Electronic Design

FFT Brings Spectrum Analysis To The Scope

Many of today's digital oscilloscopes include fast-Fourier-transform (FFT) capability for frequency-domain analysis. This feature is especially valuable for oscilloscope users who have limited or no access to a spectrum analyzer, yet occasionally require frequency-domain analysis capability.

Although a spectrum analyzer exhibits better dynamic range and less distortion, the digital oscilloscope FFT provides a cost-effective, space-saving alternative. For example, the Agilent 54800 Series Infinium Deep Memory Oscilloscope with FFT can compute both magnitude and phase. Its signal-to-noise ratio is at least 70 dB.

A number of its features assist in spectral analysis. There are controls for adjusting the memory depth, sampling rate, vertical scale, and horizontal scale of the FFT. Also, automatic measurements and markers are included for measuring spectral peak frequencies and magnitudes, plus deltas between peaks.

Other features, although designed primarily for time-domain analysis, are useful for FFTs as well. Designers can annotate display traces and save them to a file, as well as save and recall the oscilloscope setup. Users may chain together functions to perform complex tasks like computing the average, maximum, or minimum of several FFT spectrums. Additionally, measurement statistics are available for computing the mean and standard deviation of a measurement over several acquisitions.

Some of the newest-generation oscilloscopes with FFTs have deep memories too. This allows users to increase the record length of the FFT, which improves the frequency-spectrum estimate. Longer record lengths also provide finer frequency resolution and better dynamic range. By upgrading the processor speed and improving the efficiency of the FFT algorithm, these new-generation oscilloscopes can perform FFTs on long records very quickly.

TAGS: Components
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