Agilent Technologies 54621A, Руководство пользователя

Страница: 198 / 276

5- 28
Making Measurements
FFT Measurement
2 Press the More FFT softkey to display additional FFT settings.
Scale
allows you to set your own vertical scale factors for FFT expressed in
dB/div (decibels/division). Press the Scale softkey, then turn the Entry knob
to rescale your math function.
Offset
allows you to set your own offset for the FFT. The offset value is in dB
and is represented by the center graticule of the display. Press the
Offset
softkey,
then turn the Entry knob to change the offset of your math function.
Window selects a window to apply to your FFT input signal:
Hanning
- window for making accurate frequency measurements or for
resolving two frequencies that are close together.
Flat Top - window for making accurate amplitude measurements of frequency
peaks.
Rectangular
- good frequency resolution and amplitude accuracy, but use only
where there will be no leakage effects. Use on self-windowing waveforms
such as pseudo-random noise, impulses, sine bursts, and decaying sinusoids.
3 To make cursors measurements, press the Cursors key and set the Source
softkey to
Math
.
Use the X1 and X2 cursors to measure frequency values and difference between
two frequency values (
∆
X). Use the Y1 and Y2 cursors to measure amplitude
in dB and difference in amplitude (
∆
Y).
FFT Scale FFT Offset
Return to
previous menu Window
Scale and offset considerations
If you do not manually change the FFT scale or offset settings, when you turn the
horizontal sweep speed knob, the span and center frequency settings will
automatically change to allow optimum viewing of the full spectrum. If you do
manually set scale or offset, turning the sweep speed knob will not change the span
or center frequency settings, allowing you see better detail around a specific
frequency. Pressing the FFT
Preset softkey will automatically rescale the waveform
and span and center will again automatically track the horizontal sweep speed
setting.