User Manual

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21.3 Reference
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f(t
)
Input signa
l
Time domain Frequency domain
t
ω
F(ω
)
0
Fma
x
Sampling
function
Fs > 2･Fma
x
t
Fs > 2･Fma
x
t
Fs > 2･Fma
x
Sampling
function
Fs < 2･Fma
x
t
Fs < 2･Fma
x
t
Fs < 2･Fma
x
Overlapped parts seem folded back
Fs/
2
Section
observed o
n
screen
Aliasing
When the frequency of the signal to be measured approaches the sampling
frequency, beyond a certain point the measured signal frequency will be lower
than the actual signal frequency. In such a case, frequency components that
do not exist will appear in the waveform along the frequency axis. This
phenomenon is called aliasing, and it occurs if sampling is carried out at a
frequency lower than the so-called Nyquist frequency determined by Nyquist’s
sampling theorem.
Sampling theorem
Fs = 2･Fmax
Fmax: Highest frequency component to be measured
Fs: Sampling frequency (Nyquist frequency)
・In order to be able to restore the original waveform from the sampling data,
the sampling frequency must be at least twice as high as the signal frequency.
・If sampling is carried out at a frequency lower than the Nyquist frequency,
frequency components above 1/2 of the sampling frequency will be aliased to
lower frequencies, and the measured signal will appear to contain frequency
components that actually do not exist.