AEMC OX 7102 III, User Manual

Page: 81 / 226

Oscilloscope Mode - The "Horizontal" Menu
SCOPIX IV - 81
Oscilloscope Mode (cont'd)
The finite duration of the study interval results in a convolution in the signal
frequency domain with a function sinx/x.
This convolution modifies the graphic representation of the FFT because
of the lateral lobes characteristic of the sinx/x function (unless the study
interval contains an whole number of periods).
Four types of window selections are available: the menus appear directly
on selection of the FFT menu.
Effects of under-sampling on frequency representation:
If the sampling frequency is not correctly adjusted (less than or twice the
maximum frequency of the signal to be measured), the high-frequency
components will be under-sampled and appear in the graphic
representation of the FFT by symmetry (aliasing).
•
The "Autoset" function is active. This prevents the phenomenon above
and adapts the horizontal scale: the representation is more legible.
•
The "Zoom" function is active.
The "
"
symbol in front of one of the options indicates the function
selected.
"EXTENDED
ACQUISITION
MEMORY"
option operation
If the "EXTENDED ACQUISITION MEMORY" option is installed
(see §. Menu " ? " "Options"):
• The acquisition memory is extended from 2,500 samples to 50,000
samples.
• On the time base range [1 ns, 5 ms], the signal image is obtained by the
aggregate of several acquisitions and reconstruction. Deactivate the
"HORIZ " "Repetitive Signal" option if the signal observed is not
periodical, acquired and synchronous with the trigger.
• On the time base range [10 ms, 200 s], 50,000 samples are obtained in
one acquisition.
Type of window
Width of main
lobe
Max. amplitude of
secondary lobe
(compared with main
lobe)
Rectangular window
- 13 dB
4 π /N
Hanning window
- 32 dB
8 π /N
Hamming window
- 43 dB
8 π /N
Blackman window
- 94 dB
12 π /N
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