TDS2MM Measurement Module
TDS 200-Series Extension Module Instructions
31
Setting Up the Time-Domain Waveform
It is important to correctly set up the time-domain (YT) waveform to
produce a useable FFT waveform. Follow these steps to set up the
YT waveform:
1. Press AUTOSET to display a YT waveform.
2. Position the YT waveform vertically at center screen (zero
divisions) to get a true DC value. A standard FFT computation
yields a DC value that is twice as large as it should be with
respect to the other frequencies. The TDS2MM Module corrects
this error.
3. Position the YT waveform horizontally so that the waveform area
of interest is contained in the center eight divisions. The
TDS2MM Module uses the center 2048 points of the time-do-
main waveform to transform into an FFT waveform.
4. Set the YT waveform VOLTS/DIV so that the signal does not go
off screen. (Off screen waveform peaks can result in FFT
waveform errors).
5. Set the YT waveform SEC/DIV to provide the resolution you
want in the FFT waveform. It is important to show a number of
signal cycles if possible. Setting the time/division to a faster
setting means the FFT waveform shows a larger frequency range,
has less frequency resolution and reduces aliasing (refer to
Aliasing on page 38 for more information).
In many cases, you can produce a useful FFT waveform even if the
YT waveform is not triggered. This is especially true if your signal is
periodic or random (noisy). However, transient or burst waveforms
should be triggered and positioned as close as possible to center
screen.
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