Aliasing - TiePie Handyscope HS4 DIFF-5MHz Instrument Manual

The sampling frequency must be higher than 2 times the highest
frequency in the input signal. This is called the Nyquist fre-
quency. Theoretically it is possible to reconstruct the input signal
with more than 2 samples per period. In practice, 10 to 20 sam-
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
...
-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
Figure 3.8: The effect of the sampling frequency
Table 3.2: Aliasing
1250 Hz signal
-1000 + 1250 = 250
0 + 1250 = 1250
1000 + 1250 = 2250
2000 + 1250 = 3250
-1250 Hz signal
-1000 - 1250 = -2250
0 - 1250 = -1250
1000 - 1250 = -250
2000 - 1250 = 750
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Introduction