How to make the zplane function to plot on a existing axes?

The zplane function always create a new plot. HOW to let it on a exisiting axes? Or any workaround?

Answer 1

[Paul]

You can specify an existing axis via a third argument. I didn't see this on the doc page, but it does show up in the help,

help zplane

 ZPLANE Z-plane zero-pole plot.
    ZPLANE(Z,P) plots the zeros Z and poles P (in column vectors) with the 
    unit circle for reference.  Each zero is represented with a 'o' and 
    each pole with a 'x' on the plot.  Multiple zeros and poles are 
    indicated by the multiplicity number shown to the upper right of the 
    zero or pole.  ZPLANE(Z,P) where Z and/or P is a matrix, plots the zeros
    or poles in different columns using the colors specified by the axes 
    ColorOrder property.
 
    ZPLANE(B,A) where B and A are row vectors containing transfer function
    polynomial coefficients plots the poles and zeros of B(z)/A(z).  Note
    that if B and A are both scalars they will be interpreted as Z and P.
 
    [HZ,HP,Hl] = ZPLANE(Z,P) returns vectors of handles to the lines and 
    text objects generated.  HZ is a vector of handles to the zeros lines, 
    HP is a vector of handles to the poles lines, and Hl is a vector of 
    handles to the axes / unit circle line and to text objects which are 
    present when there are multiple zeros or poles.  In case there are no 
    zeros or no poles, HZ or HP is set to the empty matrix [].
 
    ZPLANE(Z,P,AX) puts the plot into the axes specified by the handle AX. 
 
    [HZ,HP,Hl] = ZPLANE(D) plots the poles and zeros of digital filter D. 
    You design a digital filter, D, by calling the designfilt function.
 
    % Example 1:
    %   Design a lowpass FIR filter with normalized cut-off frequency at 
    %   0.3 and show its zero-pole plot.
 
    b=fircls1(54,0.3,0.02,0.008);
    zplane(b)                       % zero-pole plot for filter
 
    % Example 2: 
    %   Design a 5th order lowpass elliptic IIR filter and show its 
    %   zero-pole plot.
 
    [b,a] = ellip(5,0.5,20,0.4);
    zplane(b,a)                     % zero-pole plot for filter
 
    % Example 3:
    %   Design a lowpass Butterworth IIR filter and show its Zero-Pole
    %   plot.
 
    [z,p] = butter(4,0.15);
    zplane(z,p)                     % zero-pole plot for filter
 
    % Example 4:
    %   Use the designfilt function to design a highpass IIR digital filter 
    %   with order 8, passband frequency of 75 KHz, and a passband ripple 
    %   of 0.2 dB. Sample rate is 200 KHz. Visualize the zero-pole plot.
   
    D = designfilt('highpassiir', 'FilterOrder', 8, ...
             'PassbandFrequency', 75e3, 'PassbandRipple', 0.2,...
             'SampleRate', 200e3);
 
    zplane(D)
 
    See also FREQZ, GRPDELAY, IMPZ, FVTOOL.

    Documentation for zplane
       doc zplane

The following code works fine on my local installation. It results in a figure with three lines and the zplane pole/zero plot..Don't know why it's not working here?

figure;
rng(101);
plot(rand(3));
hax = gca;
zplane([1 2 3],[3 2 1],hax)

Here is the figure I get on my machine:

[D D]

Why is it not on the doc page... Thank you!

[Paul]

There should be link or something similar at the bottom of the doc page that allows you to provide feedback on a doc page if you wish.

Answer 2

[Voss]

I don't think you can do that with zplane(), but you can, in your own axes, fairly easily reproduce the lines that zplane() creates:

% First a zplane plot for reference:
[b,a] = ellip(4,3,30,200/500);
zplane(b,a);

% Now a new figure and axes with some stuff in it:
figure();
plot(linspace(-1,1,10),linspace(-1,1,10),'rs','LineWidth',2);
hold on

% Now, plotting what zplane(b,a) would plot:
axis equal
z = roots(b);
p = roots(a);
t = linspace(0,2*pi,1000);
blue = [0 0.447 0.741];
xl = get(gca(),'XLim');
yl = get(gca(),'YLim');
line(xl,[0 0],'Color',blue,'LineStyle',':','LineWidth',0.5);
line([0 0],yl,'Color',blue,'LineStyle',':','LineWidth',0.5);
line(cos(t),sin(t),'Color',blue,'LineStyle',':','LineWidth',0.5);
line(real(z),imag(z),'Color',blue,'LineStyle','none','Marker','o','MarkerSize',7);
line(real(p),imag(p),'Color',blue,'LineStyle','none','Marker','x','MarkerSize',8);
xlabel('Real Part');
ylabel('Imaginary Part');

[D D]

Thanks!