2
\$\begingroup\$

hope you are all doing well. Right now I am dealing with a homework and I have transfer function and I need to plot the asymptotic Bode Plots of it. I am trying to plot the Magnitude Diagram right now, and I am misevaluating the Magnitude at Low Frequencies. I calculate it as approximately 15dB, whereas MATLAB shows it 35 dB. Here is what I did:

enter image description here

The original question:

enter image description here

What am I missing? I am pretty ill right now, so I am probably missing out a detail, even though I checked it multiple times.

\$\endgroup\$
12
  • 2
    \$\begingroup\$ +1 for identifying this as homework and showing what you have tried. \$\endgroup\$ Commented Apr 18, 2021 at 1:31
  • \$\begingroup\$ question. what is log_10 of 6? What is 20 times that? \$\endgroup\$ Commented Apr 18, 2021 at 1:34
  • 2
    \$\begingroup\$ I assume that you are trying to plot the asymptotic bode for the open loop system. In that case, there is a pole at the origin and so the zero frequency gain is infinite. You can check by substituting s=0 in the third equation. Due to the pole at origin, the asymptote is a sloping line with —20 dB/decade slope until it reaches the first corner frequency. You can verify by substituting 0.00001 rad/s, 0.0001 rad/s etc for s in the third equation. Matlab result may be wrong. What does dcgain(openlooptransferfunction) give a as output in matlab? \$\endgroup\$
    – AJN
    Commented Apr 18, 2021 at 2:15
  • 1
    \$\begingroup\$ If your original question is answered, you can post it as an answer yourself. It is allowed on this site. \$\endgroup\$
    – AJN
    Commented Apr 18, 2021 at 3:24
  • 1
    \$\begingroup\$ @jonk In the lessons we dealt with the open loop transfer functions, therefore I assumed that this one require me to deal with the open loop system too. I asked my friends about it and they confirmed me. However, you might be right, I will ask my lecturer about it. Thanks for the heads up! \$\endgroup\$
    – kucar
    Commented Apr 18, 2021 at 12:21

1 Answer 1

0
\$\begingroup\$

I got what was I missing thanks to everyone who commented. I was not taking the pole at the origin into account. However, the magnitude of this pole starts with a slope of -20dB/decade at infinity, and at w = 0.1 rad/s, it is at 20 dB magnitude. Adding that 20 dB to the 15 dB I had already calculated, yields the starting magnitude as 35 dB, just like on MATLAB. Big thanks to everyone who helped!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.