HP 54540 Series - Page 180

Math/FFT Menu
FFT Softkeys
The memory bar above the graticule indicates what portion of the acquisition
record is used for the FFT time record. The memory bar shows the FFT
record whenever a FFT menu is selected.
An FFT with an input record of N points is transformed into a frequency
record of N points. Because half of the points are above the Nyquist
frequency and provide redundant information, they are not used, so the
frequency points are half that of the input. The FFTs are computed on
records that are in powers of two.
If the number of FFT points is 512, 256 additional frequency points are
created by interpolating between the actual points to give 512 frequency
points. The number of FFT points and the resultant frequency points is
listed below.
Input points Frequency points
512 (repetitive) 512
512 (real time) 512
1024 512
2048 1024
4096 2048
8192 4096
16384 8192
32768 16384
Sensitivity Softkey
The sensitivity softkey scales the function vertically by using software
expansion. It does not affect the hardware settings. The scaling units are in
dB per division. For example, if the scale is set to 10 dB/div, and a peak is
two divisions high, you know that the amplitude of the frequency peak is
20 dB. This setting is for ease of viewing and making measurements on the
function.
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