Bad Fast Fourier Transfer (FFT) data captures can cause you to misunderstand the true performance of your high-speed or RF analog-to-digital converter (ADC). Some engineers like to coherently sample and capture data (which means to have the analog input frequency and clock frequency locked in phase with a reference frequency), while others do not necessarily need to. However, in either setup, there can be a need to refresh some details to present good data in your next design review.
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Although there seems to be a lot of material about coherent and non-coherent sampling methods online, there is little guidance as to why you can use one sampling method versus the other. Most of the information that is available does not include the steps necessary to accurately take a measurement. There are details to keep in mind.
This article discusses the differences between coherent and non-coherent sampling in terms of an ADC test setup.