TiePie Handyscope HS4 DIFF-5MHz Instrument Manual page 16

signals with a frequency ranging from 0 to 500 Hz. This means
that from the resulting frequencies in the table, we can only see
the 250 Hz signal in the sampled data. This signal is called an
alias of the original signal.
If the sampling frequency is lower than twice the frequency of the
input signal, aliasing will occur. The following illustration shows
what happens.
In figure 3.9, the green input signal (top) is a triangular signal with
a frequency of 1.25 kHz. The signal is sampled with a frequency of
1 kHz. The corresponding sampling interval is 1/1000Hz = 1ms.
The positions at which the signal is sampled are depicted with
the blue dots. The red dotted signal (bottom) is the result of the
reconstruction. The period time of this triangular signal appears
to be 4 ms, which corresponds to an apparent frequency (alias) of
250 Hz (1.25 kHz - 1 kHz).
To avoid aliasing, always start measuring at the highest sam-
pling frequency and lower the sampling frequency if required.
12
Chapter 3
Figure 3.9: Aliasing