Math FFT
116 TDS1000/2000-Series Digital Oscilloscope User Manual
Setting Up the Time-Domain Waveform
Before you use FFTmode, you need to set up the time-domain (YT)
waveform. To do so, follow these steps:
1. Push AUTOSET to display a YT waveform.
2. Turn the VERTICAL POSITIO N knob to move the YT
waveform to the center vertically (zerodivisions).
This ensures that the FFT will show a true DC value.
3. Turn the HORIZONTALPOSITION knob to position the part
of the YT waveform that you want to analyz e in the center eight
divisions of the screen.
The oscilloscope calculatesthe FFT spectrum using the center
2048 points of the time-domain waveform.
4. Turn the VOLTS/ DIV knob to ensure that the entire waveform
remains on the screen. The oscilloscope may display erroneous
FFT results (by adding high frequency components) if the ent ire
waveform is not visible.
5. Turn the SEC/DIV knob to provide t he resolution you want in the
FFT spectrum.
6. If possible, setthe oscilloscope to display many signal cycles.
If you turn the SEC/DIV knob to select a faster setting (fewer
cycles), the FFT spectrum shows a larger frequency range, and
reduces the possibilityof FFT aliasing, described on page 122.
However,the oscilloscope also displays less frequency resolution.