LeCroy 9300C Series, Operator'S Manual

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C –3
Using FFT
Figure C–2
An FFT operation on an N point time-domain signal may thus be
compared to passing the signal through a comb filter consisting
of a bank of N/2 filters. All the filters have the same shape and
width and are centered at N/2 discrete frequencies. Each filter
collects the signal energy that falls into the immediate
neighborhood of its center frequency. Thus it can be said that
there are N/2 frequency bins. The distance in Hz between the
center frequencies of two neighboring bins is always the same:
∆
f.
Power (Density) Spectrum
Because of the linear scale used to show magnitudes,
lower amplitude components are often hidden by larger
components. In addition to the functions offering magnitude and
phase representations, the FFT option offers power density and
power spectrum density functions, selected from the “FFT result”
menu shown in the figures. These latter functions are even
better suited for characterizing spectra. The power spectrum (V
2
)
is the square of the magnitude spectrum (0 dB m corresponds to
voltage equivalent to 1 mW into 50
Ω
.) This is the