88 Agilent InfiniiVision 3000 X-Series Oscilloscopes User's Guide
4 Math Waveforms
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 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.
FFT 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 resolution (the quotient of the sampling rate and
the number of FFT points) 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. Because the FFT spectrum is limited by
this frequency, any higher components are displayed at a lower (aliased)
frequency.
Nyquist Frequency and Aliasing in the Frequency Domain
The Nyquist frequency is the highest frequency that any real-time digitizing oscilloscope
can acquire without aliasing. This frequency is half of the sample rate. Frequencies above
the Nyquist frequency will be under sampled, which causes aliasing. The Nyquist
frequency is also called the folding frequency because aliased frequency components fold
back from that frequency when viewing the frequency domain.
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