Signal Processing & FFT Analysis Flashcards

What does the Nyquist frequency represent?

Answer: Half the sampling rate, above which aliasing occurs

The Nyquist frequency is half the digital sampling rate and represents the highest frequency that can be accurately captured without aliasing.

In a vibration spectrum, what is a sideband?

Answer: A frequency peak located a fixed interval above or below a carrier frequency

Sidebands are spectral peaks spaced symmetrically around a carrier frequency and indicate amplitude or frequency modulation, commonly associated with gear mesh or bearing faults.

What is the purpose of a Flat Top window in FFT analysis?

Answer: To provide accurate amplitude measurements at the cost of frequency resolution

The Flat Top window provides the most accurate amplitude readings because it has a very flat passband in the frequency domain, making it ideal for calibration and amplitude-critical measurements.

What is cepstrum analysis primarily used for in machinery diagnostics?

Answer: Detecting families of equally spaced harmonics or sidebands

Cepstrum analysis is particularly useful for identifying regularly spaced harmonics or sidebands in a spectrum, which are characteristic of gear defects and bearing faults.

What does 'overlap processing' refer to in FFT averaging?

Answer: Reusing a portion of each time record in the next record to improve averaging efficiency

Overlap processing reuses a percentage (typically 50–75%) of each time block in the subsequent block, allowing more averages to be computed from the same total data length.

Which signal parameter is measured in a time waveform but NOT directly visible in an FFT spectrum?

Answer: Phase relationships between vibration components

Phase information is lost in a standard FFT magnitude spectrum; a time waveform preserves the temporal relationships between signal components that are essential for balancing and other phase-dependent analyses.