Fast Fourier Transforms
3–38
TDS 420A, TDS 430A, TDS 460A & TDS 510A User Manual
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, 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 3–28. 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 frequen-
cies 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 to set phase suppression 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.
To Next Page
Loading...
