Need help getting oscillator circuit to drive 2.4MHz piezo

Discussion in 'Analog & Mixed-Signal Design' started by MatCat, Nov 7, 2018.

  1. MatCat

    Thread Starter New Member

    Aug 18, 2012
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    [​IMG]
    I am trying to recreate this circuit based on this patent located here: https://patents.google.com/patent/US3989042

    I tried this circuit with the following components:
    R1: 1K (Though I tried a 10K pot from 10k to the point of it blowing up)
    C1: 560pF
    L1: 8.2uH
    TD: 20mm Piezo with resonate frequency of 2.4MHz ~100KHz
    Q1: BU406
    C2: I tried a few different values, 33nF, 1.2nF, 100nF, 10nF, 47nF
    L2: 100nH
    C3: 4.7uF

    The circuit will suck down plenty of current but is not oscillating, any help would be GREATLY appreciated!
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
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    What kind of 2.4 MHz "piezo"? - without more details, the description is largely meaningless. Can you post a datasheet?
     
  3. MatCat

    Thread Starter New Member

    Aug 18, 2012
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  4. MatCat

    Thread Starter New Member

    Aug 18, 2012
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    The only data I can find on any 20mm 2.4MHz piezo is the MIST-PZT4-20*2400-FLAT, which specifies:

    Resonant Frequency : 2.4MHz+ - 0.05MHz

    Resonant Impedance : ≤2 ohm

    Static capacitance Cs: 1600pF±20%@1kHz Voltage Input: 48V Max Power Input
     
  5. ebp

    Well-Known Member

    Feb 8, 2018
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    It does appear trying to find any actual data may be futile. There are other similar products with resonant frequencies around 1.6 MHz.

    If you have a suitable signal generator and and oscilloscope, you could try to find the resonant frequency by applying a moderate AC drive voltage (a volt or two is probably sufficient) through a resistor (perhaps 1k) and look at the amplitude of the voltage across the transducer. There may well be multiple resonant frequencies and there is probably a main "anti-resonant" frequency close to the resonant. If you don't have a signal generator, you might be able to get some useful info by applying a square wave with fast rise and fall of perhaps a few tens of kilohertz, again through a resistor, and looking carefully for ringing after the edges of the excitation waveform.

    Just found this
    https://www.americanpiezo.com/knowledge-center/piezo-theory/determining-resonance-frequency.html
    which may be helpful

    I have to run off for a few minutes to take care of something. I'll look more closely at the doc linked and come back if I have any further comments.
     
  6. MatCat

    Thread Starter New Member

    Aug 18, 2012
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    Well the piezo is supposed to be 2.4MHz resonate frequency with +/- 0.05KHz error rate, so we KNOW what the resonate frequency IS, its a matter of getting this circuit which is based on a colpitts oscillator .... oscillating :). The idea is that the circuit is supposed to oscillate AT the resonate frequency by being a part of the oscillator topology.
     
  7. MatCat

    Thread Starter New Member

    Aug 18, 2012
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    I do have my scope with me, but I am on a business trip and do not have my siggen with me , however I am pretty confident that its resonate frequency really will be close to 2.4MHz, I have done crystral oscillator circuits in the past that could take any crystal from a range of a few MHz up to about 40MHz without changing the side components, I don't really understand how this would be much different other then efficiency being more important because of the high power draw (it should be roughly 25 watts when running correctly.).
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    2,019
    Try reducing C2 to, say, ~1n.
     
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