Using Features for Advanced Applications
TDS 684A, TDS 744A, & TDS 784A User Manual 3–157
For records with a 100 K length (TDS 700A models with Option 1M only),
set the trigger position to 5%. Use the horizontal position knob to move the
trigger T on screen to the center horizontal graticule line.
Do not use the 15 K (all models), 30 K (TDS 700A models), 75 K, or 130 K
(TDS 700A models equipped with Option 1M) to impulse test using FFTs.
These record lengths do not allow easy alignment of the zero reference point
for phase measurements.
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 VRMS. 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 To Create an FFT that begins on page 3–146. 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
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.