plotting equation which is function of 2 variable

I have one equation in terms of 2 variable A,B, I want to plot magnitude vs phase for different value of A and B .

i don't know how to do this . i was trying but it won't work

for A = -20:1:20

for B = -20:1:20

g=(1+i*B)/(1+i*A);

M=20*log10(abs(g)); %Magnitude matrix of Compensator

M = round(M,2);

P =rad2deg(angle(g)); %Phase Matrix of compensator

P = round(P,2);

plot(P,M);

hold on

end

end

trying to plot this figure

then

Find value of A and B for particualr value of phase and magnitude.

Answer 1

[Cris LaPierre]

Your code is close. I would take advantage of meshgrid instead of the for loops. Here's what I get making that change. Also, there is no need to round.

[A,B] = meshgrid(-20:1:20);
g=(1+i*B)./(1+i*A);
M=20*log10(abs(g));                 %Magnitude matrix of Compensator
P=rad2deg(angle(g));                %Phase Matrix of compensator
plot(P,M,'k')

[Cris LaPierre]

Looking at your plot, you don't want even spacing in you A & B values. Hard code the values displayed on the plot.

[A,B] = meshgrid([-80 -33 -14 -6 -2.7 -1.2 -0.5 0 .5 1.2 2.7 6 14 33 80]);
g=(1+i*B)./(1+i*A);
M=20*log10(abs(g));                 %Magnitude matrix of Compensator
P =rad2deg(angle(g));                %Phase Matrix of compensator
plot(P,M,'k',P,-M,'k')

[RJS]

Thanks @Cris LaPierre

how do reverse that is Finding value of A and B for particualr value of phase and magnitude.

[Cris LaPierre]

The trick for me was to realize that the lines of B are the negative of the magnitudes of A, so I just plotted +M and -M against phase.

plot(P,M,'k',P,-M,'k')
%           (^,^^) ->2nd plot using negative magnitude.

You could compute it if you want using g=(1+i*A)./(1+i*B);

[RJS]

yes @Cris LaPierre but I also want to obtain values of A and B for particular magnitude and phase value .....Consider I have magnitude 10dB and phase 11 deg find value single value of A and B from that chart you have drawn to give magnitude nad phase requirment.

[Cris LaPierre]

Perhaps try run your equations in reverse?

[RJS]

I am not getting how to do?

syms a b

g=(1+i*b)/(1+i*a)

E = [20*log10(abs(g)) == 10, rad2deg(angle(g)) == 12];

S = solve(E,a,b);

Warning: Unable to find explicit solution. For options, see help.

> In solve (line 317)

empty a and b

[Cris LaPierre]

Perhaps try adding an assumption that a and b are real.

syms a b real

g=(1+i*b)/(1+i*a);
E = [20*log10(abs(g)) == 10, rad2deg(angle(g)) == 12];
S = vpasolve(E,a,b)

S = struct with fields:
    a: 3.1836585625055579563412070062939
    b: 10.505085360250508538864040773687

Now test the result to see if it results in the expected M and P values.

A=double(S.a);
B=double(S.b);
g=(1+i*B)./(1+i*A);
M=20*log10(abs(g)) % Should be 10

M = 10.0000

P =rad2deg(angle(g)) % Should be 12

P = 12.0000

[RJS]

Yes i did