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20 DESIGN007 MAGAZINE I APRIL 2020 To control the impedance of high-speed sig- nal interconnects, one first needs to predict the impedance of a specific multilayer stack- up configuration. A precision field solver is arguably the most accurate way to calculate the single-ended, edge-coupled, and broad- side-coupled differential impedance. Once the board is fabricated, the transmission lines need to be physically measured to determine the actual impedance to qualify the board (Figure 1). The most common method for measuring PCB trace impedance is to use a time-domain reflectometer (TDR). This measures the imped- ance in the time domain. However, a far more accurate method is to use a vector network an- alyzer (VNA), which operates in the frequen- cy domain. The VNA sweeps through a range of frequencies determining the impedance and signal losses at particular frequencies. However, VNAs are expensive, delicate instru- ments and not as robust as a TDR (Figure 2), which allows unskilled personnel to operate Predicting and Measuring Impedance Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: The iCD Stackup Planner coupled to the Zmetrix TDR and Zscan software. Figure 2: Zmetrix impedance test systems.

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