Help plotting frequency versus voltage

I've been searching around for longer than I'd like to admit trying to figure out how to make a plot of frequency vs voltage. I have a series RLC circuit in which the max voltage was taken for 5 different resistances. I need to mark on both sides of the curve at 0.707 times the max voltage. I have all of these voltage values along with their corresponding frequencies but I have no idea how to create a graph in MATLAB that does this. I found this code that comes close to what I want to do but every time I try to get rid of the 6dB marker I just get errors or a blank graph.

if true
  % code
end
fyy = linspace(0, 50, 250);                             % Create Data
Pyy = 0.5*exp(-(fyy-15).^2) + 0.3*exp(-(fyy-40).^2);        % Create Data
[pk,loc] = findpeaks(Pyy,'Npeaks',1,'SortStr','descend');
db3c = 10^(-3/10);                                      % Relative Magnitude At -3dB
db6c = 10^(-6/10);                                      % Relative Magnitude At -6dB
ofst = 10;
for k1 = 1:length(pk)
  varmtx = [Pyy(loc(k1)-ofst:loc(k1)); fyy(loc(k1)-ofst:loc(k1)); Pyy(loc(k1):loc(k1)+ofst); fyy(loc(k1):loc(k1)+ofst)];
  dBpts(k1,1:2) = interp1(varmtx(1,:), varmtx(2,:), pk(k1)*[db6c db3c], 'linear','extrap');
  dBpts(k1,3:4) = interp1(varmtx(3,:), varmtx(4,:), pk(k1)*[db6c db3c], 'linear','extrap');
end

if true
  % code
end
figure(1)
plot(fyy, Pyy)
hold on
for k1 = 1:length(pk)
  plot(dBpts(k1,:), pk(k1)*[db6c db3c db6c db3c], 'r+')
end
hold off
grid
for k1 = 1:length(pk)
  fprintf(1, '\n\t-6dB frequencies = %.3f, %.3f\n', dBpts(k1,[1 3]))
  fprintf(1, '\n\t-3dB frequencies = %.3f, %.3f\n', dBpts(k1,[2 4]))
end

Could someone please help me with this? It's supposed to look like a bell curve with the mean being the resonant frequency, and the frequencies that match up with the -3dB voltage would indicate the bandwidth.

Answer 1

[Star Strider]

If you want to ‘get rid’ of the -6 dB marker, one easy way to do it without disrupting the rest of your code is to define it as ‘NaN’:

[pk,loc] = findpeaks(Pyy,'Npeaks',1,'SortStr','descend')
db3c = 10^(-3/10);                                      % Relative Magnitude At -3dB
db6c = 10^(-6/10);                                      % Relative Magnitude At -6dB
db6c = NaN;                                             % <— INSERT THIS ASSIGNMENT HERE

That worked when I used it with your code.

[Brian Hoblin]

Thanks, that did get rid of that. Where in the code do I need to edit to get the -3db frequencies to equal 764 and 3609? I changed Line 2 to

if true
  % code
Pyy = 0.693*exp(-(fyy-15).^2);
end

because my measurement peaked at 0.693V but as soon as I mess with the other parameters I gets errors. I need the curve to be centered at 1721 Hz and the -3db markers to be located at (764,0.490) and (3609,0.490). How would I do that?

[Star Strider]

My pleasure.

I have no idea.

None of your peaks are anywhere near those frequencies. You need to look through your initial calculations to see what the problem is.