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Fast Fourier Transforms
Reference
3Ć48
H 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.
H For records with 30 K, 50 K, or 60 K lengths (not all lengths are available
for all TDS models Ċ 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.
H Trigger on the input impulse.
Adjust Phase Suppression Ċ 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
.Ifthe
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 3Ć37. Select dBV
RMS (side) for the Set FFT Vert Scale to (side).
2. Press CURSOR ➞ Mode (main) ➞ Independent (side) ➞ FuncĆ
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
frequencies of interest above the cursor but places other magnitudes
completely 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 supĆ
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.