Using Features for Advanced Applications
TDS 684A, TDS 744A, & TDS 784A User Manual
3–149
Once you have displayed an FFT math waveform, use cursors to measure its
frequency amplitude or phase angle.
1. Be sure MORE is selected in the channel selection buttons and that the FFT
math waveform is selected in the More main menu.
2. Press CURSOR ➞ Mode (main) ➞ Independent (side) ➞ Func-
tion (main) ➞ H Bars (side).
3. Use the general purpose knob to align the selected cursor (solid line) to the
top (or to any amplitude on the waveform you choose).
4. Press SELECT to select the other cursor. Use the general purpose knob to
align the selected cursor to the bottom (or to any amplitude on the waveform
you choose).
5. Read the amplitude between the two cursors from the D: readout. Read the
amplitude of the selected cursor relative to either 1 V
RMS
(0 dB), ground
(0 volts), or the zero phase level
(0 degrees or 0 radians) from the @:
readout. (The waveform reference indicator at the left side of the graticule
indicates the level where phase is zero for phase FFTs.)
Figure 3–89 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 D: 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
3–147 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 D: 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.
To Take Cursor
Measurements of an FFT
