Agilent Technologies InfiniiVision 5000 Series User Manual

6 Measurements and Math Functions
236 InfiniiVision Oscilloscopes User’s Guide
FFT Measurement
FFT is used to compute the fast Fourier transform using analog input channels or an
arithmetic operation g(t). 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 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 M.
FFT units will be displayed in dBm when channel units is set to Volts and channel
impedance is set to 50.
FFT units will be displayed as dB for all other FFT sources or when a source channel’s
units has been set to Amps.
DC Value
The FFT computation produces a DC value that is incorrect. It does not take the offset at
center screen into account. The DC value is not corrected in order to accurately represent
frequency components near DC.
Aliasing
When using FFTs, it is important to be aware of frequency aliasing. This requires that the
operator have some knowledge as to what the frequency domain should contain, and also
consider the sampling rate, frequency span, and oscilloscope vertical bandwidth when
making FFT measurements. The FFT sample rate is displayed directly above the softkeys
when the FFT Menu is displayed.
Aliasing happens when there are frequency components in the signal higher than half the
sample rate. Since the FFT spectrum is limited by this frequency, any higher components
are displayed at a lower (aliased) frequency.

Agilent Technologies InfiniiVision 5000 Series Specifications

General

Brand	Agilent Technologies
Model	InfiniiVision 5000 Series
Category	Test Equipment
Language	English