5-25
Making Measurements
FFT Measurement
FFT Measurement
FFT is us ed to c omput e the f ast Fo urier trans form u sing a nalog input chann els
or math functions 1 + 2, 1 – 2, or 1 * 2. FFT takes the digitize d time record of
the sp ecif ied sour ce an d tr ansf orms it t o the freq uen cy do main . Wh en th e FFT
function is selected, the FFT spectrum is plotted on the oscilloscope display as
mag n i t ud e i n dB V v e rs u s f re q u e n cy . T h e r ea d o u t f o r t he h o r iz o n t a l a x i s c h a n g es
from time to frequency (Hertz) and the vertical readout changes from volt s to
dB.
Use the FFT f unction to find c rosstal k problem s, to fin d distor tion pro blems in
analog waveforms caused by amplifier non-line arity, or for adjusting analog
filters.
FFT Units 0 dBV is t he a mpl it ude of a 1 Vr ms s inu soi d. Whe n th e FFT so urc e
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 MΩ.
For the 54640-series, FFT units will be displayed in dBm when channel units is
set to Volts and channel impedance is set to 50Ω. For the 54620-series, if you
want the result to be in dBm, you must connect a 50Ω load (10100C or
equivalent) to the analog channel input, and th en 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 t ake the off set at cen ter scr een int o ac co unt . Th e DC val ue is n ot c orr ect ed
in order to accurately represent frequency components near DC.
Aliasing When using FFTs, it is important to be aware of frequency aliasing.
This re quire s that the op erato r have some k nowle dge as to wha t the f reque ncy
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.