5.2 FFT 
Using FFT (Fast Fourier Transform) mathematical operations, the time 
domain signal (YT) can be converted into frequency domain signal. The 
following types of signals can be easily observed with using FFT:
Harmonic content and distortion in measurement system
   Performance of noise in DC power supply
   Vibration Analysis
 
FFT frequency spectrum
Press MATH button, then press F1 to select FFT to enter the FFT menu.
FFT menu 
Descriptions
Set any one of the CH1,CH2 for FFT source
Use Hamming Window Function
Use Blackman Window Function
Use Rectangle Window Function
Use Hanning Window Function
Set vertical unit as linear or dB (log)
Options
FFT
CH1, CH2
Hamming
Blackman
Rectangle
Hanning
Vrms dBVrms, 
Functions
Type
Source 
Window
Vertical Unit
FFT Tips
Signals with DC components might cause mistakes or inaccuracies in FFT 
calculation. In order to reduce error, it is adviced to set the channel to AC.
In order to reduce interference or noise from isolated events, the capture 
mode can be set to average.
UTD2000CEX-II User Manual