Goal seeking target impedance with W1/W2 Constant Pitch

Sometimes, when specifying controlled impedance structures in Si8000m / Si9000e / Speedstack you may need to goal seek a target impedance, varying trace width while holding the trace pitch constant.

The trace pitch for differential structures can depend upon whether you are the designer or fabricator.

Designer: a designer has the capability of altering the X Y trace layout considerably with their PCB layout system so they can actually change the pitch between the pair of traces. Therefore, they tend to be able to alter either the W1 or S1 or both. So goal seeking often doesn’t require them to use the constant pitch option as they have the flexibility to alter the pitch.

Fabricator: a fabricator's job is to manufacture the design as supplied by the designer. They don’t usually alter the X Y layout of the traces but can increase or decrease the trace widths (W1 / W2) whilst maintaining the pitch between the traces. They commonly use the Goal Seeking with Constant Pitch as they can’t increase or decrease S1 without altering the trace pitch. As S1 increases, W1 has to decrease and vice versa.

In a differential trace, the trace pitch is the distance from the center of one trace to the center of the other trace, as shown in Figure 1 below.

Figure 1 Pitch distance – the center of one trace to the center of the other trace

From this diagram the formula for the pitch can be discerned as:

or

When goal seeking with option W1/W2 Constant Pitch, the sum of W1 and S1 is held constant while increasing or decreasing the W1 and S1 parameters as needed.

Example: When you want to goal seek constant pitch but don’t know W1 or S1 values to use but know the trace pitch

1. Divide the known trace pitch in half and place the same value in both W1 and S1.  In this example the trace pitch is 10 mils, so specify 5 mils in both W1 and S1, and the W2 etch factor 1 mil below W1. See W1, W2 and S1 below.

2. Select the Calculate button next to W1/W2 Constant Pitch and click OK

3. The Goal Seek code determines the constant pitch by adding W1 + S1 ( 5 + 5 = 10 mils ). It then maintains this pitch by increasing / decreasing the W1 and S1 parameters. As W1 changes, W2 is auto adjusted to maintain the 1 mil etch factor.

So in order to achieve 100 ohms with a 10 mils trace pitch, the goal seeker has determined W1 = 6.7485 and S1 = 3.2515.

To confirm the trace pitch: W1, 6.7485 + S1, 3.2515 = 10 mils.

Adding Trace Pitch to the Technical Report

Trace Pitch can be a selected column in the Controlled Impedance Table in the Technical Report in Speedstack as shown below. Use the Select Controlled Impedance Table Columns dialog to add the Trace Pitch (S1 + W1) column to the Technical Report Impedance Data Columns – Figure 2.

Select Controlled Impedance Table Columns dialog

The Trace Pitch (S1 + W1) column is added to Speedstack's Technical Report Controlled Impedance table.

Figure 2 Trace Pitch – a selectable column in the Speedstack Technical Report Controlled Impedance Table