Wideband FFT analysis An increasing number of today’s signals have modulation present that can increase the spectral width to hundreds of megahertz or even multiple gigahertz. If spectral widths of signals are beyond around 500 MHz, then spectrum analyzers or vector signal analyzers available today do not have enough analysis bandwidth to make meaningful measurements. In such cases, an oscilloscope or digitizer is required that has enough analysis bandwidth for the application. The carrier frequency of a signal of interest also is important. The carrier frequency of the signal under test plus half the spectral width of that signal must be less than or equal to the oscilloscope bandwidth for the oscilloscope to be used on its own for the measurement. A wideband signal frequency domain measurement will now be considered. The signal under test is a 600-MHz RF pulse train, with 4-μs-wide RF pulses repeating every 20 μs. There is a linear frequency modulation of the signal that chirps the carrier frequency from 300 MHz at the start of the RF pulse envelope to 900 MHz at the end of the pulse envelope. To make a basic FFT measurement of the RF pulse, the first step is to get a clean time- domain capture of a pulse from the signal on screen. The scope is reset to a known condition by pressing Default Setup. Then Auto Scale is pressed, and the time/division setting is adjusted to bring one main RF pulse on screen. The basic default rising-edge trigger is further qualified with trigger holdoff. This ensures that a trigger doesn’t happen mid-pulse since that would create instability in the captured trace. The trigger holdoff is set to something slightly longer than the width of the RF pulse. The RF pulse is 4 μs wide so a trigger holdoff of 5 μs works well.
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