Question 1

How do I choose between Hanning, Rectangular, and Hamming windows?

Accepted Answer

The choice depends on the application. Rectangular window: use when the signal fits exactly within the window (impulse response, coherent measurement of steady-state sinusoids). It has the maximum spectral leakage. Hanning window: most commonly used for general vibration and noise spectrum analysis. Good balance between leakage reduction and amplitude accuracy. Hamming window: slightly lower sidelobes than Hanning, often used in communications signal processing. Hanning is the default choice for CAE vibration analysis.

Question 2

What is spectral leakage?

Accepted Answer

Spectral leakage occurs when the signal frequency does not fall exactly on an FFT bin frequency (k × Δf). Energy 'leaks' into adjacent bins, making the peak appear broader. The rectangular window has maximum leakage. Applying window functions like Hanning reduces sidelobe levels, but slightly reduces frequency resolution.

Question 3

How is THD (Total Harmonic Distortion) calculated?

Accepted Answer

THD is the ratio of harmonic distortion to the fundamental: THD = √(V₂² + V₃² + V₄² + ...) / V₁ × 100[%]. This simulator identifies the fundamental as the highest-amplitude component and treats the rest as harmonics. THD is used to evaluate waveform quality in amplifiers, power supplies, and motor control.

Question 4

How is frequency resolution Δf determined?

Accepted Answer

Frequency resolution Δf = f_s / N, where f_s is the sampling frequency and N is the FFT size. For example, with f_s = 1000 Hz and N = 1024, Δf ≈ 0.977 Hz. To improve resolution, increase N (longer observation time) or lower f_s. Lowering f_s also reduces the maximum analyzable frequency (Nyquist frequency), so a tradeoff exists.

FFT Spectrum Analyzer Simulator

Theory