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Bode Plot..

  1. Jan 22, 2016 #1
    I am ploting bode of resistor connected with DC source.So no freqvancy at all!!! The bode magnitude plot should be(or it
    should start from) 20*log(jw)=20*log(0) (jw=0 because we have dc source) what should be the value??for step of 5v.... what
    should bode look like??
  2. jcsd
  3. Jan 22, 2016 #2


    Staff: Mentor

    Maybe it would look like a point. I really don't know because Bode plots are not intended for DC.

    Why are you doing that?
  4. Jan 22, 2016 #3
    yes true bodes are not for DC..... But can you tell me what is the frequency of DC I mean 0 or Infinity???

    Or suppose I have square wave of some frequency....then what would be the transfer function?? for R it would be L^-1(r)=1 or r and for input i think it should be 1/s.....
  5. Jan 22, 2016 #4
    also if you can tell me.....this..

    If we have LTI system and if we apply one frequency say 5HZ the in output will we get same frequency or we may get different
    frequency?? yes amplitude and phase may change that i undestand..
  6. Jan 22, 2016 #5


    Staff: Mentor

    The frequency of DC is zero. A square wave cane be represented as a Forier sum of many frequencies. Google Fourier series.
  7. Jan 23, 2016 #6
    ok i know fourier series..... So you mean we have to get fourier series of the signal and then once we get it.... we take laplace transform of that....?? So that we can analyse bode......
  8. Jan 23, 2016 #7


    Staff: Mentor

    That is still not appropriate for Bode plots. A Bode plot shows the response of a system as a function of frequency. Continuous changes in frequency.

    A Fourier series of a square wave gives you values at discrete integer multiples of the fundamental frequency. When plotted, you would have a series of isolated points, not lines.

    Once you have a specific signal, it doesn't make sense to make a Bode plot of it. Perhaps you are thinking of power spectral density.
  9. Jan 23, 2016 #8
    I am thinking of circuit which has only Resistance and a supply......so Laplace of resistance of 1ohm is always 1 so we only need to take laplace of input to analyse a bode.....
  10. Jan 24, 2016 #9
    hey i am stuck with the question of bode of circuit having resistance connected to sinusoidal source.....

    What i am trying to get is bode of that circuit.... so I take laplace of sin=1/(s^2+1).... Now this is our T.f. If i put diffrent values of frequency do i get the bode magnitude plot??
  11. Jan 24, 2016 #10


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    If your circuit has an input location and an output location, then it has a Bode plot. Otherwise, it doesn't. An input location assumes that you can inject any frequency at that point.
  12. Jan 24, 2016 #11
    that's ok!! My input is Sin wave(Vin) and output is voltage across R(vout).....
  13. Jan 24, 2016 #12


    Staff: Mentor

    A voltage source connected to a resistor has the input terminals and output terminals at the same place. Your transfer function is 1 no matter what the voltage and no matter what the resistance.. A bode plot of 1 is a horizontal line of magnitude 1 for all frequencies. That's pointless.

    I think you have some conceptual problems about transfer functions, circuits, and Bode plots.

    Can you tell us what you are trying to accomplish?
  14. Jan 24, 2016 #13
    you are right!! I understood the things now.... sorry for some silly questions that mess you up!!!
    Thanks for reply.
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