FFT Measurement

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.

FFT Operation (Manual Controls)
FFT Units
FFT Measurement Hints
Commands and Queries

FFT Operation

Press the Math key, press the FFT softkey, then press the Settings softkey to display the FFT menu.

Source — selects the source for the FFT.
Span — sets the overall width of the FFT spectrum that you see on the display (left to right). Divide span by 10 to calculate the number of Hertz per division. It is possible to set Span above the maximum available frequency, in which case the displayed spectrum will not take up the whole screen. Press the Span softkey, then turn the Entry knob to set the desired frequency span of the display.
Center — sets the FFT spectrum frequency represented at the center vertical grid line of the display. It is possible to set the Center to values below half the span or above the maximum available frequency, in which case the displayed spectrum will not take up the whole screen. Press the Center softkey, then turn the Entry knob to set the desired center frequency of the display.

Press the More FFT softkey to display additional FFT settings.

Scale — lets you 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 — lets you set your own offset for the FFT. The offset value is in dB and is represented by the center horizontal grid line of the display. Press the Offset softkey, then turn the Entry knob to change the offset of your math function.

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.

FFT Units

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.

FFT Measurement Hints

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.

Commands and Queries

Syntax	Options and Query Returns
`:FUNCtion:DISPlay {{0\|OFF}\|{1\|ON}}`	{0 \| 1} Switch display off and on.
`:FUNCtion:DISPlay?`	{0 \| 1} Switch display off and on.
`: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?`	<frequency> ::= the current center frequency in NR3 format.
`:FUNCtion:SPAN <span>`	<span> ::= the current frequency span in NR3 format.
`:FUNCtion:SPAN?`	<span> ::= the current frequency span in NR3 format.
FFT Vertical Controls
`:FUNCtion:OFFSet <offset>`	<offset> ::= the value at center screen in NR3 format.
`:FUNCtion:OFFSet?`	<offset> ::= the value at center screen in NR3 format.
`: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?`	<window> ::= {RECTangular \| HANNing \| FLATtop}

For return values,

NR1 specifies integer data.
NR3 specifies exponential data in floating point format (for example, -1.0E-3).

Reference

Agilent Application Note 243, “The Fundamentals of Signal Analysis”.

Maintained by John Loomis, last updated 20 February 2009