LP filter transfer function + Matlab Plot

Discussion in 'General Electronics Chat' started by Shagas, May 8, 2014.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello ,

    Could someone help me out plotting a Bode and nyquist diagram in matlab
    of a simple R/c low pass filter?
    I'm making a presentation of my semestral work in a few weeks of a very basic oscilloscope that I made in java and a small part of the presentation is the input analog ciruitry.

    So first I have to find the transfer function. I've looked on this page and it says that the transfer function will be

    1/(jwRC + 1)

    I understand how they derived that but I don't understand what the j and w will be. Do I consider w as 2pi/T - (2*pi*F) ? What about the j , and how would I plot this function in matlab?

    Also , I have two r/c stages for a steeper low pass but If I remember from the signals course lecture I can just multiply the two transfer functions that are in the S domain to get the total transfer function.
    (I know that two cascaded R/C stages aren't optimal and that I should put a buffer in between but for simplicity sake I just cascaded them without a buffer)

    Thanks in advance
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    I dont use Matlab, but these are the two questions you asked.

    In this context the 'j' indicates advance the phase by 90 degrees.

    The 'w' = 2 * pi * f

    E
     
  3. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Ok thanks for the answer ,
    I will wait for someone who can advise how to punch that in to matlab
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    Does Matlab have an AC analysis function, similar to LTSpice.??

    Thats the method used in LTS.
    E
     
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
    549
    75
    For the Bode plot you can use the filter coefficients and the freqz function (it also creates a phase plot), but it would be easiest to just create an array for frequencies and plot the magnitude of the transfer function against that array.

    I have no experience with Nyzquist diagrams.
     
    Shagas likes this.
  6. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    I think I'll try it using an array of frequencies and ploting the h(s) against it,
    thanks
     
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