Low Pass Filter

Discussion in 'General Electronics Chat' started by Dritech, May 13, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    Hi all,

    I was designing a low pass filter on Matlab and when I zoomed on the output I noticed an unstable output at start and a phase shift.
    What causes the unstable output on start-up and the phase shift in filters? Is there a way to reduce these?
    (In red is the output of the filter and the blue signal is the input)

    Thanks in advance.
  2. Papabravo


    Feb 24, 2006
    I don't see any unstable output. You should review the definition of that term. What you are seeing at the beginning is the transient response. What you see later is the steady state response. We need more details on your design to answer your question about what you might be able to do.
    1. Is the filter active or passive?
    2. Schematic of the design?
    3. Bill of materials?
  3. crutschow


    Mar 14, 2008
    Filters have phase shift. It's part of doing the filter function and can't be avoided.
    Why is the phase of concern to you?

    The resolution of your simulation is low, giving sharp peaks on the sinewave. You need to adjust the simulation parameters to improve that.
    Papabravo likes this.
  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    Notice that what you call the unstable part is the same frequency as your main signal. This is the first clue that it is not unstable. It is the effect of group delay, another filter characteristic.

  5. Papabravo


    Feb 24, 2006
    To be unstable the amplitude of the response would have to increase without bound as t →∞.
    No bounded response can be characterized as unstable.