Question 1

What is the best free online FFT spectrum analyser?

Accepted Answer

This FFT Spectrum Analyser is a free browser-based tool that requires no installation. It supports CSV upload from oscilloscopes, manual data input, four window functions (Flat-top, Hann, Hamming, Rectangular), harmonic extraction up to 10th order, and THD calculation.

Question 2

Which window function should I use for THD measurement?

Accepted Answer

Flat-top window is the best choice for THD measurement because it provides the highest amplitude accuracy (less than 0.01 dB error) regardless of whether the signal frequency aligns with FFT bins. This makes harmonic amplitude measurements reliable and THD calculations accurate.

Question 3

How do I calculate THD from oscilloscope data using FFT?

Accepted Answer

Upload your oscilloscope CSV file to this tool, select Flat-top window function, and click Analyse. The tool automatically computes FFT, extracts harmonic amplitudes (1x to 10x), and calculates THD using the formula: THD = sqrt(A2² + A3² + ... + A10²) / A1, where A1 is the fundamental amplitude.

Question 4

What is spectral leakage in FFT analysis?

Accepted Answer

Spectral leakage occurs when a signal frequency does not align exactly with an FFT bin. FFT assumes the signal repeats infinitely, so a discontinuity at the boundaries causes energy to spread across all frequency bins, distorting the spectrum. Window functions solve this by smoothly tapering the signal to zero at both ends.

Question 5

What is the difference between Hann and Flat-top window functions?

Accepted Answer

Hann window provides good frequency resolution and low sidelobes (-31 dB), making it suitable for general spectrum analysis. Flat-top window sacrifices frequency resolution for superior amplitude accuracy (less than 0.01 dB), making it ideal for precise harmonic amplitude measurement and THD calculation.

Question 6

Can I analyse motor back-EMF with this FFT tool?

Accepted Answer

Yes. This tool is designed for motor back-EMF harmonic analysis. Upload oscilloscope measurements as CSV or paste raw data, select Flat-top window for accurate amplitude measurement, and the tool will identify the fundamental frequency and calculate each harmonic component up to the 10th order along with total THD.