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building a sine wave oscillator

Discussion in 'Electronic Design' started by Joe, Mar 6, 2008.

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  1. Joe

    Joe Guest

    Hello to the group,

    I am looking for some ideas on how to build a 24khz sine wave
    generator to drive an ultrasonic piezo transducer.

    Or, if someone knows of a way that I can change a square wave from a
    555 to a sine wave, that would work too.

    I have googled and found something called a wien oscillator, which
    uses a light bulb as a non linear element. I am just wondering if
    anyone knows of a way to do it without having to use a light bulb.

    Any ideas, suggestions, links greatly appreciated.

    TIA,

    Joe
     
  2. The harmonics of the 24kHz do little except draw some
    capacitive current. You can limit them with a series
    resistor, or resonate the piezo with a series inductor and
    really reduce the harmonics. I doubt there is a good reason
    to produce a pure sine wave. A 555 timer produced square
    wave may be good enough.

    What voltage do you need driving the piezo?

    What is the piezo capacitance?
     
  3. Al

    Al Guest

    Check this out for a phase shift osc.

    http://en.wikipedia.org/wiki/Phase-shift_oscillator

    That will be your starting point.

    Al
     
  4. John Fields

    John Fields Guest

     
  5. D from BC

    D from BC Guest

    I have an offbeat idea.. :)

    Use a computer sound card with 2 tone into a frequency mixer.
    http://en.wikipedia.org/wiki/Frequency_mixer
    Say..
    10khz + 11 khz for 21khz
    Perhaps use a multiplier chip..
    Then filter everything below 21khz.


    D from BC
    British Columbia
    Canada.
     
  6. Phil Allison

    Phil Allison Guest

    "John Fields"
    Joe

    ** Betcha it's a " woofer stopper ".





    ....... Phil
     
  7. Charles

    Charles Guest

    The Wien bridge oscillator can be amplitude/gain limited using a diode/FET
    combination to dynamically adjust the loop gain.

    However, a square wave might be OK. Also, a low-pass filter might be OK if
    the harmonics are going to cause problems for your application.
     
  8. Guest

    If you can live with low level harmonics, you could use a Baxandall
    class-D oscillator, which has the advantage of being remarkably
    efficient, if your piezoelectric transducer looks more like a
    capacitor than a resistor (that is lightly damped rather than heavily
    damped by medium into which you are driving your ultrasound).

    I posted a discussion of the circuit on my web-site some time ago

    http://home.planet.nl/~sloma000/Baxandall parallel-resonant Class-D oscillator1.htm
     
  9. John Fields

    John Fields Guest

     
  10. Fred Bloggs

    Fred Bloggs Guest

    I think the industry has since advanced to Class E operation, same idea
    but with much better efficiency:
    http://www.freepatentsonline.com/5179511.pdf
    The OP is clearly musing over a from_scratch build of an ultrasonic
    range finder, he would be better off acquiring a driver on the surplus
    market.
     
  11. Joe

    Joe Guest


    Hi John,

    The piezo that I am working with is an "in air", ie, it is built for
    in the air use. No specs as it was a bargain from Allelectronics.
    Eventually, though, I will be working with an underwater 24khz
    transducer (PZT) to transmit a 24khz carrier and digital modulation
    across the depths of Boston harbor. I am practicing with the "in air"
    model. You are right, a square wave from the 555 works OK, but my
    understanding of piezo is that it expands and contracts as the
    electric field across it changes, so to maximize the performance, a
    sine wave would probably be better. The voltage I am using right now
    is 12Vpk, but I know it can go up to 20Vrms, as most of them can. I
    know because I tried using sine waves from my function generator, and
    a power tranny with a cap (trial and error for the best output
    signal), and when I cranked the supply up just over 28Vpk, it made a
    popping sound, and some smoke was let out, and then it didn't work
    anymore. I have no specs yet on the underwater transducer that I will
    be using as I have not decided which company to go with. They are
    difficult to find. Airmar makes them, and a company called sensortech
    up in Canada makes them also.
     
  12. Joe

    Joe Guest

    Hello John,

    Right now, I am practicing, Eventually, this will be used in an
    underwater acoustic modem.

    Joe
     
  13. Joe

    Joe Guest

    Hello D,

    Actually, I am using the mpy634 multiplier chip (Burr Brown, now TI)
    and a 555 generated carrier, and the modulation is coming from a
    function generator meant to simulate audio data.
    This will have to be a portable application eventually, and the
    underwater transducers normally dissipate a couple of hundred watts
    (low duty cycle though). It works ok, but I would like to maximize the
    performance of the eventual instrument.

    Joe
     
  14. Then you can lock the 555 very close to the LC resonance by
    taping the piezo voltage with a high impedance RC low pass
    filter (say, 100k, 100pF) and connect the capacitor voltage
    to trigger and threshold. Pick the inductor to set the
    frequency. Something around a few milli henries should work.
     
  15. I think you will find that the at resonance properties of a
    piezo in air will be quite different than one in water.
    The corners of the square wave require a lot of current from
    the driver. Adding a series inductor reduces those corner
    currents. Adding a resonant inductor, so that the
    electrical resonance of the inductor and the capacitance of
    the piezo approximately matches the drive frequency,
    effectively removes most of the harmonic current from the drive.
    The series resonating inductor will make the piezo voltage
    higher than the square wave voltage.
    Expect to have to make significant design changes when you
    go from piezo in air to piezo in water. I think a piezo in
    water has a significantly less mechanically resonant
    operation, since water is a much better mechanical impedance
    to the element, so it carries energy away from it more
    efficiently. To get a similar electrical resonance, you may
    need to parallel it with a good quality capacitor and lower
    the inductance to see similar electrical resonant Q (at a
    higher drive current).
     
  16. VWWall

    VWWall Guest

    The main problem will be driving whatever transducer you select. You
    will need to match the transducer to the driving amplifier. For maximum
    efficiency, the driving impedance needs to be the conjugate of the load.

    You will need to measure the transducer characteristics under water and
    driven at the carrier frequency and waveform you will use.
     
  17. Guest

    And a much better chance of melting the switching transistor. "Much
    better efficiency" is an intersting claim - the Baxandall circuit
    typically offers better than 90% efficiency - Jim Williams claims 92%
    in

    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1033,P1093,D4154
    A resonant driver isn't all that appropriate in that context.
     
  18. Joe

    Joe Guest

    Hi John,

    Well, sheepishly, I must say, I screwed up my algebra again. The
    capacitance is looking like around 82pf at 24khz.

    Joe
     
  19. Joe

    Joe Guest

    LOL, Joseph, so you caught my algebra goofup too. There is no excuse,
    I know, so I am not going to try and make any.
    I will not use a light bulb. There are other nonlinear elements I can
    use. I DO know the principles that the wien oscillator works by.
    Also, there ARE other oscillators I can use, which I am just now
    discovering

    Thank you for pointing this out.

    Joe
     
  20. Joe

    Joe Guest

    Dear M...

    I already have a hydrophone. That is what I am going to use to receive
    the 24khz. It will receive or transmit up to 25khz, the mfr says to
    stay away from 25khz if transmitting, but I can receive at any
    frequency within its range with no problem. Been there, done that.

    As far as research, you might want to do a little yourself, start
    here: http://www.massa.com/fundamentals.htm

    The what you call 'loss' is proportional to 1/r^2, just like em waves.
    Because of the spherical or hemispherical nature of the wave
    propagating thru the water. There are also absorption losses due to
    the manganese dioxide present in sea water. At 20khz, this amounts to
    -3dB in a kiloyard (I know, unfortunately some folks still use the
    imperial system). Now the 1/r^2 I am speaking of is the acoustic
    energy flux density (aka intensity), NOT the sound pressure level. The
    sound pressure level falls off as proportional to 1/r. I would refer
    you to a great book on this very subject:
    "Principles of Underwater Sound" by Robert Urick <sp>, and also
    "Transducers and Arrays for Underwater Sound", by Sherman and Butler.
    You can take a quick peek at them at amazon.

    I know a little something about the physics of sound propagation in
    different media.

    My original question was about building a sine wave oscillator, which
    has been answered, and I wish to thank everyone who replied. I know
    what I am up against as far as sending sound through the ocean. I am
    trying to design with those principles in mind, hence, the reason I
    wanted to maximize the output from my multiplier.

    Electronics has been a hobby for me, so that is why I come here and
    ask questions on occasion. Most of the time, I just lurk and learn.
    Oh, and screw up algebra.

    Thanks again to all who replied. I believe I can make my way from
    here.

    Joe
     
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