analysisbode plotMATLAB
I have this bode diagram. I am wondering how to interpret this. We start off with 90 degree phase lag e.g. a integrator. Then at approx 12 Hz we drop another 180 degrees indicating two integrators.
Now my problem is with the ripples which start at about 60 Hz and continue to blow up till it reaches some maximum? and then it stops at 4KHz? What are these?
ps. it has to do with this question, Derive transfer function from block diagram, for which I was able to derive the frequency response.
That transfer function can be rewritten as
$$H(s)=\frac{40}{s}.\frac{1}{\frac{s}{0.25}+1}.(\frac{s}{10}+1).\frac{1}{\frac{s}{25}+1}$$
Each term leads to a asymptote with +-20 dB/decade inclination on semi-log scale. The figure below shows two magnitude plots (handwritten and software based):
Now, I leave you working on the phase plot...
You can add or subtract 360 deg (2*pi rad) as many times as needed in Bode phase graph. Right click in your graph, Properties > Options > Phase Response > Adjust phase offsets [on] > Keep phase close to [-pi] (in your case), At frequency [0.000]. Done in MATLAB R2015a.
Edit: still works in MATLAB R2020b.
Best Answer
Partial answer only:
Check the period associated with the frequency of each notch in the frequency response. You will probably find they are integer multiples of the shortest period. This is typical of a "comb filter", often implemented as the sum of a signal and a delayed version of itself (the notch corresponds to delay=half a cycle). So look for a delay line in the block diagram.