FFT is used to compute the fast Fourier transform using analog input channels. FFT takes the digitized time record of the specified source and transforms it to the frequency domain. When the FFT function is selected, the FFT spectrum is plotted on the oscilloscope display as magnitude in dBV versus frequency. The readout for the horizontal axis changes from time to frequency (Hertz) and the vertical readout changes from volts to dB.
Use the FFT function to find crosstalk problems, to find distortion problems in analog waveforms caused by amplifier non-linearity, or for adjusting analog filters.
Press the Math key, press the FFT softkey, then press the Settings softkey to display the FFT menu.
Press the More FFT softkey to display additional FFT settings.
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.
0 dBV is the amplitude of a 1 Vrms sinusoid. When the FFT
source is channel 1 or channel 2 (or channel 3 or 4 on 4-channel
models), FFT units will be displayed in dBV when channel units
is set to Volts and channel impedance is set to 1 MΩ.
FFT units will be displayed in dBm when channel units is set to
Volts and channel impedance is set to 50Ω.
FFT units will be displayed as dB for all other FFT sources or
when a source channel’s units has been set to Amps.
The number of points acquired for the FFT record is 1000 and
when frequency span is at maximum, all points are displayed.
Once the FFT spectrum is displayed, the frequency span and
center frequency controls are used much like the controls of a
spectrum analyzer to examine the frequency of interest in
greater detail. Place the desired part of the waveform at the
center of the screen and decrease frequency span to increase
the display resolution. As frequency span is decreased, the
number of points shown is reduced, and the display is
magnified.
While the FFT spectrum is displayed, use the Math and Cursors
keys to switch between measurement functions and frequency
domain controls in FFT menu.
Decreasing the effective sampling rate by selecting a slower
sweep speed will increase the low frequency resolution of the
FFT display and also increase the chance that an alias will be
displayed. The resolution of the FFT is the effective sample rate
divided by the number of points in the FFT. The actual
resolution of the display will not be this fine as the shape of the
window will be the actual limiting factor in the FFTs ability to
resolve two closely space frequencies. A good way to test the
ability of the FFT to resolve two closely spaced frequencies is to
examine the sidebands of an amplitude modulated sine wave.
FFT Units
FFT Measurement Hints
Commands and Queries
Syntax | Options and Query Returns |
---|---|
:FUNCtion:DISPlay {{0|OFF}|{1|ON}} | {0 | 1} Switch display off and on. |
:FUNCtion:DISPlay? | |
:FUNCtion:SOURce <source> | <source> ::= {CHANnel<n> | ADD | SUBT | MULT} <n> ::= 1-2 or 1-4 in NR1 format |
:FUNCtion:SOURce? | |
:FUNCtion:OPERation <operation> | <operation> ::= {SUBTract | MULTiply | INTegrate | DIFFerentiate | FFT} |
:FUNCtion:OPERation? | |
FFT Horizontal Controls | |
:FUNCtion:CENTer <frequency> | <frequency> ::= the current center frequency in NR3 format. |
:FUNCtion:CENTer? | |
:FUNCtion:SPAN <span> | <span> ::= the current frequency span in NR3 format. |
:FUNCtion:SPAN? | |
FFT Vertical Controls | |
:FUNCtion:OFFSet <offset> | <offset> ::= the value at center screen in NR3 format. |
:FUNCtion:OFFSet? | |
:FUNCtion:SCALe <scale value>[<suffix>] | <scale value> ::= integer in NR1 format <suffix> ::= {V | dB} |
:FUNCtion:SCALe? | |
:FUNCtion:RANGe <range> | <range> ::= the
full-scale vertical axis value in NR3 format. The range is 8 times the scale. |
:FUNCtion:RANGe? | |
:FUNCtion:REFerence <level> | <level> ::= the
current reference level in NR3 format. Reference and Offset are synonymous. |
:FUNCtion:REFerence? | |
FFT Window Control | |
:FUNCtion:WINDow <window> | <window> ::= {RECTangular | HANNing | FLATtop} |
:FUNCtion:WINDow? |
For return values,
NR1 specifies integer data.
Agilent
Application Note 243, “The Fundamentals of Signal Analysis”.
Maintained by John Loomis,
last updated 20 February 2009
NR3 specifies exponential data in floating point format (for example, -1.0E-3).
Reference