CVA Signal Processing & FFT Analysis 2 — Questions and Answers
Question 1: What does the Nyquist frequency represent?
- The resonant frequency of the measurement system
- Half the sampling rate, above which aliasing occurs (Correct answer)
- The lowest detectable frequency in an FFT spectrum
- The center frequency of a bandpass filter
Correct 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.
Question 2: In a vibration spectrum, what is a sideband?
- A harmonic of the fundamental frequency
- A frequency peak located a fixed interval above or below a carrier frequency (Correct answer)
- The noise floor surrounding a spectral peak
- A sub-harmonic component below running speed
Correct 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.
Question 3: What is the purpose of a Flat Top window in FFT analysis?
- To maximize frequency resolution for closely spaced peaks
- To provide accurate amplitude measurements at the cost of frequency resolution (Correct answer)
- To minimize noise floor in wideband spectra
- To prevent aliasing at high frequencies
Correct 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.
Question 4: What is cepstrum analysis primarily used for in machinery diagnostics?
- Identifying natural frequencies of structural components
- Detecting families of equally spaced harmonics or sidebands (Correct answer)
- Measuring overall vibration severity levels
- Calibrating accelerometer sensitivity
Correct 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.
Question 5: What does 'overlap processing' refer to in FFT averaging?
- Using two sensors simultaneously to improve accuracy
- Reusing a portion of each time record in the next record to improve averaging efficiency (Correct answer)
- Applying two different window functions to the same dataset
- Combining spectra from different measurement locations
Correct 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.
Question 6: Which signal parameter is measured in a time waveform but NOT directly visible in an FFT spectrum?
- Amplitude of individual frequency components
- Phase relationships between vibration components (Correct answer)
- Presence of harmonics
- Dominant frequency of vibration
Correct 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.
CVA Signal Processing & FFT Analysis 2
What does the Nyquist frequency represent?