Strange output when building an active filter

Discussion in 'General Electronics Chat' started by richiechen, Oct 8, 2012.

  1. richiechen

    Thread Starter Member

    Jan 1, 2012
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    0
    Hi all

    I was trying to build two active filters.
    The attached diagrams are the schematic and the output.

    The input is a sine wave.
    Could anyone give me an intuitive idea of how do these strange diagrams come out?


    Thank you very much.
     
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    • f3.jpg
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    Last edited: Oct 8, 2012
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Is this real or just simulation?
     
  3. richiechen

    Thread Starter Member

    Jan 1, 2012
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    It is real!
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    I see a flaw in your schematics. Take a look at the wire that cross the connection between R1_2 and R1_3 This wire miss a dot. So it is not connected at the crossing point as it should be. If you have made a PCB. This can have a lot to say ;)
     
  5. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    Thanks t06afre! I will check the PCB

    If an active filter is made. How could the response of the filter being unstable?
    The input of the filter is 1kHz signal.
    The output appears that the gain of the filter keeps changing.

    I mean, the gain of the filter to a signal of a certain frequency should be constant, right?
    Why the gain varies? When the input is 1kHz signal(stable frequency).
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Have you looked at the output with no input signal?

    From the screen capture you posted,it looks like one stage is oscillating at a very much lower frequency,& amplitude modulating the input signal.

    If this is a real circuit & real Oscilloscope,try tracing the signal through from input to output to see where the problem is.
     
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