Trace Width Height Separation Zo -------------------------------------- Rev. 17-Jan-2025 Much of the data in this file uses mixed units, i.e. mm is used for Width & Separation where as mils is used for Height above Ground Plane. For these approximate checks use Dk = 3.55 in all cases. Scan the Zo of a 0.14 mm wide single ended MicroStrip bla mils above its Ground Plane: Height above Ground Plane Zo 0.5 oz Zo 1 oz ------------ ---------- -------- 3.0 mils 49.20 44.24 3.5 55.22 50.26 3.8 58.44 53.48 4.0 60.44 55.48 4.5 65.04 60.08 10.0 96.24 91.29 20.0 123.31 118.35 50.0 159.11 154.14 Scan the Zo of Edge Coupled Microstrip Differential Pair 0.14 mm width 0.40 mm centers 0.26 mm air gap bla mils above its Ground Plane Height above Ground Plane Zo 0.5 oz Zo 1 oz ------------ ---------- -------- 3.0 mils 96.61 86.87 3.5 107.23 97.60 3.8 112.63 103.07 4.0 115.89 106.37 4.5 123.02 113.64 5.0 128.98 119.73 10.0 157.83 149.69 20.0 174.14 167.13 30.0 181.97 175.49 40.0 187.80 181.61 50.0 192.68 186.67 55.0 194.88 188.94 From this it is clear that the bulk of the trace to trace coupling is through their coupling to Ground and NOT directly from their Edge to Edge coupling, i.e. this geometry is only lightly or moderatly Edge to Edge Coupled Pair - basically as we want. As a double check on this - see the effect of close spacing: Scan the Zo of Edge Coupled Microstrip Differential Pair with a much smaller air gap between them 0.14 mm width 0.24 mm centers 0.10 mm air gap bla mils above its Ground Plane Height above Ground Plane Zo 0.5 oz Zo 1 oz ------------ ---------- -------- 3.5 92.41 84.11 4.0 98.29 90.22 4.5 103.08 95.22 10.0 129.12 122.46 20.0 148.60 142.62 50.0 176.57 171.07 Even with this very close trace to trace spacing most of the coupling is through the Ground Plane and not through the Edge to Edge Coupling. Finally look at an asymmetric Strip Line Scan the Zo of a 0.14 mm wide single ended Strip Line that is a nominal distrace from one of its Ground Planes as a function of the distace to the further Ground Plane Distance to the Close Ground Plane is always 3.5 mils Recall that 0.5 oz 0.14 mm wide MicroStrip 3.5 mils to a single Ground Plane is 55.22 Ohm Zo Distance to the Far Ground Plane Zo 0.5 oz ------------ ---------- 3.5 33.57 5.0 36.93 10.0 40.84 12.0 41.50 15.0 42.15 20.0 42.80 50.0 43.98 100.0 44.37 200.0 44.57 500.0 44.68 Recall that 0.5 oz 0.14 mm wide MicroStrip 3.5 mils to a single Ground Plane is 55.22 Ohm Zo and that 0.5 oz 0.14 mm wide MicroStrip 50 mils to a single Ground Plane is 159.11 Ohm Zo 55.22 parallel 159.11 is 40.99 Ohm i.e. NOT 43.98 The formula that DK shows can not be correct as it is not symmetric in the two distances - but their result is. Must check another reference. Scan the Zo of Edge Coupled StripLine Differential Pair 0.14 mm width 0.40 mm centers 0.26 mm air gap bla mils to each of its Two Ground Planes: Distance to each of Two Ground Plane Zo 0.5 oz ------------ ---------- 3.5 mils 66.65 4.0 74.07 5.0 86.21 7.0 103.36 10.0 119.39 15.0 134.77 Recall that the Zo of the same geometry Differential Microstrip Pair is: for 3.5 mils --> 107.23 Ohm and for 5.0 mils --> 128.98 Ohm So adding a 2nd Gnd Plane but putting both Gnd Planes twice as far away from the Diff Pair as in the case of the Microstrip Diff Pair results in a Lower Zo i.e. the two Gnd Planes twice as far away results in a Diff Zo that is about 94 % of the Zo of a single Gnd Plane (in the range of interest). For a single ended StripLine between Two Ground Planes that are a fixed distance apart (20 mils) what is the Zo as a function of the asymmetric spacing of the trace to the Two Ground Planes. Trace width 0.14 mm. Distance between the Two Ground Planes fixed at 20 mils. Distance to the Two Ground Plane Zo 0.5 oz ------------- ---------- 10 - 10 mils 65.09 8 - 12 64.72 5 - 15 53.85 3 - 17 37.13 So as expected the highest Zo is in the middle.