You can speed up the computation by selecting a Transform Size smaller
than 25000. The 9400A will select for you the effective Transform Size,
N, equal to or smaller than your selected value, depending on the actual
number of points in the time domain record.
11.6 FFT/9400A Glossary
Aliasing
If the input signal to a sampling acquisition system contains components
whose frequency is greater than the Nyquist frequency (half the sampling
frequency) which results in less than two samples per signal period,
these components will be aliased. That is, their contribution to the
sampled waveform will be indistinguishable from that of the components
below the Nyquist frequency.
In the 9400A, the FFT definition menu displays the effective Nyquist
frequency. You should select the time base and Transform Size resulting
in a Nyquist frequency higher than the highest significant component in
the time domain record.
To help you choose suitable settings for FFT analysis, a table of
Nyquist frequencies is given in Section 11.8.
Coherent Gain
The coherent gain of a filter corresponding to each window function is
1.0 (0 dB) for the Rectangular window and less for other windows.
defines the loss of signal energy due to the multiplication by the
window function. In the 9400A this loss is compensated. Table 11.3 lists
the values for the windows implemented.
ENBV (Equivalent Noise Bandwidth)
For a filter associated with each frequency bin, ENBW is the bandwidth
of an equivalent rectangular filter (having the same gain at the center
frequency) which would collect the same power from a white noise signal
as the filter considered. In Table 11.3, ENBW is listed for each window
function implemented and is given in bins.
Filters
Computing an N-point FFT is equivalent to passing the time domain input
signal through N/2 filters and plotting the outputs of the filters along
the frequency axis. The spacing of filters is Af = I/T and the bandwidth
depends on the window function used (see Frequency bins).
11-16
Fast Fourier Waveform Processing
Option (WP02, V 2.06FT)
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