# uC-adjustable harmonic oscillator circuit @100W

I'm looking for a circuit that performs a harmonic oscillation while its resonance frequency can be controlled with a microcontroller (preferably via an analog signal). The circuit has to drive a set of ultrasonic transducers with a maximum power of 100 W at ~ 50 V and a maximum frequency of 100 kHz.

Till now I'm using a h-bridge circuit for my low-power setup (using the 0D24K2 transducers) which dissipates most of the power by (dis-)charging the transducers' capacitance. I can't find any datasheet, but it might be similar to When I now increase power and frequency, power dissipation will increase as well. Another disadvantage is that driving the transducers with a square wave generates unwanted higher harmonics, but I'd like to have a signal with a pure spectrum.

Some solutions that came to my mind are using:

• a LC resonant circuit, but right now I'm unsure how to control its frequency with a microcontroller.

• an oscillating circuit with operational amplifiers. I guess it might be difficult to generate the needed power and frequency at acceptable cost.

• Seems like an X-Y question to me. The question I read is "How do I produce a 100W, 50V, 100KHz sine wave." But that makes it a design request, which are not answered here. I would make a run-of-the-mill class AB amplifier for 50V and >100KHz. Jul 27, 2019 at 13:56
• Varactor diodes in parallel will shift capacitance to alter your frequency, they are aimed at higher frequency but enough of then would work here as well. What is the f range you want to shift though? Jul 27, 2019 at 14:02
• @Oldfart Thanks for your suggestion! I'm not looking for a ready-to-go design, but a hint in which direction I should go. The ideas I had seem to have their disadvantages, so I wonder what kind of circuit would be straight forward. Please note, that I also need to tune the frequency in a range of +/-10% of its resonance. Jul 27, 2019 at 14:02
• @MadHatter Thanks for introducing this kind of component to me, I didn't know it! Jul 27, 2019 at 14:05
• Drive a set of ultrasonic transducers ? A pair? An array? There may be an arrangement of driving transducers where the annoying capacitances cancel...but their phases might be fouled (likely not what you want). Jul 27, 2019 at 14:06

There’s a good reason why piezo type transducers are driven brutally with a square wave and that is because response and decay times are minimised. So, if you don’t care much about the attack and decay times of energy you are trying to transmit then incorporate the device into an RLC circuit where it (the device) forms the bulk of the C element.

If you want fast response and an unambiguous off state then be brutal and drive with a square wave.

• Thanks for your answer. Those higher harmonics (odd multiples of the fundamental) are far away from the piezo's resonance which have a bandwidth of only a few kHz. So they will be damped by the piezos anyway, resulting in a more or less pure sine (right?). Just another question: will those harmonics (or driving piezos out of resonance in general) be harmfull to them? Jul 28, 2019 at 11:48
• Link to the piezo in question and provide any relevant details like mechanical resonating elements (Helmholtz stuff etc.) for me to provide a proper comment. Jul 28, 2019 at 11:53
• Unfortunately I didn't find any datasheet. It's the transducer described here: rcgroups.com/forums/showthread.php?1037515-Meet-the-Matsushita Jul 28, 2019 at 12:10
• I need a data sheet. Jul 28, 2019 at 12:44

Before you decide how you are going to drive the ultrasonic element you need to understand what the device model looks like. Then you need to decide whether they are in series or parallel resonance (of the multiple elements). You should start reading here and here, to gain this understanding.

It will be difficult to drive the elements without either a series resistor or inductor as the amplifer damping will lower the Q of the output circuit. This commercial offering shows the output drive configuration:

From here on page 112

At 1-5W range you could get by with a series R, but at 100W you will certainly need a series inductor. You don't need the main transformer, but you certainly need a series element and the adjusting capacitors for your task.

• Alright, my task seems far more difficult than I expected. Thanks for those links, I'll go through that, but probably the advantages I had in mind are not worth the effort... Jul 28, 2019 at 11:42