This WebWorksheet demonstrates the principle of digital aliasing.
A sinusoid is sampled at a constant rate, defaulted to 100 Hz. The
slider controls the frequency of the sinewave.

W1 displays the sampled sinewave.

W2 compares the sampled sinewave to the original continuous waveform.
The samples are plotted as solid red circles.

W3 computes the Discrete Fourier Transform of the sampled
sinewave using the FFT function. The transform magnitude is displayed. The
location of the peak of the transform represents the measured frequency
value of the sinusoid.

As long as F, the frequency of the sinewave is less than one
half the sample frequency, 1/2 F_{s} (50 Hz), the
calculated frequency compares well with the actual frequency. However,
once the frequency of the sinewave exceeds this value, the measured
frequency reports a value less than the actual frequency. The measured
frequency aliases the actual value to a lower frequency. This result
can also be seen in W2, where the sampled waveform appears as a
slower sinewave than the original continuous waveform.

The frequency value 1/2 F_{s} is referred to as the Nyquist
Frequency. In general, the sample rate of a digital system must be at
least twice as high as the highest frequency produced by the system to
prevent digital aliasing.