Iwatsu DS-8814, Operation Manual

Page: 130 / 141

F-17
3. DC Component of FFT Waveform
With this product’s FFT, the value of the DC component (0 Hz) is accurately computed
only when the offset voltage value is set to 0V. (The offset voltage value is not considered
for use to increase the gain resolution for other than the DC component.)
4. Frequency Range of FFT Waveform
With a digital oscilloscope, differing from a dedicated spectrum analyzer, the frequency
axis cannot be set to an arbitrary range.
The frequency axis range of the FFT waveform is fixed by setting the sampling rate,
memory length (Length) and number of data points for the FFT (F Points) of this product.
The frequency axis ranges from 0Hz (DC) to the Nyquest frequency (1/2 the FFT
sampling rate).
<Reference>
1) Verify the signal band to analyze.
→
4. Frequency Range of FFT
Waveform
2) Determine the window function.
→
5. FFT Window Selection
3) Determine the number of computation
points.
→
6. F Points Selection and Effect
4) Obtain the spectrum.
The figure at left shows the FFT result for a
100kHz sine wave that was acquired with
TIME/DIV set at 10us/div (10MS/s).
When Length of ACQUISITION menu is short
(1024), the FFT sampling rate is 10.0 MS/s.
The computed results will be 1/2 of the number
of data points, 512 data points. 500 of these data
points will be displayed in the screen.
Procedure for FFT Process
0kHz
X1/2=5MHz (Nyquest frequency)
X10=4.882MHz
Within screen
500 data points
12 data points
Outside screen