Tel: 886.2.2175 2930 Email: [email protected]
29
The Math FFT function has three FFT Window options. There is a trade-off between frequency resolution and
amplitude accuracy for each type of window. You shall determine which one to choose according to the object you
want to measure and the source signal characteristics.
Window
Measurement
Characteristics
Hanning
Periodic Waveform
Better frequency, poorer amplitude accuracy than Flattop
Flattop
Periodic Waveform
Better amplitude, poorer frequency accuracy than Hanning
Rectangular
Pulse or Transient
Waveform
Special-purpose window applicable to discontinuous waveforms. This is actually the same as
no windows.
Bartlett
Blackman
Best magnitude,worst
frequency resolution
Single frequency waveforms, to Find higher order harmonics.
5.3.1.4
FFT Aliasing
Problems occur when the time-domain waveform acquired by the oscilloscope contains frequency components
higher than the Nyquist frequency. The frequency components above the Nyquist frequency will be undersampled
and displayed as lower frequency components that ‘fold back’ from the Nyquist frequency. These erroneous
components are called aliases.
5.3.1.5
Eliminating Aliases
To eliminate aliases, use the following methods.
Turn the SEC/DIV knob to set a faster sample rate. Because the Nyquist frequency increases as you increase
the sample rate, the aliased frequency components will be displayed correct. If too many frequency components
appear on the screen, you may use the FFT Zoom option to magnify the FFT spectrum.
If there is no need to observe the frequency components above 20MHz, set the Bandwidth Limit option to
Limited.
Filter the signal input from outside and limit the bandwidth of the source waveform to lower than the Nyquist
frequency.
Identify and ignore the aliased frequencies.
Use zoom controls and cursors to magnify and measure the FFT spectrum.
5.3.1.6
Magnifying and Positioning FFT Spectrum
You may scale the FFT spectrum and use cursors to measure it through the FFT Zoom option which enables the
horizontal magnification. To vertically magnify the spectrum, use the vertical controls.
Horizontal Zoom and Position