CPW - Polygon's Arc Approximation

The arc approximation for polygons needs to be considered when creating Coplanar Waveguide controlled impedance traces (CPW).

click on images to view

The default Arc Approximation for Polygons is 0.5mil.


Example: The rules call for clearance of 13.6 mils



click on images to view (Gap = Pitch minus Width) = 13.6mils

The measured gap > Report > Measure Primitives

14,7 mils ?

Solution:

Reduce the Arc Approximation for the polygons poured adjacent to the CPW traces.

Re-pour the polygons and measure again.

Result 13.601mils.

Much better !

Controlled Impedance Formulas

You must use planes in your design if you wish to use Controlled Impedance Routing Features in Altium.

The downside to using planes in Altium is that you can not route traces that belong to nets within the plane.

Traces on planes are used to create:

Plane to Plane Clearances or Slots in the Plane.
Pull Back Copper from the Board Outline.
Create Split Planes and Nested Planes.

The upside is you get to use automatically controlled impedance line widths.

Equations for Microstrip (Simple trace on Plane):

CharacteristicImpedance = (60/SQRT(Er*(1-EXP(-1.55*(0.00002+TraceToPlaneDistance)/TraceToPlaneDistance))))*LN(5.98*TraceToPlaneDistance/(0.8*TraceWidth+TraceHeight)) TraceWidth = ((5.98*TraceToPlaneDistance)/EXP(CharacteristicImpedance/(60/SQRT(Er*(1-EXP(-1.55*(0.00002+TraceToPlaneDistance)/TraceToPlaneDistance)))))-TraceHeight)/0.8

Equations for Stripline (Simple trace asymmetrically placed between two planes):

CharacteristicImpedance = (80/SQRT(Er))*LN((1.9*(2*TraceToPlaneDistance+TraceHeight)/(0.8*TraceWidth+TraceHeight)))*(1-(TraceToPlaneDistance/(4*(PlaneToPlaneDistance-TraceHeight-TraceToPlaneDistance)))) TraceWidth = ((1.9*(2*TraceToPlaneDistance+TraceHeight))/(EXP((CharacteristicImpedance/(80/SQRT(Er)))/(1-(TraceToPlaneDistance/(4*(PlaneToPlaneDistance-TraceHeight-TraceToPlaneDistance))))))-TraceHeight)/0.8