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APRIL 2020 I DESIGN007 MAGAZINE 25 ±5%, but that comes at a premium due to re- duced yields. Also, controlled impedance interconnects are intended to achieve target impedance when the transmission lines are loaded with ICs and powered. These conditions do not prevail on a bare board, so these measurements are likely to read higher than that predicted. Fortunately, providing the impedance is in the ballpark, the most important factor from a PCB design perspective is the constant value of impedance along the transmission line—no discontinuities or reflections. However, imped- ance becomes more critical as frequency in- creases and wavelengths become close to trace lengths. Key Points • A TDR measures the impedance in the time domain. However, a far more accurate method is to use a VNA, which operates in the frequency domain. • TDRs are the de facto standard, as VNAs are expensive, delicate instruments and not as robust. • A TDR applies a very fast pulse to an impedance test coupon via a controlled impedance cable and matching impedance probe and measures the reflected signal and graphs the impedance. • The key attribute of a TDR to consider is the pulse rise time, as this determines the measured resolution. • Impedance test coupons are generally 150 mm long, but with a 20-ps pulse, trac- es can be tested down to 20 mm in length, allowing the operator to test actual on-board traces—not just test coupons. • Test coupons are placed outside the board outline, which can make the impedance higher due to increased resin on the panel edge. • There is also a multitude of fabrication variables to account for—hence a tolerance of ±10%, which is the IPC standard for controlled impedance measurement. DESIGN007 Further Reading • B. Olney, "Beyond Design: Controlled Impedance De- sign," The PCB Design Magazine, May 2015. • A. Bahl, "How TDR Impedance Measurements Work," November 28, 2017. • Tektronix, "TDR Impedance Measurements: A Founda- tion for Signal Integrity," 2008. • E. Bogatin, "TDR and VNA Techniques for PCB Charac- terization," IPC APEX EXPO 2002. Barry Olney is managing director of In- Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that special- izes in board-level simulation. The com- pany developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at To read past columns or contact Olney, click here.

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