Calculating Differential Pair Rule Stack Values from a Target Differential Impedance Value

For a Differential pair rule stack that you select in the Rule Editor dialog, Differential Pairs tab, this topic explains how to calculate the following:

• The track width, differential impedance and delay per unit length, based on target values for differential impedance and track spacing that you specify.
• The track spacing, differential impedance and delay per unit length, based on target values for target differential impedance and track width that you specify.

1. In eCADSTAR PCB Editor, launch the Rule Editor dialog by clicking Home > Design Rules > Rule Editor. Alternatively, click Design > Design Rules > Rule Editor.
2. In the Rule Editor dialog, select the Differential Pairs tab.
3. In the Differential Pairs tab, select a differential pair rule stack and click Calculate differential impedance -> Track Width. The Calculate Differential Impedance -> Track Width dialog is displayed.

Calculating Track Width using Target Values for Differential Impedance and Track Spacing

1. In the Calculate Differential Impedance -> Track Width dialog, Calculation Mode field, select Track width. The Target differential impedance [Ω] and Track spacing columns are made available.
2. In the Rounding precision for track width box, set the value by which the track width is incremented in order to reach the specified Target differential impedance and Track spacing values. When you click Calculate Track Width, the Track width value is automatically incremented by this value until Differential impedance calculated value is as close as possible to the Target differential impedance value. The most accurate results are produced by specifying smaller values.

 

3. In the Calculate Differential Impedance -> Track Width dialog, Differential Impedance table, enter a value in the Target Differential impedance [Ω] box for a particular conductor layer, and press Return. This is the required differential impedance for this layer in the relevant differential pair rule stack.
4. Enter a value in the Track spacing box for the same conductor layer, and press Return. This is the required track spacing for this layer in the relevant differential pair rule stack.
5. Click Calculate Track Width. eCADSTAR attempts to calculate the track width, differential impedance and delay using the Target Differential impedance [Ω], Track spacing and Rounding precision for track width values that you specify. If acceptable values cannot be found using the specified values, then an error message is displayed and is displayed in the Track spacing, Differential impedance calculated value [Ω] and Delay columns.
6. To execute the track width calculation again, enter new values in the Target Differential impedance [Ω], Track spacing and Rounding precision for track width boxes, and click Calculate Track Width.
7. When you are satisfied with the results, click Apply calculated results to diff. pair rule stack. The Track spacing value that you specify, as well as the calculated values are added to the Differential Pairstab, Differential pair rule stack table.
8. Click Apply in the Rule Editor dialog to save the settings.

Calculating Track Spacing using Target Values for Differential Impedance and Track Width

1. In the Calculate Differential Impedance -> Track Width dialog, Calculation Mode field, select Track spacing. The Target differential impedance [Ω] and Track width columns are made available.
2. In the Rounding precision for track spacing box, set the value by which the track spacing is incremented in order to reach the specified Target differential impedance [Ω] and Track width values. When you click Calculate Track Spacing, the Track spacing value is automatically incremented by this value until Differential impedance calculated value is as close as possible to the Target differential impedance value. The most accurate results are produced by specifying smaller values.

 

 

3. In the Calculate Differential Impedance -> Track Width dialog, Differential Impedance table, enter a value in the Target Differential impedance [Ω] box for a particular conductor layer, and press Return. This is the required differential impedance for this layer in the relevant differential pair rule stack.
4. Enter a value in the Track width box for the same conductor layer, and press Return. This is the required track width for this layer in the relevant differential pair rule stack.
5. Click Calculate Track Spacing. eCADSTAR attempts to calculate the track spacing, differential impedance and delay using the Target Differential impedance [Ω], Track width and Rounding precision for track spacing values that you specify. If acceptable values cannot be found using the specified values, then an error message is displayed and is displayed in the Track spacing, Differential impedance calculated value [Ω] and Delay columns.
6. To execute the track spacing calculation again, enter new values in the Target Differential impedance [Ω], Track width and Rounding precision for track width boxes, and click Calculate Track Spacing.
7. When you are satisfied with the results, click Apply calculated results to diff. pair rule stack. The Track width value that you specify, as well as the calculated values are added to the Differential Pairs tab, Differential pair rule stack table.
8. Click Apply in the Rule Editor dialog to save the settings.

Checking a Track spacing value after changing the Units value

1. After following the above procedure to calculate a track spacing value for a particular conductor layer in a differential pair rule stack, note the Track spacing value in the Rule Editor dialog and then close the dialog.
2. On the Status Bar, select a different value for Units.
3. Launch the Rule Editor dialog.
4. In the Differential Pairs tab, notice that the Track spacing value is shown using the new units.
5. Select a differential pair rule stack, and click Calculate differential impedance -> Track Width.
6. In the displayed Calculate Differential Impedance -> Track Width dialog, notice that the Rounding precision for track spacing value must now be specified using the new units.
7. Click Close in the Calculate Differential Impedance -> Track Width  dialog.
8. Click Cancel in the Rule Editor dialog.