Using Features for Advanced Applications
3–156 TDS 684A, TDS 744A, & TDS 784A User Manual
Nyquist Frequency
Point
Frequency
Amplitude
Actual FrequenciesAliased Frequencies
Figure 3–92: How Aliased Frequencies Appear in an FFT
When you set up an FFT math waveform to display the phase angle of the
frequencies contained in a waveform, you should take into account the reference
point the phase is measured against. You may also need to use phase suppression
to reduce noise in your FFTs.
Establish a Zero Phase Reference Point. The phase of each frequency is measured
with respect to the zero phase reference point. The zero reference point is the
point at the center of the FFT math waveform but corresponds to various points
on the source (time domain) record. (See Figure 3–90 on page 3–151.)
To measure the phase relative to most source waveforms, you need only to center
the positive peak around the zero phase point. (For instance, center the positive
half cycle for a sine or square wave around the zero phase point.) Use the
following method:
First be sure the FFT math waveform is selected in the More menu, then set
horizontal position to 50% in the Horizontal menu. This positions the zero
phase reference point to the horizontal center of the screen.
In the Horizontal menu, vary the trigger position to center the positive peak
of the source waveform at the horizontal center of screen. Alternately, you
can adjust the trigger level (knob) to bring the positive peak to center screen
if the phase reference waveform has slow enough edges.
When impulse testing and measuring phase, align the impulse input into the
system to the zero reference point of the FFT time domain waveform:
Set the trigger position to 50% and horizontal position to 50% for all record
lengths less than 15 K.
Considerations for PhaseDisplays