Language Reference

FFT

Interpreting

Displayed

Results

wavelength resolution, but also produces the highest side lobes for periodic signals.

The FLATTOP window has the greatest wavelength uncertainty of the windows, but it has outstanding side lobe suppression and amplitude ￿atness. Use FLATTOP to transform periodic signals.

The HANNING window is a traditional passband window found in most real-time analyzers. The HANNING window o￿ers a compromise between the FLATTOP and UNIFORM windows. The HANNING window amplitude uncertainty is 01.5 dB and its 3-dB bandwidth is 40% of the FLATTOP bandwidth. Use the HANNING window when transforming periodic or random data.

The HAMMING window has a bandwidth between the HANNING and FLATTOP windows, with more uniform side-lobe suppression than the HANNING window, but less suppression than either HANNING or FLATTOP for signals farther out of the passband.

The FFT results are displayed on the spectrum analyzer in a logarithmic scale. The frequency at the left graticule line is 0 Hz. The maximum frequency (Fmax) at the right graticule line equals the inverse of the sample rate, divided by two. Thus, a 20-millisecond sweep time yields a display spanning 20 kHz:

Fmax = 1/(sample rate) 4 2

Fmax = 1/[(sweep time) 4 N] 4 2

Fmax = 1/[20 ms 4 800] 4 2

Fmax = 20 kHz

Where

N equals number of elements in trace array containing amplitude information. 800 is assumed.

Fourier transforms of the prede￿ned weighting algorithms are shown on the following graphs. Use these graphs to estimate resolution and amplitude uncertainty of a Fourier-transform display. Each horizontal division of the graphs equals 1/(sweep time) or Fmax 4 400, which can be calculated from the previous equations.

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