ples per period are recommended to be able to examine the signal
thoroughly.
3.3.1
Aliasing
When sampling an analog signal with a certain sampling frequency,
signals appear in the output with frequencies equal to the sum and
difference of the signal frequency and multiples of the sampling
frequency. For example, when the sampling frequency is 1000 Hz
and the signal frequency is 1250 Hz, the following signal frequencies
will be present in the output data:
Multiple of sampling frequency
1250 Hz signal
-1250 Hz signal
...
-1000
-1000 + 1250 =
250
-1000 - 1250 = -2250
0
0 + 1250 = 1250
0 - 1250 = -1250
1000
1000 + 1250 = 2250
1000 - 1250 =
-250
2000
2000 + 1250 = 3250
2000 - 1250 =
750
...
Table 3.3: Aliasing
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.
10
Chapter 3
Summary of Contents for Handyscope TP450
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