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Leakage
Observe the Power Spectrum of a sinusoidal waveform having an integral
number of periods in the time window (i.e. the source frequency equals
one of the bin frequencies), using the Rectangular window. The spectrum
contains a sharp component whose value reflects accurately the source
waveform's
amplitude.
For other
input
frequencies
this spectral
component is lower and broader.
The broadening
of the base of the peak, stretching
out into many
neighboring bins is termed the leakage. It is due to the relatively high
side lobes of the filter associated with each frequency bin.
The filter side lobes and the resulting leakage are reduced when one of
the available
window
functions
is applied.
The best reduction
is
provided by the Blackman-Harris and the Flat Top windows. However, this
reduction is offset by a broadening of the main lobe of the filter.
Numbers
of Points
In the 9400A, FFT is computed over the number of points (Transform Size)
selected in the Redefine Menu. The effective number of points, N, is
displayed at the bottom of the FFT Redefine Menu screen. It is always a
sub-multiple
of the number of points actually displayed
in the time
domain.
FFT generates N/2 spectrum points as output. These points are expanded
by linear interpolation
to 12500 points (I0000 points in the case of
10 MHz/div).
Nyquist Frequency
Equal to one half of the total sampling frequency.
Also, f Nyquist = Af * N/2.
In the 9400A, the value of Nyquist frequency is displayed at the bottom
of the FFT Redefine Menu screen.
Picket Fence Effect
Observe again the Power Spectrum of a sinusoidal
waveform having an
integral number of periods in the time window (i.e. the source frequency
equals one of the bin frequencies), using the Rectangular window. The
spectrum is a sharp peak whose value reflects accurately
the source
waveform's amplitude. For other input frequencies the spectrum peak is
lower and broader.
11-18
Fast Fourier Waveform Processing
Option (WP02, V 2.06FT)
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