Fast Fourier Transforms
3–32
TDS 420A, TDS 430A, TDS 460A & TDS 510A User Manual
Figure 3–18 shows the cursor measurement of a frequency magnitude on an
FFT. The @: readout reads 0 dB because it is aligned with the 1 V
RMS
level.
The : readout reads 24.4 dB indicating the magnitude of the frequency it is
measuring is –24.4 dB relative to 1 V
RMS
. The source waveform is turned
off in the display.
The cursor units will be in dB or volts for FFTs measuring magnitude and in
degrees or radians for those FFTs measuring phase. The cursor unit depends
on the selection made for Set FFT Vert Scale to (side). See step 7 on
page 3–29 for more information.
6. Press V Bars
(side). Use the general purpose knob to align one of the two
vertical cursors to a point of interest along the horizontal axis of the
waveform.
7. Press SELECT to select the alternate cursor.
8. Align the selected cursor to another point of interest on the math waveform.
9. Read the frequency difference between the cursors from the : readout. Read
the frequency of the selected cursor relative to the zero frequency point from
the @: readout.
The cursor units will always be in Hz, regardless of the setting in the Time
Units side menu. The first point of the FFT record is the zero frequency
point for the @: readout.
Figure 3–18: Cursor Measurement of an FFT Waveform
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