National Instruments 370753C-01 Manual De Usuario

-0.52263 

0.00336213 + 3.75217 j

0.00336213 - 3.75217 j

-11.4841 

Two of the poles of the closed-loop system are now unstable.

A key characteristic of the linear, time-invariant systems represented in 
Xmath is that the transfer function between a system input and a system 
output is just the Fourier transform of the response at that output to a delta 
impulse at that input. The power spectral density of a time series is defined 
as the Fourier transform of the autocorrelation function of the series. 

Given these two concepts, you can obtain the power spectral density of the 
output of a linear, time-invariant system just by knowing the power spectral 
density of the input and the system’s transfer function [Leo89], [GrD86]. 
Representing the transfer function by H(q) and the power spectral densities 
of the input and output as S

(q) and S

(q), respectively:

You also can obtain the cross-power spectral densities:

These results indicate that you can shape the spectrum of a linear system’s 
output by using an input with an appropriate spectrum. Alternatively, you 
can choose a system to give you the output spectrum you want, given a 
fixed set of input data. When you use linear systems in transfer-function 
form for such applications, you generally refer to them as filters rather than 
systems.

[Ypsd,Yspec] = psd(Sys,{Uspec})

The 

psd( )

 function computes the power spectral density and 

cross-spectral density of a system’s outputs as a function of frequency, 
given the frequency-dependent input power spectral density matrices. The 
input parameter 

Uspec

 is a PDM where domain contains the frequency 

( )

( )

2

( )

=

( )

( )S

( )

=

( )

H∗ q

( )S

( )

=