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Oscilloscope Mode (cont'd)
Min/Max
Acquisition
Averaging
No averaging
Average rate 2
Average rate 4
Average rate 16
Average rate 64
FFT
(Fast Fourier
Transform)
Description
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Use this mode to view the extreme values of the signal acquired between
2 acquisition memory samples.
This mode:
detects wrong representation due to under-sampling
•
displays short-term events (Glitch, ≥ 2 ns).
•
Whatever time base is used and the corresponding sampling speed, short-
term events (Glitch, ≥ 2 ns) are displayed.
The "
" symbol indicates that the "Min/Max Acquisition" mode is active.
Selection of a coefficient to calculate an average for the displayed samples.
For instance, this is a way of attenuating random noise observed in a signal.
For the averaging coefficient to be taken into account for representation of the
signal, the "Repetitive signal" option must be selected.
The calculation is performed using the following formula:
Pixel
= Sample*1/Average rate + Pixel
N
with:
Value of new sample acquired at abscissa t
Sample
Ordinate of pixel with abscissa t on the screen, at moment N
Pixel N
Ordinate of pixel with abscissa t on screen, at moment N-1
Pixel N-1
The " " symbol indicates the averaging coefficient selected.
The Fast Fourier Transform (FFT) is used for calculating the discrete
representation of a signal in a frequency domain from its discrete
representation in the time domain.
FFT can be used in the following applications:
•
measurement of the different harmonics and the distortion of a signal,
•
analysis of a pulse response,
•
search for noise source in logic circuits.
The FFT is calculated over 2500 points.
The Fast Fourier Transform is selected by the FFT icon in the control area.
When the trace is zoomed, the FFT applies to the zoomed part of the trace
(next to the time base adjustment).
The Fast Fourier Transform is calculated using the equation:
N
−
1
1
2
∑
*
x n
( ) * exp −
X (k) =
N
N
n
= −
2
with:
x (n): a sample in the time domain
X (k): a sample in the frequency domain
N: resolution of the FFT
n: time index
k: frequency index
The displayed trace represents the amplitude in V or dB of the various signal
frequency components according to the scale selected (linear or logarithmic).
The DC component of the signal is removed by software.
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Oscilloscope Mode - The "Horizontal" Menu
(1-1/Average rate)
N-1
2
π
nk
⎛
⎞
j
⎜
⎟
for k ∈ [0 (N – 1) ]
⎝
⎠
N
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