Another way to recognize aliasing is to realize that higher order harmonics usually have decreasing magnitudes compared to lower order
harmonics. Thus, if you see a series of increasing harmonic magnitude values as frequency increases then you can suspect that they
may be aliased. In the spectral math waveform, the actual higher frequency components are under sampled, and therefore they appear as
lower frequency aliases that "fold back" around the Nyquist point. (See the next figure.) Y
ou may test by increasing the sample rate and
observing if aliases unwrap to different frequency positions.
If you have a variable-frequency signal source, another way to observe aliasing is to adjust the frequency slowly while watching the
spectral display
. If some of the harmonics are aliased, you will see the harmonics decreasing in frequency when they should be increasing
or vice versa.
Blackman-Harris FFT window concepts
FFT windows have various resolution bandwidths and scallop losses (see the figure below). Choose the one that best allows you to view
the signal characteristics you are interested in.
The Blackman-Harris window has a low amount of energy leakage compared to the other windows. Its best use is for single frequency
signals to look for higher order harmonics.
Blackman-Harris window
Measurement concepts
2 Series MSO MSO24 and MSO22 251
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