If a device's additive phase-noise contribution is known, engineers can more easily select individual components for their signal chain. They will feel confident that those devicesin aggregatemeet the phase-noise requirements for their complete system. In an application note titled, "The Residual Phase Noise Measurement," Analog Devices' David Brandon and John Cavey describe a technique to evaluate device-under-test (DUT) noise by removing external noise sources. A residual phase-noise setup is used to isolate and measure the additive phase-noise contribution of a device. Such a setup allows noise sources external to the DUT, such as power supplies or input clocks, to be canceled from the measurement.

In the experiment, two DUTs with the same part number were clocked by a single 1-GHz oscillator. Both devices were set up to divide the clock-source frequency by 4, thereby producing a 250-MHz output. To minimize the downconverted signal that appears at DC, the two output signals were adjusted so that they shifted in relative phase by 90 deg. When two different clock sources were measured, they were shown to have very different phase-noise characteristics. Using a residual phase-noise setup, however, neither clock source impacted the DUTs' additive phase noise.

To emphasize the attributes of the residual phase-noise setup, the document includes actual phase-noise measurement plots of a clocked device. It also demonstrates how a device's additive phase noise can be used to identify the source of noise-related issues in the signal chain. With a combination of residual and absolute phase-noise measurements, engineers can identify the dominant noise source in a system design.

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