Signal Processing & FFT Analysis Flashcards

What is a waterfall plot (cascade diagram) used for?

Answer: Showing multiple FFT spectra stacked over time or speed to reveal changing frequency content

A waterfall plot displays a series of FFT spectra arranged in sequence (time or speed), allowing analysts to observe how the frequency content of a machine changes as operating conditions vary.

What does 'zoom FFT' or 'zoom analysis' provide?

Answer: Higher frequency resolution within a selected narrow frequency band

Zoom FFT increases frequency resolution within a selected band of interest, enabling analysts to resolve closely spaced spectral components that would merge in a full-range spectrum.

What is the relationship between time record length and frequency resolution in FFT analysis?

Answer: Longer time records produce finer (better) frequency resolution

Frequency resolution (Δf) equals 1/T, where T is the time record length, so increasing the time record directly improves the ability to distinguish closely spaced frequencies.

In synchronous time averaging (STA), what is the primary purpose?

Answer: To eliminate asynchronous noise and isolate components locked to shaft rotation

Synchronous time averaging phase-locks averaging to a tachometer signal so that random, non-synchronous components cancel out while shaft-synchronous components reinforce, isolating gear or shaft defects.

What is the unit of the Y-axis in a power spectral density (PSD) plot?

Answer: g² / Hz

PSD normalizes the spectral amplitude by the frequency resolution bandwidth, yielding units of amplitude² per Hz (e.g., g²/Hz), which allows comparison of spectra with different resolutions.

Which measurement quantity is obtained by integrating an acceleration signal once?

Answer: Velocity

Single integration of acceleration with respect to time yields velocity, while a second integration yields displacement.