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With increasing clock speed in high-speed digital design, proper impedance matching between the printed circuit board trace and the clock driver output is crucial for signal integrity design. Adding a series output resistor is a common technique to match a clock driver output to a high-speed signal trace for its simplicity and ease of implementation. The selection of the value of the series output resistor requires known output impedance of the clock driver, which is not always readily available. Based on an in-circuit measurement approach, a method to extract the output impedance of any clock driver under its actual operating condition has been developed. With the known output impedance of the clock driver, precise selection of the series resistor value for impedance matching purpose has become a relatively simple task without the usual trial-and-error process.