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Fast Fourier Transforms
Reference
For records with a 15 K length, set the trigger position to 33%. Use the
horizontal position knob to move the trigger T on screen to the center
horizontal graticule line.
For records with 30 K, 50 K, or 60 K lengths (not all lengths are available
for all TDS models — consult your User manual), set the trigger position
to 16.6%,10%, or 8.3%, respectively. Use the horizontal position knob to
move the trigger T on screen and to the center horizontal graticule line.
Trigger on the input impulse.
Adjust Phase Suppression — Your source waveform record may have a
noise component with phase angles that randomly vary from −pi to pi. This
noise could make the phase display unusable. In such a case, use phase
suppression to control the noise.
You specify the phase suppression level in dB with respect to 1 V
RMS
. If the
magnitude of the frequency is greater than this threshold, then its phase
angle will be displayed. However, if it is less than this threshold, then the
phase angle will be set to zero and be displayed as zero degrees or radians.
(The waveform reference indicator at the left side of the graticule indicates
the level where phase is zero for phase FFTs.)
It is easier to determine the level of phase suppression you need if you first
create a
frequency
FFT math waveform of the source and then create a
phase
FFT waveform of the same source. Do the following steps to use a
cursor measurement to determine the suppression level:
1. Do steps 1 through 7 of
Operation
that begins on page 3-39. Select dBV
RMS (side) for the Set FFT Vert Scale to (side).
2. Press CURSOR ➞ Mode (main) ➞ Independent (side) ➞ Func-
tion (main) ➞ H Bars (side). Use the general purpose knob to align the
selected cursor to a level that places the tops of the magnitudes of fre-
quencies of interest above the cursor but places other magnitudes com-
pletely below the cursor.
3. Read the level in dB from the @: readout. Note the level for use in step
5.
4. Press MORE (main) ➞ Change Waveform Definition menu (side).
Press Set FFT Vert Scale to (side) repeatedly to choose either Phase
(rad) or Phase (deg).
5. Press Suppress Phase at Amplitudes (side). Use the general purpose
knob (or keypad if your oscilloscope is so equipped) to set phase sup-
pression to the value obtained using the H Bar cursor. Do not change the
window selection or you will invalidate the results obtained using the
cursor.
FFT Windows
To learn how to optimize your display of FFT data, read about how the FFT
windows data before computing the FFT math waveform. Understanding FFT
windowing can help you get more useful displays.