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Making Measurements
FFT Measurement
FFT Measurement
FFT is used to compute the fast Fourier transform using analog input channels
or math functions 1 + 2, 1 – 2, or 1 * 2. 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
magn it ud e i n d BV ve rs us fr equ en cy. Th e r ea do ut fo r t he h orizo nt al ax is ch an ge s
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 the 54624A), FFT units will be
displayed in dBV when channel units is set to Volts and channel impedance is
set to 1 M
Ω.
For the 54640-series, FFT units will be displayed in dBm when channel units is
set to Volts and channel impedance is set to 50
Ω. For the 54620-series, if you
want the result to be in dBm, you must connect a 50
Ω load (10100C or
equivalent) to the analog channel input, and then perform the following
conversion:
dBm = dBV + 13.01
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 effective 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 effective sample rate. Since the FFT spectrum is limited by this
frequency, any higher components are displayed at a lower (aliased) frequency.