Ripples in stopband of filter

Discussion in 'The Projects Forum' started by nazcov, Jan 20, 2013.

  1. nazcov

    Thread Starter New Member

    Jan 20, 2013
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    0
    Hi everyone

    I was hoping on some help to a project I have done this semester.

    The circuit is actually done and working but there are some minor complications.

    I have uploaded a picture of the part in the circuit which I'm curious about (It's a lowpass filter with cutoff frequency 500Hz) along with the LTSpice file.

    So in my stopband there are some ripples that i cant explain. They seem to be there in my HP filter also, but much less (that is also only a 3rd order). Along with that, when I test the filter, it has a cutoff-freq at 500Hz damped by 4.5dB, and the HP is at its normal 3dB (Both are at 3dB in the simulation).

    Hope you can help.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    It's the analysis steps you are using. Change the analysis from 10 steps/decade to 1000 steps/decade.

    The difference between simulation and test may be the tolerance of your actual components. The simulation uses perfect components, of course.
     
  3. nazcov

    Thread Starter New Member

    Jan 20, 2013
    8
    0
    Thank you very much, that worked. I figured it might be the tolerance of the components since the capacitors is 10%.

    Another question if OK - When i put my HP(3rd order) and LP(5th order) together to a bandpass filter, the phase makes some waves in the simulation. Is it because of the difference in order? I figured that was why.

    Thank you once again
    Sonny
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    You can try changing one or more of the capacitor values in the simulation by 10% to see the affect.

    Yes, the phase-shift shown is likely normal for your circuit. The phase shift of complex circuits can vary significantly up and down with frequency as you pass the various pole and zero frequencies.
     
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