User Manual

Analyzing a wave
form
Table 2: FFT windows (cont.)
Window type Window ‘shape’
Hamming
This is a very good window for resolving frequencies that are very close to the same value
with somewhat improved amplitude accuracy over the rectangular window. It has a slightly
better frequency resolution than the Hanning.
Use H amming for m easuring sine, periodic, and narrow band random noise. This window
works on transients or bursts w here the signal levels before and after the event are
significantly different.
Blackman-Harris
This is the best w indow for measuring the amplitude of frequencies but worst at resolving
frequencies.
Use Blackman-Harris for measuring predominantly single frequency waveforms to look for
higher order harmonics.
FFT and display waveform aliasing
FFT waveform problems occur when the oscilloscope acquires a signal that contains frequency components that are greater
than the Nyquist frequency (the Nyquist frequency is 1/2 of the sample rate). The frequency components that are above the
Nyquis
t frequency are undersampled and appear to "fold back," or reflect, around the right edge of the graticule, showing as
lower-frequency components in the FFT waveform. These incorrect components are called aliases.
You can use the following methods to reduce or eliminate aliases:
Increase the sample rate by adjusting the Horizontal Scale front-panel knob to a faster frequency setting. Since you
increase the Nyquist frequency as you increase the horizontal sampling rate (samples/second), the aliased frequency
components should appear at their proper frequency. If the increased number of frequency components shown on the
screen makes it difficult to measure individual components, use the Horizontal Scale side-menu button in the FFT menu
to display more detail of the FFT waveform.
Use a filter on the input signal to limit the signal to frequencies below that of the Nyquist frequency. If the frequency
components you are interested in are below the built-in 20 MHz bandwidth setting, push the Vertical Menu button
and set the channel bandwidth to 20 MHz.
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