Hioki 8847, Instruction Manual

Page: 354 / 372

Appendix 5 FFT Definitions
A
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Imaging ______________________________________________________
When the instrument is set to a measurement frequency range that requires a
higher sampling rate than the maximum capability of the input module, interme-
diate data points are interpolated between successive data samples. In this
case, the time-domain waveform exhibits a stair-step shape. When FFT analysis
is performed in this situation, non-existent high frequency spectral components
appear. This phenomena is called zero-order hold characteristic imaging.
The following figures show the time-domain waveform and spectrum of a sine
wave applied to the Model 8968 High Resolution Unit.
To avoid imaging phenomena when analyzing waveforms with the FFT function,
verify the maximum sampling frequency of the input module before measuring.
Spectral Imaging
Time-domain waveform
in the 8 MHz frequency range
(sampling frequency = 20 MHz)
Spectrum
The highest sampling frequency of the
Model 8968 is 1 MHz, so the same input
data value is used for each block of 20
samples, resulting in a stair-step wave-
form.
When FFT processing is performed on
a stair-step waveform, the resulting
spectrum shows non-existent compo-
nents.
In this case, the spectral components
above 1 MHz / 2 = 500 kHz are theoret-
ically meaningless.
Spectrum
Here, the frequency range matches the
sampling frequency of the Model 8968 so
no interpolation is performed on the time-
domain data.
Time-domain waveform
in the 400 kHz frequency range
(sampling frequency = 1 MHz)