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Oscillator and audio harmonic filtering...

Discussion in 'Misc Electronics' started by J.Shrum, Jun 29, 2006.

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  1. J.Shrum

    J.Shrum Guest

    I'm currently working on a project that requires recording multiple audible
    frequencies to the same audio file. Basically I'm recording the tone and
    monitoring the freq spectrum for freq changes over a long period of time.
    I'm using a 555 to produce the tone, since its frequency changes as the
    condition of what I'm monitoring changes.
    My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
    am wanting to remove these harmonics so I can use that bandwidth for other
    tones simultaneously. Therefore I'd be able to monitor several sensors at
    once... each sensor consisting of the same 555 circuit of course.
    I realize I could use a notch filter on every oscillator, but I'm wanting to
    see if there is a different way of filtering the harmonics, or even a way to
    produce the tones w/out even creating the harmonics.
    I was considering using an electret mic element coupled w/ a transducer
    assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
    know if this will work.

    Any thoughts? Any help would be appreciated.

    Thanks as always
  2. Luhan

    Luhan Guest

    Better to use a sinewave generating IC. Intersil 8038 as I recall;
    maybe some others available today.

  3. Guest

    You dont need a notch filter just a low pass RC filter. If you can
    ensure you have a square wave output the even harmonics are supressed
  4. Tim Wescott

    Tim Wescott Guest

    Google on "Wien Bridge Oscillator". Built right one of those would have
    very low harmonics so you could really pack the tones in, and a
    diode-stabilized version, while not having the very lowest harmonics,
    would be smaller than the harmonic filter you'd need to put after a 555.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  5. Joerg

    Joerg Guest

    You either need to generate a sine wave to start with or use the output
    of your square wave oscillator and filter it right at the source. Either
    with LC or with an active filter around opamps. I don't think a
    microphone/speaker combo will cut it, plus it would likely consume more
    space than a real filter and pick up surrounding audio noises.
  6. Mochuelo

    Mochuelo Guest

    Using square waves for that is one of the best ways to make your life
    harder unnecessarily. I would use sine waves generated and added
    digitally (with an MCU or a DSP). You said "audible" frequencies, so
    you should be able to also include some noise shaping, in case you
    need higher resolution. You may not even need a DAC. Just a PWM output
    and an external filter could work.

    However, I think the whole strategy is bad. If the only information
    that your tones carry is their frequency, why don't you just store,
    for each tone, a value that is proportional to its frequency? Your
    file will be much smaller.

    What do your sensors provide? A variable voltage? A wave with variable
    frequency? In the first case, you should digitize the voltage and
    store it. In the second case, you should measure the frequency (easy
    with an MCU) and store that measurement. Forget about storing the
    whole wave.

  7. Joerg

    Joerg Guest

    Hello Tim,
    Such as this one here:
  8. Guest

    Take a look on XR2206 , generate a sinewave with low distortion.

  9. With a square wave, the first harmonic is the third. Depending on the
    magnitude of the expected frequency deviation, you might be able to put
    the frequencies of several sensors in the space before the third
    harmonic of the lowest frequency one.
  10. J.Shrum

    J.Shrum Guest

    Excellent advice everyone. I'm going to carefully review all of the
    suggestions and decide what is most feasible.

    To describe my exact useage:
    I am using thermistors to vary the tone freq of the 555. I then record that
    over a period of hours. Then I can go back and view the spectral readout of
    the wav/mp3, and look for any temperature dips, and spikes. So far its
    worked very well, ... just gotta get rid of the harmonics so I could monitor
    more locations w/ the use of the same recorder.
  11. J.Shrum

    J.Shrum Guest

    If I understand you correctly, are you suggesting use a circuit to alternate
    between each sensor and record a 1-2 second bit of each one? I assume if
    that was done, I would be able to identify the current sample, because it
    would be the current lowest frequency in that particular time space. Did I
    understand this right?
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