Hioki MR8827, Instruction Manual

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FFT Definitions
Imaging
When the instrument is set to a measurement frequency range that requires a higher sampling rate than
the maximum capability of the module, intermediate 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.
Spectral Imaging
Time-domain waveform in the 8
MHz frequency range (sampling
frequency = 20 MHz)
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 waveform.
Spectrum
When FFT processing is performed on a
stair-step waveform, the resulting spectrum
shows non-existent components.
In this case, the spectral components
above 1 MHz / 2 = 500 kHz are theoretically
meaningless.
Time-domain waveform in the 400
kHz frequency range (sampling
frequency = 1 MHz)
Here, the frequency range matches the
sampling frequency of the Model 8968 so
no interpolation is performed on the time-
domain data.
Spectrum
To avoid imaging phenomena when analyzing waveforms with the FFT function, verify the maximum
sampling frequency of the module before measuring.