Q: What is the Nyquist frequency?

The Nyquist frequency (f_N) is defined as half the sampling frequency: f_N = f_s/2. According to the Nyquist-Shannon sampling theorem, a signal can be perfectly reconstructed only if f_s ≥ 2 × f_signal. In practice, a safety margin of 2.5 to 3 times the highest signal frequency is recommended for the sampling rate.

Q: How do you prevent aliasing in vibration measurements?

Two main countermeasures exist. First, use an anti-aliasing filter: apply a low-pass filter before A/D conversion to cut frequency components above f_s/2. Second, choose a sufficiently high sampling rate: select f_s at least 2.56 times (and ideally 5 times) the highest frequency of interest. FFT analyzers typically auto-set the sampling rate to 2.56 × f_span.

Q: How does this tool apply to CAE analysis?

Key applications include: (1) designing measurement conditions for modal testing, verifying sufficient f_s for target mode frequencies; (2) setting time-history output intervals in LS-DYNA or Abaqus to capture high-frequency responses; (3) setting sensor sampling rates for structural health monitoring; and (4) optimizing data acquisition settings for fatigue test measurements.

Question 1

What is aliasing in signal processing?

Accepted Answer

Aliasing occurs when the sampling frequency is less than twice the signal frequency (f_s < 2 × f_signal). The signal is incorrectly reconstructed as a lower frequency called the alias frequency. For example, if f_signal = 80 Hz and f_s = 100 Hz, the alias frequency is |80 - 100| = 20 Hz. In vibration measurements, aliasing appears as spurious frequency peaks that can lead to incorrect mode identification.

Question 2

What is the Nyquist frequency?

Accepted Answer

The Nyquist frequency (f_N) is defined as half the sampling frequency: f_N = f_s/2. According to the Nyquist-Shannon sampling theorem, a signal can be perfectly reconstructed only if f_s ≥ 2 × f_signal. In practice, a safety margin of 2.5 to 3 times the highest signal frequency is recommended for the sampling rate.

Question 3

How do you prevent aliasing in vibration measurements?

Accepted Answer

Two main countermeasures exist. First, use an anti-aliasing filter: apply a low-pass filter before A/D conversion to cut frequency components above f_s/2. Second, choose a sufficiently high sampling rate: select f_s at least 2.56 times (and ideally 5 times) the highest frequency of interest. FFT analyzers typically auto-set the sampling rate to 2.56 × f_span.

Question 4

How does this tool apply to CAE analysis?

Accepted Answer

Key applications include: (1) designing measurement conditions for modal testing, verifying sufficient f_s for target mode frequencies; (2) setting time-history output intervals in LS-DYNA or Abaqus to capture high-frequency responses; (3) setting sensor sampling rates for structural health monitoring; and (4) optimizing data acquisition settings for fatigue test measurements.

Sampling Theorem & Aliasing Visualizer

Theory