CVA Vibration Measurement & Analysis Techniques 2

Question 1

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

Accepted Answer

2.5 Hz

Answer

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

Question 2

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

Accepted Answer

0.4 s

Answer

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

Question 3

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

Accepted Answer

2.56 times

Answer

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

Question 4

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

Accepted Answer

Flat top

Answer

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

Question 5

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

Accepted Answer

About 16% (≈1.4 dB)

Answer

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

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