repfreq

Arguments

sys

A siso or simo linear dynamical system, in state space, transfer function or zpk representations, in continuous or discrete time.

fmin,fmax

two real numbers (lower and upper frequency bounds)

frq

real vector of frequencies (Hz)

step

logarithmic discretization step

splitf

vector of indexes of critical frequencies.

repf

vector of the complex frequency response

Description

repfreq returns the frequency response calculation of a linear system. If sys(s) is the transfer function of Sys, repf(k) equals sys(s) evaluated at s= %i*frq(k)*2*%pi for continuous time systems and at exp(2*%i*%pi*dt*frq(k)) for discrete time systems (dt is the sampling period).

db(k) is the magnitude of repf(k) expressed in dB i.e. db(k)=20*log10(abs(repf(k))) and phi(k) is the phase of repf(k) expressed in degrees.

If fmin,fmax,step are input parameters, the response is calculated for the vector of frequencies frq given by: frq=[10.^((log10(fmin)):step:(log10(fmax))) fmax];

If step is not given, the output parameter frq is calculated by frq=calfrq(sys,fmin,fmax).

Vector frq is split into regular parts with the split vector. frq(splitf(k):splitf(k+1)-1) has no critical frequency. sys has a pole in the range [frq(splitf(k)),frq(splitf(k)+1)] and no poles outside.

Examples

A=diag([-1,-2]);B=[1;1];C=[1,1];
Sys=syslin('c',A,B,C);
frq=0:0.02:5;w=frq*2*%pi; //frq=frequencies in Hz ;w=frequencies in rad/sec;
[frq1,rep] =repfreq(Sys,frq);
[db,phi]=dbphi(rep);
Systf=ss2tf(Sys)    //Transfer function of Sys
x=horner(Systf,w(2)*sqrt(-1))    // x is Systf(s) evaluated at s = i w(2)
rep=20*log(abs(x))/log(10)   //magnitude of x in dB
db(2)    // same as rep
ang=atan(imag(x),real(x));   //in rad.
ang=ang*180/%pi              //in degrees
phi(2)
repf=repfreq(Sys,frq);
repf(2)-x

See also

• bode — Bode plot
• freq — frequency response
• calfrq — frequency response discretization
• horner — polynomial/rational evaluation
• nyquist — nyquist plot
• dbphi — frequency response to phase and magnitude representation

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