Math FFT
122
TDS1000/2000-Series Digital Oscilloscope User Manual
The Math FFT function i ncludes three FFT Window options. The re i s
a trade-off between frequency resolution and amplitude accuracy
with each type of window. What you want to measure and your
source signal charact eristics help you to determine which window to
use.
Window Measure Characteristics
Hanning Periodic
waveforms
Better frequency, poorer magnitude
accuracy than Flattop
Flattop Periodic
waveforms
Better magnitude, poorer frequency
accuracy than Hanning
Rectangular Pulses or
transients
Special-purpose window for waveforms that
do not have discontinuities. This is essential-
ly the same as no window
FFT Aliasing
Problems occur when the oscilloscope acquires a time-domain
waveform containing frequency com ponents that are greater than the
Nyquist frequency (refer to Nyquist Frequency on page 117). The
frequency components that a re a bove th e Nyquist frequency are
undersampled, appearing a s lower frequency components that “fold
back” around the Nyquist frequency. These incorrect components are
called aliases.
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