MATLAB: Find and plot 3dB and 6dB downpoints of spectral peak frequency in a PSD

lobe widthpsdspectral peak frequency

I have a PSD and find its peak with:
 Pyy, fyy]= periodogram(filterLow_hyd1,[],[],fs);
 [pk, loc] = findpeaks(Pyy,'Npeaks',1,'SortStr','descend');
 Peak_frequency = fyy(loc);
 pkdB = 10 * log10(pk);
How do I find its 3dB/6dB downpoints?

Best Answer

This is robust in the event you want to find the -3dB and -6dB points on more than one peak:
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
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
Related Question