space to prevent arcing. Learning about this part of the tool will make life easier during routing wherever shapes are in the picture. Figure 2. The first page of the Spacing worksheet selector allows the creation of new Spacing Constraint Sets. Right Mouse on the name of an existing constraint set in order to copy it for revisions. Each constraint set will define spacing requirements sorted 11 different ways. The Line values are shown but the Shape values are the definitive ones as far as generating copper pour. Each attribute (Lines, Pins, Vias, etc.) has its own tab that interacts with the others. This helps with management of the air gaps all around us. Intra-Pair Spacing It's not uncommon to express the air-gap as a function of line width. The typical air-gap between controlled impedance traces will often be three times the line width or five times if you want to be conservative. It's not really the line width that matters as much as the distance in the Z-axis to the reference plane. Given a typical dielectric constant, the line width and the dielectric thickness are fairly close in value. The width of the line stands in as a shorthand for this rule of thumb. The practical effect of the rule is that we don't want long runs of parallelism when we're routing a bus. The grain of salt to take from this rule is that there are going to be times when the design rules cannot be met due to spacing limitations around fine pitch devices. Inter-Pair Spacing Differential pairs have a gap between the positive and negative members; the interpair gap. Here, it's a matter of whether the pair is closely or loosely coupled. Closely coupled differential pairs have a gap that is equal to or less than their line width. Conversely, loosely coupled pairs have an air-gap that is greater than the trace width by some measure. On one hand, the tightly coupled diff-pair will use less space and excel at rejecting common mode noise. In addition, the bends have minimal effect on skew between the positive and negative connections which should make phase matching possible with less uncoupled length. 3 www.cadence.com Controlling Air Gaps Using OrCAD X and Allegro X Tools