Setup to test a filter

Discussion in 'General Electronics Chat' started by atferrari, Apr 5, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I have never done this before.

    Today I started to test the state variable filter in the attached circuit.

    My setup is just a (brand new) function generator sweeping from 30 Hz up to 30 KHz, 1 V pk-pk - 0 offset. Output impedance set to 50 ohms and signal applied to R8 which not yet connected to the preamp.

    Soon I realized that I needed to trigger the scope (generator's 2nd channel, delivering pulse with the same period of the sweeping).

    Basically I get a response that barely looks what I could expect for HP, BP and LP (albeit irregular and asymetric) but I cannot find anything that I could call a notch.

    Op amps polarization checks as +4,4V, in case you ask.

    Eventually I could take some pictures tomorrow.

    My questions: am I doing right?

    How to measure fc? I suspect there is an obvious way but I cannot think of any right now. I have no markers available.

    Comments and suggestions will be aprreciated.
     
    Last edited: Apr 5, 2014
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    If you pass white noise through the filter and observe the output in the frequency domain, you can see the filter response easily.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    Post a picture of the oscilloscope sweep.
     
  4. #12

    Expert

    Nov 30, 2010
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    Use the signal generator to drive the horizontal sweep at a logarithmic rate, if it has that ability. Mine does, and I built it from a kit about 40 years ago.
     
  5. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Hola crut

    After a long time trying, I learnt that my DSO has a strange way of changing settings, even if I do not use the autoset mode.

    The circuit is assembled in a very cramped perfboard. Maybe there is something wrong that I am not seeing.

    This morning I assembled the same circuit in a protoboard :eek:, brought back in service my old and friendly analog scope and enabled the generator's sync output.

    The notch is there, as we use to say, "big as a house" together with other expected outputs.

    Being my first time with this, I feel happy. :) :p

    Thanks for replying.
     
  6. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Now that I have it working, have to see how. Is it just using the FFT function?
     
  7. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Yes, logarithmic sweep.
     
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  8. tshuck

    Well-Known Member

    Oct 18, 2012
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    ...That'll do it...
     
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