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Using Features for Advanced Applications

3–206

TDS 500C, TDS 600B, & TDS 700C User Manual

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 To Create an FFT that begins on page 3–193. 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

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.

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.

Windowing Process. The oscilloscope multiplies the FFT time domain record by

one of four FFT windows before it inputs the record to the FFT function. Figure

3–99 shows how the time domain record is processed.

FFT Windows

Tektronix TDS 600B User Manual

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