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 Nyquist 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.
Analyzing a waveform
TBS2000B Series Oscilloscopes User Manual 119
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