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2-pole active filter question — Parallax Forums

2-pole active filter question

FovakisFovakis Posts: 8
edited 2013-06-30 10:36 in Robotics
Hello! I am desiging in PSpice a 2-pole active filter. the values are taken from a wikipedia example to be sure about the fcut.

http://i1284.photobucket.com/albums/a578/fovos1/activefilter2polesallenkeytopology_zps30b85696.png

this is the circuit http://en.wikipedia.org/wiki/File:Sallen-Key_Lowpass_Example.svg

and the fcut=16KHz. but when i am doing it in pspice the fcut is at 10khz.

i tried with other values and again problem. i read about FREQUENCY SCALING FACTOR accoring to Butterworth filter but i don't understand
why for example wiki has wrong fcut. what is this value?

at pages 9 to 10 texas instruments has some tables with FSF depending of what filter design you like.BUT ONLY ABOUT LOW PASS FILTERS....
http://www.ti.com/lit/an/sloa049b/sloa049b.pdf

please someone to explain.thanks a lot :)

Comments

  • TtailspinTtailspin Posts: 1,326
    edited 2013-06-30 08:18
    Welcome to the forums Fovakis.
    I don't want to see your third attempt at getting help to go unnoticed.

    I think this thread should be moved to the general discussion section. I am sure you will get the help you need.
    (I am starting to think they should change the name of this forum section from Projects, to Finished Projects...)

    Honestly, I have no idea what you are asking, I'm sure it's important, I just don't have the knowledge, I wish I could help. The best I can do, is suggest you request your thread be moved to General Discussion...:smile:


    -Tommy
  • Beau SchwabeBeau Schwabe Posts: 6,559
    edited 2013-06-30 09:56
    This might be a rounding error.... look at the 'green' trace in your image, notice where the sample points are. Looks like the convergence with the sample point is at about -3.5dB and not -3dB. At -3.5dB it might be 16kHz.

    Is there a way you can increase your resolution in the simulation?
  • FovakisFovakis Posts: 8
    edited 2013-06-30 10:30
    Ttailspin wrote: »
    Welcome to the forums Fovakis.
    I don't want to see your third attempt at getting help to go unnoticed.

    I think this thread should be moved to the general discussion section. I am sure you will get the help you need.
    (I am starting to think they should change the name of this forum section from Projects, to Finished Projects...)

    Honestly, I have no idea what you are asking, I'm sure it's important, I just don't have the knowledge, I wish I could help. The best I can do, is suggest you request your thread be moved to General Discussion...:smile:


    -Tommy

    No problem about the move to General Discussion ~: )
  • FovakisFovakis Posts: 8
    edited 2013-06-30 10:36
    Beau Schwabe (Parallax) wrote: »
    This might be a rounding error.... look at the 'green' trace in your image, notice where the sample points are. Looks like the convergence with the sample point is at about -3.5dB and not -3dB. At -3.5dB it might be 16kHz.

    Is there a way you can increase your resolution in the simulation?

    Hi and thanks for the reply.it is not a problem of cursor or resolution my friend :P
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