Low Pass Response in LTSpice

Discussion in 'General Electronics Chat' started by Sparky49, Dec 9, 2012.

  1. Sparky49

    Sparky49 Thread Starter Active Member

    Jul 16, 2011
    Hi guys. :)

    I've found this awesome program called LTSpice which lets you model circuits, and I've been playing around with it today.

    I put in a simple low pass filter, and when I tested it, I got the response.

    May I ask what causes the sudden change in attenuation?

    Also, I am not aware of an equation to find the gradient of the response - does anyone know one?

    Many thanks for your time,


    Attached Files:

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  2. Papabravo

    Papabravo AAC Fanatic!

    Feb 24, 2006
    Michigan, USA (GMT-5)
    It is a nice discovery. Some of us have known about it for almost a decade or more.

    It is a "feature" of the capacitor that you cannot chnge the voltage across it instantaneously. As the frequency of the generator goes up tha capactitor cannot keep up with the input because it takes time to move charge onto and off of the plates.
  3. crutschow

    crutschow AAC Fanatic!

    Mar 14, 2008
    L.A. USA Zulu -8
    The equation for the filter is here. The reason your response looks rather odd is the the rolloff frequency of your filter is .006Hz and you are plotting the circuit with a linear frequency scale out to 20kHz. Typically you use a log frequency scale (octave or decade in LTspice) to plot frequency response. So on your simulation command, change the Type of Sweep to Octave or Decade, the Number of Points to 10, the Start Frequency to .0001 and the Stop Frequency to 10, to get a more normal looking plot.
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