Using FFT in Digital Storage Oscilloscopes (DSO)

Many modern Digital Storage Oscilloscopes (DSOs) have advanced mathematical operations available that typically include varying degrees of FFT analysis of the observed waveform. Fast Fourier Transform (FFT) allows the frequency content of a signal to be displayed along with the amplitude of these various frequency components.

Since the FFT operation is a mathematical operation, it requires a large amount of computing time to properly display the information. For these reasons, FFT operation in a DSO is not equivalent to a spectrum analyzer, but oftentimes it is sufficient for many applications.

FFT analysis of a signal is useful for tracking down EMI/RFI problems and for detecting spurious oscillations in circuitry. It is also useful for displaying the harmonic content of various waveforms and tracking down problems in switching power supplies. Distortion created by nonlinear operation of analog circuitry can easily be shown using FFT analysis. This phenomenon is essentially impossible to view using a standard time domain representation of the signal as is shown with a standard DSO display of the waveform.

FFT analysis is an extremely useful tool available to the technician or engineer. It can provide insight into the operation of a circuit and help determine which components or operating parameters are causing problems. Since this function is entirely a mathematical operation, it is implemented in the firmware of the DSO and therefore does not require additional hardware to implement, and therefore does not increase the cost of the unit.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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