Vibration Measurement & Analysis Techniques Flashcards

An analyzer is set to Fmax = 1000 Hz with 400 lines of resolution. What is the frequency resolution (bin width)?

Answer: 2.5 Hz

Resolution equals Fmax divided by the number of lines, so 1000/400 = 2.5 Hz.

Using the same setup (Fmax 1000 Hz, 400 lines), how long does it take to acquire one time block, ignoring overlap?

Answer: 0.4 s

Time block length equals lines divided by Fmax, so 400/1000 = 0.4 s, the reciprocal of the 2.5 Hz resolution.

Most FFT analyzers sample at what multiple of Fmax so the anti-aliasing filter has room to roll off?

Answer: 2.56 times

A 2.56× sampling rate gives margin above the Nyquist minimum of 2× for a practical anti-aliasing filter roll-off.

Which window gives the best amplitude accuracy for a steady sinusoid that falls between two FFT bins?

Answer: Flat top

The flat top window's wide, flat main lobe keeps amplitude error very small, though it gives up frequency resolution.

In the worst case, a Hanning-windowed sine wave that falls exactly between two bins can be under-reported by about how much?

Answer: About 16% (≈1.4 dB)

Hanning picket-fence (scalloping) loss is about 1.42 dB, which is roughly a 16% amplitude error.

When should the uniform (rectangular) window be used?

Answer: For transient signals that fit completely inside the time record, such as an impact response

If a signal starts and ends at zero within the time block there is no leakage to fix, so no tapering is needed.

Which averaging type is the right choice for capturing the highest amplitude at each frequency during a coast-down?

Answer: Peak hold averaging

Peak hold keeps the maximum value seen in each bin, which captures resonance peaks as the speed sweeps through them.