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Using Features for Advanced Applications
TDS 500C, TDS 600B, & TDS 700C User Manual
3–201
The following topics contain information to help you display your FFT properly.
Adjust for a Non-Clipped Display. To properly display your FFT waveform, scale
the source waveform so it is not clipped.
H You should scale and position the source waveform so it is contained on
screen. (Off-screen waveforms may be clipped, resulting in errors in the FFT
waveform).
Alternately, to get maximum vertical resolution, you can display source
waveforms with amplitudes up to two divisions greater than that of the
screen. If you do, turn on Pk-Pk in the measurement menu and monitor the
source waveform for clipping.
H Use vertical position and vertical offset to position your source waveform.
As long as the source waveform is not clipped, its vertical position and
vertical offset will not affect your FFT waveform except at DC. (DC
correction is discussed below.)
Adjust Offset and Position to Zero for DC Correction. Normally, the output of a
standard FFT computation yields a DC value that is twice as large as it should be
with respect to the other frequencies. Also, the selection of window type
introduces errors in the DC value of an FFT.
The displayed output of the FFT on TDS oscilloscopes is corrected for these
errors to show the true value for the DC component of the input signal. The
Position and Offset must be set to zero for the source waveform in the Vertical
menu. When measuring the amplitude at DC, remember that 1 VDC equals
1 V
RMS
and the display is in dB.
Most often, you will want to use a short record length because more of the FFT
waveform can be seen on screen and long record lengths can slow oscilloscope
response. However, long record lengths lower the noise relative to the signal and
increase the frequency resolution for the FFT. More important, they might be
needed to capture the waveform feature you want to include in the FFT.
Offset, Position, and Scale
Record Length