TiePie Handyscope HS3 User Manual page 12

Multiple of sampling frequency
...
-1000
0
1000
2000
...
As stated before, when sampling a signal, only frequencies lower
than half the sampling frequency can be reconstructed. In this
case the sampling frequency is 1000 Hz, so we can we only observe
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 3.4, the green input signal (top) is a triangular signal
with a frequency of 1.25 kHz. The signal is sampled with a fre-
quency of 1 kHz. The corresponding sampling interval is 1/1000Hz
8
Chapter 3
1250 Hz signal
-1000 + 1250 = 250
0 + 1250 = 1250
1000 + 1250 = 2250
2000 + 1250 = 3250
Table 3.2: Aliasing
Figure 3.4: Aliasing
-1250 Hz signal
-1000 - 1250 = -2250
0 - 1250 = -1250
1000 - 1250 = -250
2000 - 1250 = 750