Name

nyquist — nyquist plot

Calling Sequence

nyquist( sl,[fmin,fmax] [,step] [,comments] )
nyquist( sl, frq [,comments] )
nyquist(frq,db,phi [,comments])
nyquist(frq, repf [,comments])

Parameters

sl

syslin list (SIMO linear system in continuous or discrete time )

fmin,fmax

real scalars (frequency bounds (in Hz))

step

real (logarithmic discretization step)

comments

string vector (captions).

frq

vector or matrix of frequencies (in Hz) (one row for each output of sl).

db,phi

real matrices of modulus (in Db) and phases (in degree) (one row for each output of sl).

repf

matrix of complex numbers. Frequency response (one row for aech output of sl)

Description

Nyquist plot i.e Imaginary part versus Real part of the frequency response of sl.

For continuous time systems sl(2*%i*%pi*w) is plotted. For discrete time system or discretized systems sl(exp(2*%i*%pi*w*fd) is used ( fd=1 for discrete time systems and fd=sl('dt') for discretized systems )

sl can be a continuous-time or discrete-time SIMO system (see syslin). In case of multi-output the outputs are plotted with different symbols.

The frequencies are given by the bounds fmin,fmax (in Hz) or by a row-vector (or a matrix for multi-output) frq.

step is the ( logarithmic ) discretization step. (see calfrq for the choice of default value).

comments is a vector of character strings (captions).

db,phi are the matrices of modulus (in Db) and phases (in degrees). (One row for each response).

repf is a matrix of complex numbers. One row for each response.

Default values for fmin and fmax are 1.d-3, 1.d+3 if sl is continuous-time or 1.d-3, 0.5/sl.dt (nyquist frequency) if sl is discrete-time.

Automatic discretization of frequencies is made by calfrq.

Examples

 
clf();
s=poly(0,'s');
h=syslin('c',(s^2+2*0.9*10*s+100)/(s^2+2*0.3*10.1*s+102.01));
comm='(s^2+2*0.9*10*s+100)/(s^2+2*0.3*10.1*s+102.01)';
nyquist(h,0.01,100,comm);
h1=h*syslin('c',(s^2+2*0.1*15.1*s+228.01)/(s^2+2*0.9*15*s+225))
clf();
nyquist([h1;h],0.01,100,['h1';'h'])
clf();nyquist([h1;h])
 

See Also

bode, black, calfrq, freq, repfreq, phasemag