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Design007-Apr2020

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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 icd.com.au. To read past columns or contact Olney, click here.

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