HP 54540 Series - Page 181

Math/FFT Menu
FFT Softkeys
Offset Softkey
The of fset softkey positions the function vertically on the screen. Offset is
the value at the center of the graticule area. If you adjust the offset so that a
peak is at the vertical center of the graticule area, then you know that the
peak magnitude is the offset value. For example, if the peak of the spike is at
the vertical center of the graticule area, and the offset reading is ~16.2 dBm,
then you know that the peak magnitude is -16.2 dBm.
Window Softkey
The window softkey allows you to select from three windows: rectangular,
Hanning, and flattop. The FFT operation assumes that the time record
repeats indefinitely. Unless there is an integral number of cycles of the
sampled waveform in the record, a discontinuity is created at the end of the
record. This is referred to as leakage. In order to minimize spectral leakage,
windows that approach zero smoothly at the beginning and end of the signal
are employed as filters to the acquired data record.
Windows work by weighting points in the middle of the waveform record
higher than those at the ends of the record. For example, a Hanning window
looks like the first half of a sine wave. The Hanning window multiplies the
points in the center of the record by 1 and multiplies the points at the start
and the end of the record by zero.
The rectangular window multiplies all the points in the record by 1. The
rectangular window is used for transients signals and signals where there are
an integral number of cycles in the time record. The Hanning window is used
for frequency resolution and general purpose use. It is good for resolving two
frequencies that are close together or for making frequency measurements.
The flattop window is the best window for making accurate amplitude
measurements of frequency peaks.
More Softkey
The more softkey returns to the previous Math/FFT softkey menu.
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