Frequency resolution
Frequency resolution is the sample rate divided by FFT length. A smaller frequency spacing requires a longer record. The acquisition must remain sufficiently stable during that record.
- Sample rate
- FFT length
- Record duration
- Displayed frequency span
Window selection
A window reduces leakage when the record does not contain an exact integer number of signal cycles. Hann is a practical general-purpose choice. Rectangular is appropriate only when the signal is periodic within the record or transient analysis requires it.
- Hann window
- Hamming window
- Rectangular window
- Coherent-gain correction
Averaging
Averaging reduces random variation and makes stable spectral components easier to see. The selected method should match the objective and must not hide short-duration events.
- Linear averaging
- Exponential averaging
- Peak hold
- Overlap
Amplitude scaling
The spectrum must state whether amplitudes are RMS, peak or peak-to-peak and whether the display is linear or logarithmic. Engineering units and reference values must remain consistent.
- RMS spectrum
- Peak spectrum
- Acceleration, velocity or displacement
- dB reference
Interpreting peaks
A spectral peak should be related to speed, bearing geometry, gearmesh, electrical frequency or structural resonance before a fault conclusion is made.
- Running speed and harmonics
- Sidebands
- Bearing frequencies
- Gearmesh frequency
- Structural modes