User manual

Operator's Manual

3. Choose an Output type.

4. Optionally, choose a weighting Window. See below for more information about FFT weighting windows.

5. Depending on your Output Type selection, you may also make selections for :

l Group Delay Shift

l Line Impedence. By default, the FFT function assumes that the oscilloscope is terminated in 50

Ohms. If an external terminator is being used, this setting can be changed to properly calculate the

FFT based on the new termination value.

6. Check the Suppress DC box to make the DC bin go to zero. Otherwise, leave it unchecked.

Choosing a Window

The choice of a spectral window is dictated by the signal's characteristics. Weighting functions control the

filter response shape, and affect noise bandwidth as well as side lobe levels. Ideally, the main lobe should be

as narrow and flat as possible to effectively discriminate all spectral components, while all side lobes

should be infinitely attenuated. The window type defines the bandwidth and shape of the equivalent filter to

be used in the FFT processing.

Rectangular windows provide the highest frequency resolution and are useful for estimating the type of

harmonics present in the signal. Because the rectangular window decays as a (sinx)/x function in the

spectral domain, slight attenuation will be induced. Functions with less attenuation (Flat Top and Blackman-

Harris) provide maximum amplitude at the expense of frequency resolution, whereas Hamming and Von Hann

are good for general purpose use with continuous waveforms.