User`s guide

Spectrum Scope
5-410
the number of samples on which to perform the FFT. The block zero pads or
truncates every channel’s buffer to N
fft
before computing the FFT.
The number of spectra to average is set by the
Number of spectral averages
parameter. Setting this parameter to 1 effectively disables averaging; See
Short-Time FFT for more information.
In order to correctly scale the frequency axis (i.e., to determine the frequencies
against which the transformed input data should be plotted), the block needs
to know the actual sample period of the time-domain input. This is specified by
the
Sample time of original time series parameter, T
s
.
When the
Inherit sample time from input check box is selected, the block
computes the frequency data from the sample period of the input to the block.
This is valid when the following conditions hold:
•The input to the block is the original signal, with no samples added or deleted
(by insertion of zeros, for example).
•The sample period of the time-domain signal in the simulation is equal to the
period with which the physical signal was originally sampled.
One example when these conditions do not hold, is such as when the input to
the block is not the original signal, but a zero-padded or otherwise rate-altered
version. In such cases, you should specify the appropriate value for the
Sample
time of original time-series
parameter.
The
Frequency units parameter specifies whether the frequency axis values
should be in units of Hertz or rad/s, and the
Frequency range parameter
specifies the range of frequencies over which the magnitudes in the input
should be plotted. The available options are
[0..Fs/2], [-Fs/2..Fs/2], and [0..Fs],
where F
s
is the time-domain signal’s actual sample frequency. If the
Frequency units parameter specifies Hertz, the spacing between frequency
points is 1/(N
fft
T
s
). For Frequency units of rad/sec, the spacing between
frequency points is 2π/(N
fft
T
s
).
Note that all of the FFT-based blocks in the DSP Blockset, including those in
the Power Spectrum Estimation library, compute the FFT at frequencies in the
range [0,F
s
). The Frequency range parameter controls only the displayed
range of the signal.